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High-yield and Selective Photoelectrocatalytic Reduction of CO2 to Formate by Metallic Copper Decorated Co3O4 Nanotube Arrays Qi Shen, Zuofeng Chen, Huang Xiaofeng, Meichuan Liu, and Guohua Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00066 • Publication Date (Web): 06 Apr 2015 Downloaded from http://pubs.acs.org on April 12, 2015
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High-yield and Selective Photoelectrocatalytic Reduction of CO2 to Formate by Metallic Copper Decorated Co3O4 Nanotube Arrays
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Qi Shen, Zuofeng Chen, Xiaofeng Huang, Meichuan Liu, Guohua Zhao*
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*Department of Chemistry, Shanghai Key Lab of Chemical Assessment and
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Sustainability, Tongji University, 1239 Siping Road, Shanghai 200092, China
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ABSTRACT: Carbon dioxide (CO2) reduction to useful chemicals is of great
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significance to global climate and energy supply. In this study, CO2 has been
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photoelectrocatalytically reduced to formate at metallic Cu nanoparticles (Cu NPs)
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decorated Co3O4 nanotube arrays (NTs) with high yield and high selectivity of nearly
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100%. Noticeably, up to 6.75 mmol·L-1·cm-2 of formate was produced in 8 h
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photoelectrochemical process, representing one of the highest yields among literature
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reports. The results of SEM, TEM and photoelectrochemical characterization
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demonstrated that the enhanced production of formate was attributable to the
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self-supported Co3O4 NTs/Co structure and the interface band structure of Co3O4 NTs
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and metallic Cu NPs. Furthermore, possible two electron reduction mechanism on the
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selective PEC CO2 reduction to formate at the Cu-Co3O4 NTs was explored. The first
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electron reduction intermediate, CO , was adsorbed on Cu in the form of Cu-O. 2 ads
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With carbon atom suspended in solution, CO is readily protonated to form 2ads
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HCOO‒ radical. And HCOO‒ as product rapidly desorbs from the copper surface with
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a second electron transfer to the adsorbed species.
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1. INTRODUCTION
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The global energy consumption has been increasing dramatically in the past two
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decades. While largely met by fossil fuels, the rapidly increasing global consumption
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for this limited resource has generated growing concern over their future availability.
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Meanwhile, carbon dioxide (CO2) emissions from the use of fossil fuels have an
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adverse and irreversible impact on global climate. Global warming resulted from the
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significant rising in the atmospheric CO2 level has become the most serious
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environmental concern.1,2 The fixation and transformation of CO2 into high
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value-added hydrocarbon fuels is one of the best solutions to problems of both the
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energy supply and global warming.3,4
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Photocatalytic (PC) reduction of CO2 into hydrocarbon fuels, as a promising
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approach for realizing sustainable development has been persistently drawing
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attention,5,6 but typically suffers from slow kinetics, poor product selectivity, and
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mechanistic complexity.7 Single electron reduction of CO2 to •CO2– occurs at −1.90 V
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vs. normal hydrogen electrode (NHE) requiring highly reducing equivalents.8 In this
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process, a high reorganization energy exists arising from the significant structural
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change associated with the transformation from linear CO2 to bent •CO2–.
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Proton-coupled multi-electron reduction could avoid the high-energy 1e‒ intermediate
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which requires building up multiple redox equivalents at single sites or clusters. This
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approach, however, typically leads to a poor product selectivity of CO2 reduction.9
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The products of CO2 reduction vary with the applied potentials. In this regard,
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photocatalysis with judicious potentials biased may drive reactions to achieve desired
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products. The applied potential by electrocatalytic (EC) process can not only
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accelerate the separation of photoinduced carriers, but also extend the relaxation time
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of the excited state of molecular CO2, and thus increase the chance of C=O bond
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breaking and subsequent hydrogenation. On the other hand, the light irradiation can
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assist to lower the electrochemical barrier and promote the electrode kinetics.
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Photoelectrocatalysis (PEC) process, combining the merits of both EC and PC, thus
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represents a more promising approach to reduce CO2 efficiently and selectively than
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individuals.10 Combining PEC CO2 reduction, ideally driven by solar light, with
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energy extraction by combustion or fuel cell applications would provide a renewable,
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environmentally friendly basis for use of carbon-based fuels in a new energy future.
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Recent studies on metal oxide based photoelectrodes, such as TiO211, Mg-Doped
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CuFeO212, SrTiO35, demonstrate outstanding CO2 reduction performance. However,
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the catalytic activity and selectivity of these photocatalysts are still not satisfied. The
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products were almost C1 or C2 compounds. Besides, H2 was largely produced as a
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by-product usually. As a p-type semiconductor oxide with a band gap of 2.07 eV,
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Co3O4 has been used as a visible-light driven photocathode to PEC product HCOO‒
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from CO2.13 However, the catalytic performance of pure Co3O4 is far from satisfaction
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due to the slow separation of photo-generated electron-hole pairs within this material.
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And now anodic oxidation technology has been applied to in situ growth of
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one-dimensional (1D) Co3O4 nanotubes (NTs) layer on Co foils, which shows
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promising
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pseudo-capacitive material for supercapacitors.14,15 Such a 1D NTs structure is in
applications
as
an
electrocatalyst
for
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oxidation
and
a
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favor of the separation of photo-generated electron-hole pairs. Furthermore, loading
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of metallic nanoparticles or clusters appropriately as electron trap has been reported to
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improve PC performance dramatically.16-19. Among different metals, Cu is unique in
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its ability toward CO2 reduction with products distribution dependent on applied
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potentials.20,21 The work function of Cu (4.65 eV22) is lower than the Fermi level of
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Co3O4 (-6.1eV23). When in contact, the band edges of Co3O4 bend downward and
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photo-induced electrons can transfer facilely from Co3O4 to Cu to drive the PEC
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process. Therefore, the Co3O4 NTs and Cu NPs composite and their interface may
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serve as an outstanding PEC platform for CO2 reduction.
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We report here highly efficient and selective PEC CO2 reduction to formate at a
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metallic Cu decorated Co3O4 NTs under visible-light irradiation. A well-aligned,
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upright 1D Co3O4 NTs has successfully grown at a Co foil substrate. The NTs layer
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has a high specific surface area and allows for facile electron transfer, making it an
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excellent substrate for Cu NPs loading and CO2 reduction. Moreover, the nanotube
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structure can also trap the incident light efficiently by internal multi-scattering. The
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yield of formate was comparatively high related to those in the literature and its
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selectivity was close to 100%. Possible mechanism of the selective PEC CO2
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reduction to formate at the Cu-Co3O4 NTs was explored.
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2. EXPERIMENTAL SECTION
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2.1. Materials. Cobalt (99.5%) was purchased from Aladdin Industrial Inc., and
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ammonium fluoride (NH4F), glycerol, ethylene glycol (HOCH2CH2OH), Cu2SO4
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were all analytical reagent and purchased from Sinopharm Chemical Reagent Co.,
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Ltd., SCRC, China.
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2.2. Preparation of Cu-Co3O4 NTs Electrode. Cobalt foils were polished to a
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mirror finish before use. Before anodization, the foils were cleaned via ultrasonication
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in acetone and ethanol successively, followed by drying the samples in the flow of N2.
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Electrochemical anodization was performed using a DC power supply (Sovotek,
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E5200-3 75v/2A DC Power Supply, DaHua Electronic, China) in a two-electrode
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configuration with a platinum foil as counter electrode. Anodization was carried out at
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0 oC and 50 V for 8 h to grow Co3O4 NTs layers in electrolytes including ethylene
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glycol and glycerol (1/3, v/v) with 3 M H2O and 0.54 M NH4F.14 After the
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anodization process, the samples were rinsed with ethanol and then dried in air,
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followed by heat-treating at 350 oC for 30 min in air.
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Cu NPs filling Co3O4 NTs were decorated by pulsed galvanostat method under
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high current conditions.22 The electrodepositions were carried out in a conventional
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three electrodes system using CHI 660C with the electrolytes solution containing 0.4
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M CuSO4 and 3 M lactic acid at room temperature. The concentrated lactic acid acted
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as a complex agent for stabilizing the copper ions.24 And then the pH of the solution
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was adjusted with NaOH to 7.00. Pulsed approach was rationally designed with 1 s
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negative current pulse time and 7 s delay time with a constant current density of 50
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mA·cm‒2. To obtain a uniform deposition of Cu NPs into the nanotubes, the pulse
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cycles performed at 10, 20 and 30 cycles were investigated.
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2.3. Characterization. The morphology of the as-prepared electrodes was
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characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi
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S-4800, Japan) and transmission electron microscopy (TEM, JEM-2100, JEOL,
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Japan). X-ray diffractometer (XRD, D/max2550VB3+/PC, Rigaku International
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Corporation, Japan) was carried out to determine the crystalline structures of Co3O4
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and Cu-Co3O4. Surface elemental analysis of Co3O4 and Cu-Co3O4 were performed
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on an X-ray photoelectron spectroscopy (XPS, AXIS Ultra HSA, Kratos Analytical
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Ltd., UK). The binding energies obtained in the XPS were all corrected by referencing
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the carbon 1s peak to 284.7 eV. The optical absorption properties were investigated by
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AvalightDHS UV-Vis absorbance measurements (UV-DRS, Avantes, Netherlands).
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All PEC measurements were performed on a CHI 660C electrochemical
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workstation (CH Instruments, Inc.) using a conventional three-electrode system with
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the as-prepared electrode as working electrode, saturated calomel electrode (SCE) as
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reference electrode, and a platinum foil as counter electrode in 0.1 M Na2SO4 aqueous
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solution. All the potentials were referred to SCE unless stated otherwise. Linear
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sweep voltammetry (LSV) was performed from the potential range of -0.2 to -1.1 V at
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a scan rate of 0.05 V·s-1 in order to avoid the oxidation of Cu NPs. The amperometric
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i−t curve was recorded at a constant potential of −0.9 V with an interval of 200 s for
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light on/off under the light intensity of 20 mW·cm−2 (light source: LA-410UV with
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UV cutoff, Hayashi, Japan).
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2.4. PEC Reduction of CO2. The PEC reduction of CO2 was performed in a
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homemade 250 mL gastight chamber with 100 mL 0.1 M Na2SO4 as electrolyte
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solution, as seen in Scheme 1b. Before reduction, high purity CO2 (99.99%) gas was
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bubbled for 30 min at a flow rate of 20 mL·min-1 into the solution until the
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concentration of CO2 reached saturation and the dissolved oxygen was removed
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completely. The concentration of free CO2 measured by acid-base titration was
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0.0331 M and the pH of the solution was 4.11. The potential during the PEC reduction
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was kept constant at −0.9 V under 10 mW·cm−2 irradiation (light source:
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PLS-SXE300 xenon lamp, Beijing PerfectLight Co., Ltd., China, with UV cutoff).
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The formate and methanol in the aqueous phase were determined by high
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performance liquid chromatography (HPLC). For the detection of formaldehyde, 2.0
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mL liquid product was directly mixed with 2.0 mL Nash’s reagent (25 g ammonium
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acetate, 2.1 mL glacial acetic acid, and 0.2 mL acetylacetone dissolved in 100 mL
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ultrapure water), and the mixture was shaken for 1 h. The final solution was analyzed
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by UV−vis spectroscopy (8453, Agilent). The products in the gas phase were
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determined by gas chromatograph equipped with thermal couple detector (Techcomp,
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China).
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3. RESULTS AND DISCUSSION
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3.1. Fabrication and Physicochemical Properties. Scheme 1a illustrates the
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schematic representation of the construction of 1D aligned Cu-Co3O4 NTs. Briefly,
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Co3O4 NTs was prepared by one-step anodization of a Co foil, which was then
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followed by electrochemical deposition to load Cu NPs.
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Figure 1a shows the top view and side view (inset) of SEM images of the
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nanotube arrays layer formed on a cobalt foil after 8 h anodization. As can be seen, a
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well-aligned, upright 1D nanotube arrays layer grew successfully on the substrate.
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The thickness of Co3O4 NTs layer was ca. 2.5 μm and the diameter was ca. 100 nm
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(Figure 1b, inset). Such a structure of nanotube arrays can not only provide a large
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specific surface area for deposition of Cu NPs, but also trap the incident light
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efficiently by internal multi-scattering.25 Additionally, the inside wall of nanotubes
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layer was composed of numerous Co3O4 NPs that could contribute further to an
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increasing specific surface area. Figure 1c shows the microstructure of Co3O4 NTs
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layer loaded with Cu NPs by 20 cycles of pulsed electrodeposition. Cu NPs were
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dispersed uniformly both inside and outside nanotubes. The high-resolution
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transmission electron microscopy (HRTEM) in Figure 1d shows that the diameters of
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Cu NPs were 12 - 15 nm. Under the electrodeposition condition, the original Co3O4
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nanotube structure was well retained.
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Figure 2a compares the XRD patterns of Co3O4 NTs before and after Cu NPs
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loading. Before Cu NPs loading, a set of diffraction peaks indexed to cubic phase
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Co3O4 was observed with the lattice constant a = 8.084 Å, consistent with that of
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JCPDS Card No. 42-1467. Note that the diffraction peaks at 42.0°, 44.7°and 47.6°
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arise from the underlying Co foil substrate. After Cu NPs loading onto the Co3O4 NTs,
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highly (111) and (200) preferred orientation diffraction peaks appeared, corresponding
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to the crystal planes of metallic copper. The Cu (111) crystal plane could also be
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observed from the HRTEM image of Cu-Co3O4 NTs (Figure 1d). Figure 2b shows the
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XPS spectrum of Cu-Co3O4 NTs. Two distinctive peaks without any shaken-up
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satellite peaks were observed at 932.5 eV and 952.5 eV, corresponding to the 2p3/2 and
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2p1/2 spin orbits of Cu (0).26 In addition, the Auger LMM peak at 918.8 eV in the inset
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of Figure 2b is also consistent with the presence of Cu (0).26 These results indicated
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that Cu NPs deposited onto Co3O4 NTs in the form of metallic copper, instead of CuO
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or Cu2O.
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3.2. Photoelectrocatalytic Activity toward CO2 Reduction. UV diffraction
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reflection spectroscopy (UV-DRS) was applied to investigate the light absorption
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property of Co3O4 NPs, Co3O4 NTs and Cu-Co3O4 NTs. Two broad absorption bands
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were observed at wavelength of 300 - 550 and 600 - 800 nm, respectively, for all three
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samples (Figure 3a). Compared with Co3O4 NPs, Co3O4 NTs exhibit stronger
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absorption in the visible spectral range indicative of the enhanced light harvest
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efficiency of nanotube arrays structure, note Scheme 1b. Although the absorption
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intensity of ultraviolet region was weakened slightly with Cu NPs deposited, the
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absorption in the visible spectral range became stronger and broader than the
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individual Co3O4 NTs. The increase in roughness after Cu deposition increased the
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lighted area, resulting in stronger absorption in the range of 600 - 800 nm. Further
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increase in the amount of the Cu NPs deposited results in a less porous structure
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(Figure S1). Accordingly, the absorption in the visible spectral range decreased
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(Figure S3), presumably due to a less penetration for the incident light into the
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framework. The broad and slightly enhanced absorption in the range of 510 - 550 nm
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could be attributed to the local surface plasmon resonance (LSPR) effect of Cu NPs.
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In an earlier report by Liu et al,27 Cu NPs with a diameter of 2 - 16 nm exhibit an
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LSPR absorption peak at ca. 570.0 nm.
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The band gaps of Co3O4 calculated from Tauc plot (Figure S4a) were 1.44 eV
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and 1.77 eV, which could be assigned to Co3+-O2− transition and Co2+-O2− transition,
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respectively.28 The flat-band potential of Co3O4 NTs was estimated to be -0.23 V (vs
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SCE) through Mott-Schottky plot (Figure S5a). Based on the assumption that the
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valence band position is 0.1~0.2 V lower than the flat-band potential for a p-type
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semiconductor, the valence band edge of Co3O4 NTs could be estimated to be -0.19 to
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-0.09 V vs NHE. Thus, the conduction band edge of Co3O4 NTs was located at -1.96
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to -1.86 V vs NHE. After Cu NPs loading, no obvious change on the band gap of
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Co3O4 NTs was observed. Compared to other semiconductors reported for CO2
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reduction, such as GaP (-1.18 V vs NHE),29 Zn2GeO4 (-0.70 V vs NHE),30 and
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ZnGa2O4 (-1.32 V vs NHE),31 the conduction band edge of Co3O4 or Cu-Co3O4 NTs
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is located at a comparatively negative energy level. This provides a larger driving
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force to reduce CO2 than other semiconductor photocathodes on thermodynamic
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feasibility.
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The as-prepared Cu-Co3O4 NTs exhibit excellent PEC activity toward CO2
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reduction. Figure 3b shows that, whether with illumination or not, the current profile
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of a Cu-Co3O4 NTs electrode in N2-saturated solution is relatively flat with a slight
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current increase appeared after -0.75 V, arising from background water/proton
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reduction. By contrast, in CO2-saturated solution, a dramatic current enhancement
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related to CO2 reduction was observed at the Cu-Co3O4 NTs electrode from -0.72 V
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with a current peak appearing at -0.85 V.15 It is worth noting that the onset potential
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observed here was approximately 350 mV more positive than that at a copper
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electrode20 or a copper foam electrode,32 indicating a superior electrocatalytic
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performance of Cu-Co3O4 NTs towards CO2 reduction. On the Co3O4 NTs electrode,
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the onset potential of CO2 reduction at the Co3O4 NTs electrode was -0.74 V, close to
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that at a Cu-Co3O4 NTs electrode, but the catalytic current density was only half of
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that at a Cu-Co3O4 NTs electrode. The catalytic performance is relevant to the charge
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carrier concentration of the nanotube electrodes. As determined by Mott−Schottky
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measurements (Figure S5), the charge carrier concentration of Cu-Co3O4 NTs was
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1.80 × 1024 cm-3, which was 2 orders of magnitude higher than that of Co3O4 NTs
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(4.10 × 1022 cm-3). Under visible light irradiation, the onset potential of the Cu-Co3O4
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NTs and Co3O4 NTs electrodes both shifted positively by 40 mV concomitant with
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increase in catalytic current. These results suggested that the deposition of Cu NPs
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and the introduction of visible light were in favor of CO2 reduction.
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In a more accurate way, the current transformation efficiencies (η) of PEC CO2
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reduction were calculated from LSVs of Co3O4 NTs and Cu-Co3O4 NTs according to
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the following formula: 33
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𝜂=
𝐽𝐶𝑂2 − 𝐽𝑁2 × 100% 𝐽𝐶𝑂2
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Where JCO2 is the current density of CO2 reduction, and JN2 is the current density of
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background in solution saturated with N2 that arises from hydrogen evolution.
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Because the reactions of CO2 reduction and water/proton reduction are in competition
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on the surface of these electrodes, the value of JCO2-JN2 can reflect the net current
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density of CO2 reduction. As shown in Figure 3c, the value of η increased initially
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with the applied potential reaching a maximum one. Beyond that, the value of η
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decreased because the hydrogen evolution reaction gradually became dominant. For 13
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the EC process at the Cu-Co3O4 NTs electrode, the maximum η value was 62.9% at a
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potential of -0.89 V. With the visible light applied to the EC system, it increased by
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23% to 77.5% appearing at a slightly less negative potential of -0.87 V, indicating that
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the PC process can further promote the EC CO2 reduction process. Without Cu NPs
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loading, the η value of PEC process was only 51.6%, which was just 2/3 fold than that
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at the Cu-Co3O4 NTs electrode. It indicates that deposition of Cu NPs facilitates the
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photoelectron transfer from Co3O4 to CO2.
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The amperometric i-t curves of these electrodes under chopped light irradiation
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(Figure 4) also complied with the above results. Under N2 or CO2 atmosphere, the i-t
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curve of the Co3O4 NTs electrode reached a steady state slowly by at least 80 s
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irradiation duration, attributable to the slow separation of photo-generated
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electron-hole pair within Co3O4 NTs. By contrast, at the Cu-Co3O4 NTs electrode, the
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photocurrent reached a steady state at a significantly reduced time of 50 s. The
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photocurrent density of the Cu-Co3O4 NTs electrode reached a steady level of -0.122
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mA·cm-2, which also outperformed that of the Co3O4 NTs electrode (-0.105 mA·cm-2),
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confirming that the deposited Cu NPs expedites the electron transfer leading to the
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enhanced CO2 reduction performance.
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To better understand the Cu NPs-promoted photoelectron-hole separation,
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Mott-Schottky measurements on the flat-band potentials of both electrodes were
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performed. As shown in Figure S5, the flat-band potential of Co3O4 NTs was
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estimated to be -0.232 V, and that of Cu-Co3O4 NTs 0.055 V. It is well known that
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when semiconductor and metal are in contact, the free electrons will flow between
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metal and semiconductor due to their different work functions.34 As a p-type
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semiconductor, the Fermi level of Co3O4 (-6.1 eV)23 is more negative than the work
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function of Cu (4.65 eV)22. When in contact, the electrons will flow spontaneously
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from Cu to Co3O4 until their Fermi levels are aligned (Figure S6). In the reaction
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solution, the flat-band potential of Co3O4 NTs raised from -0.232 V to 0.055 V. Upon
284
equilibrium, Co3O4 was negatively charged and Cu was positively charged at the
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Cu/Co3O4 interface. The band edge of Co3O4 bent downward because of electrostatic
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induction effect as shown in Figure S6. Such band bending can serve as an electron
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trap preventing electron–hole recombination in PC process, which resulted in an
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enhanced PEC performance.
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3.3. PEC Reduction of CO2. Sustained PEC CO2 reduction was conducted in
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100 mL CO2-saturated 0.1 M Na2SO4 solution under visible light irradiation at −0.9 V.
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At both electrodes, formate anion, HCOO−, in the liquid phase, was detected as the
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dominant product, with no evidence for other C1 products-HCHO, CH3OH and
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gaseous CO. With the Cu-Co3O4 NTs electrode, the amount of formate reached 6.75
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mmol·L-1·cm-2 in 8 h, 56% higher than that at the Co3O4 NTs electrode (4.34
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mmol·L-1·cm-2) (Figure 5). This yield is much higher than that at a Cu NPs modified
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glassy carbon electrode (0.065 mmol·L-1·cm-2) in a control experiment. It was also
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comparatively high among those reported in the literature, where the main product
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was formate/formic acid either in EC or PC process.13,35 For instance, the formate
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yield is comparable to that of Ru (II) molecular catalyst attached on semiconductor
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(InP/[MCE2-A+MCE4]).36
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There is evidence for the photoelectric synergy effect of PEC process on the
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yield enhancement. At the Cu-Co3O4 NTs electrode, the yield of formate was 4.14
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mmol·L-1·cm-2 in EC process, accounting for 3/5 that of the PEC process, while PC
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reduction yielded 1.26 mmol·L-1·cm-2 of formate, which was only 1/5 that of PEC
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process. Noticeably, the yield of formate in the PEC process was higher than the sum
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of the PC and EC processes (PC + EC). The greatly enhanced yield of PEC CO2
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reduction could be attributed to the following points: i) The growth of the Co3O4 NTs
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from the underlying Co foil substrate could minimize the contact resistance between
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the substrate and nanotube layer. ii) The well-defined 1D vertical arrays structure of
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Co3O4 NTs possesses more active sites and is favor of light harvesting. In addition,
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such a structure could also shorten the electron transfer distance, thus greatly reducing
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the probability of the recombination of the photo-induced electron and hole. iii) The
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interface band structure between Co3O4 NTs and deposited Cu NPs drives the
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photo-electrons flow to Cu NPs which also prevents the recombination of electrons
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and holes.
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The possible mechanism of selective formate production was explored. As
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mentioned above, copper is unique in its ability to electrochemically reduce CO2. At
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potentials less than -0.5 V vs NHE, only CO and HCOO‒ can be produced from CO2
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reduction at a Cu electrode.37,38 The selectivity toward CO or HCOOH depends on the
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stability of some important reaction intermediates, such as *CO2, *CO, *OCHO, and
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*HCOOH, and the surface desorption energy of *CO and *HCOOH species (*
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denotes activated state).39 At the Cu-Co3O4 NTs electrode, Cu NPs were positively
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charged near the Cu/Co3O4 NTs interface due to the flow of free electron from Cu to
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Co3O4, which made the adsorption of CO2 on Cu NPs in the form of O-Cu, as shown
325
in Figure S7b, c. PEC Tafel plot measurement was used to analyze the electron
326
transfer during CO2 reduction at the Cu-Co3O4 NTs electrode. Figure S8 shows a
327
Tafel plot with a slope of 122 mV/dec under light irradiation. It indicates that the
328
rate-determining step of CO2 reduction was likely the first electron reduction to form
329
CO 2ads
330
photo-electrons generated from Co3O4 NTs flowed to Cu NPs due to the band bending
331
between the interface of Co3O4 NTs and Cu NPs, and captured by the chemisorbed
332
CO2. The suspending carbon atom of chemisorbed CO2 could attract a proton in
333
solution, leading to chemisorbed HCOOads . DFT calculation shows that the formation
334
of a stable intermediate, *OCHO, was favorable to generate HCOO‒.38 In the
335
following step, a second electron rapidly transfers to the chemisorbed HCOOads ,
336
resulting in desorption of HCOO‒ from the electrode surface. Alternatively, the
337
anion radical may react immediately with the chemisorbed H at the CO 2 ads
338
neighboring Cu or Co3O4 NTs to form HCOO‒. In both schemes, the photogenerated
339
electron in PEC process is driven to the electrode surface to react with the carboxylic
340
acid radical, resulting in enhanced yield of formate production.
anion radical at the Cu-Co3O4 NTs. Driven by visible light, the
341
In conclusion, CO2 was effectively reduced to formate at a metallic Cu decorated
342
Co3O4 NTs electrode by PEC method. The self-supported 1D Co3O4 NTs on a Co foil
343
can minimize the contact resistance between the nanotube layer and substrate, and
344
possess more active sites. The upright nanotube structure is also in favor of light
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harvesting, allowing outstanding PEC response toward CO2 reduction. With the
346
assistance of Cu NPs, the yield of formate was further increased compared to the
347
unsophisticated Co3O4 NTs. The positively charged Cu NPs induced by its interaction
348
with Co3O4 makes the intermediate, CO , adsorb on Cu by its oxygen atom with 2 ads
349
carbon atom suspended in solution that ensures protonation of reduction intermediates,
350
resulting in a high yield and high selective synthesis of formate from CO2. This study
351
provides a new approach for CO2 fixation and transformation with low-cost materials
352
under benign conditions in environmental field.
353 354
AUTHOR INFORMATION
355
Corresponding Author
356
*Phone: (+86)21-65981180. Fax: (+86)21-65982287. E-mail: g. [email protected]
357
Notes
358 359
The authors declare no competing financial interest.
360
ACKNOWLEDGMENT
361
This work was financially supported by the National Natural Science Foundation of
362
China (NSFC, No. 21477085, 21405114, and 21277099).
363 364
SUPPORTING INFORMATION AVAILABLE
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Some supplementary data and correlation relationships related to this article. This
366
information is available free of charge via the Internet at http://pubs.acs.org. 18
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33. Li, P. Q.; Hu, H. T.; Xu, J. F.; Jing, H.; Peng, H.; Lu, J.; Wu, C. X.; Ai, S. Y.,
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Bocarsly, A. B., Mg-Doped CuFeO2 Photocathodes for Photoelectrochemical
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Reduction of Carbon Dioxide. J. Phys. Chem. C 2013, 117 (24), 12415-12422.
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36. Sato, S.; Arai, T.; Morikawa, T.; Uemura, K.; Suzuki, T. M.; Tanaka, H.; Kajino,
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38. Chen, Z.; Kang, P.; Zhang, M.-T.; Stoner, B. R.; Meyer, T. J., Cu(ii)/Cu(0)
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electrocatalyzed CO2 and H2O splitting. Energ. Environ. Sci. 2013, 6 (3), 813-817.
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39. Xiao, J.; Kuc, A.; Frauenheim, T.; Heine, T., CO2 reduction at low overpotential
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FIGURE CAPTIONS
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Scheme 1. Schematic representation of the Cu-Co3O4 NTs fabrication (a) and
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schematic mechanism of PEC reduction of CO2 on Cu-Co3O4 NTs (b).
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Figure 1. SEM images (top view) of anodic layer of the Co3O4 NTs (a) and Cu-Co3O4
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NTs by 30 cycles of pulsed current deposition (c). Inset shows the corresponding
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SEM images of side views. HRTEM images of the Co3O4 NTs (b) and the Cu-Co3O4
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NTs by 30 cycles of pulsed current deposition (d). Inset shows the corresponding
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TEM images.
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Figure 2. (a) XRD patterns of Co3O4 NTs and Cu-Co3O4 NTs. (b) XPS of Cu 2p in
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Cu-Co3O4 NTs. Inset: Auger electron spectroscopy of Cu LMM peak.
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Figure 3. (a) UV-DRS of Co3O4 NPs, Co3O4 NTs and Cu-Co3O4 NTs; inset:
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magnified area of black frame. (b) LSV of Co3O4 NTs and Cu-Co3O4 NTs electrode
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in N2-saturated and CO2-saturated 0.1 M Na2SO4 solution with the scan rate of 0.05
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V/s under light on/off; (c) Current transformation efficiency (η) of Co3O4 NTs and
509
Cu-Co3O4 NTs electrode
510
Figure 4. Amperometric i-t curves of Co3O4 NTs and Cu-Co3O4 NTs in N2 and
511
CO2-saturated 0.1 M Na2SO4 at -0.9 V with light on/off.
512
Figure 5. Formate yields under photoelectrocatalytic (PEC, solid square),
513
electrocatalytic (EC, hollow circle) and photocatalytic (PC, hollow triangle) condition
514
with the reduction time on Co3O4 NTs electrode (a) and Cu-Co3O4 NTs electrode (b).
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And the sums of yields by PC and EC (hollow square).
516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536
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Article
High-yield and Selective Photoelectrocatalytic Reduction of CO2 to Formate by Metallic Copper Decorated Co3O4 Nanotube Arrays Qi Shen, Zuofeng Chen, Huang Xiaofeng, Meichuan Liu, and Guohua Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b00066 • Publication Date (Web): 06 Apr 2015 Downloaded from http://pubs.acs.org on April 12, 2015
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High-yield and Selective Photoelectrocatalytic Reduction of CO2 to Formate by Metallic Copper Decorated Co3O4 Nanotube Arrays
5
Qi Shen, Zuofeng Chen, Xiaofeng Huang, Meichuan Liu, Guohua Zhao*
6
*Department of Chemistry, Shanghai Key Lab of Chemical Assessment and
7
Sustainability, Tongji University, 1239 Siping Road, Shanghai 200092, China
1 2 3
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ABSTRACT: Carbon dioxide (CO2) reduction to useful chemicals is of great
22
significance to global climate and energy supply. In this study, CO2 has been
23
photoelectrocatalytically reduced to formate at metallic Cu nanoparticles (Cu NPs)
24
decorated Co3O4 nanotube arrays (NTs) with high yield and high selectivity of nearly
25
100%. Noticeably, up to 6.75 mmol·L-1·cm-2 of formate was produced in 8 h
26
photoelectrochemical process, representing one of the highest yields among literature
27
reports. The results of SEM, TEM and photoelectrochemical characterization
28
demonstrated that the enhanced production of formate was attributable to the
29
self-supported Co3O4 NTs/Co structure and the interface band structure of Co3O4 NTs
30
and metallic Cu NPs. Furthermore, possible two electron reduction mechanism on the
31
selective PEC CO2 reduction to formate at the Cu-Co3O4 NTs was explored. The first
32
electron reduction intermediate, CO , was adsorbed on Cu in the form of Cu-O. 2 ads
33
With carbon atom suspended in solution, CO is readily protonated to form 2ads
34
HCOO‒ radical. And HCOO‒ as product rapidly desorbs from the copper surface with
35
a second electron transfer to the adsorbed species.
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1. INTRODUCTION
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The global energy consumption has been increasing dramatically in the past two
39
decades. While largely met by fossil fuels, the rapidly increasing global consumption
40
for this limited resource has generated growing concern over their future availability.
41
Meanwhile, carbon dioxide (CO2) emissions from the use of fossil fuels have an
42
adverse and irreversible impact on global climate. Global warming resulted from the
43
significant rising in the atmospheric CO2 level has become the most serious
44
environmental concern.1,2 The fixation and transformation of CO2 into high
45
value-added hydrocarbon fuels is one of the best solutions to problems of both the
46
energy supply and global warming.3,4
47
Photocatalytic (PC) reduction of CO2 into hydrocarbon fuels, as a promising
48
approach for realizing sustainable development has been persistently drawing
49
attention,5,6 but typically suffers from slow kinetics, poor product selectivity, and
50
mechanistic complexity.7 Single electron reduction of CO2 to •CO2– occurs at −1.90 V
51
vs. normal hydrogen electrode (NHE) requiring highly reducing equivalents.8 In this
52
process, a high reorganization energy exists arising from the significant structural
53
change associated with the transformation from linear CO2 to bent •CO2–.
54
Proton-coupled multi-electron reduction could avoid the high-energy 1e‒ intermediate
55
which requires building up multiple redox equivalents at single sites or clusters. This
56
approach, however, typically leads to a poor product selectivity of CO2 reduction.9
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The products of CO2 reduction vary with the applied potentials. In this regard,
58
photocatalysis with judicious potentials biased may drive reactions to achieve desired
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products. The applied potential by electrocatalytic (EC) process can not only
60
accelerate the separation of photoinduced carriers, but also extend the relaxation time
61
of the excited state of molecular CO2, and thus increase the chance of C=O bond
62
breaking and subsequent hydrogenation. On the other hand, the light irradiation can
63
assist to lower the electrochemical barrier and promote the electrode kinetics.
64
Photoelectrocatalysis (PEC) process, combining the merits of both EC and PC, thus
65
represents a more promising approach to reduce CO2 efficiently and selectively than
66
individuals.10 Combining PEC CO2 reduction, ideally driven by solar light, with
67
energy extraction by combustion or fuel cell applications would provide a renewable,
68
environmentally friendly basis for use of carbon-based fuels in a new energy future.
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Recent studies on metal oxide based photoelectrodes, such as TiO211, Mg-Doped
70
CuFeO212, SrTiO35, demonstrate outstanding CO2 reduction performance. However,
71
the catalytic activity and selectivity of these photocatalysts are still not satisfied. The
72
products were almost C1 or C2 compounds. Besides, H2 was largely produced as a
73
by-product usually. As a p-type semiconductor oxide with a band gap of 2.07 eV,
74
Co3O4 has been used as a visible-light driven photocathode to PEC product HCOO‒
75
from CO2.13 However, the catalytic performance of pure Co3O4 is far from satisfaction
76
due to the slow separation of photo-generated electron-hole pairs within this material.
77
And now anodic oxidation technology has been applied to in situ growth of
78
one-dimensional (1D) Co3O4 nanotubes (NTs) layer on Co foils, which shows
79
promising
80
pseudo-capacitive material for supercapacitors.14,15 Such a 1D NTs structure is in
applications
as
an
electrocatalyst
for
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and
a
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favor of the separation of photo-generated electron-hole pairs. Furthermore, loading
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of metallic nanoparticles or clusters appropriately as electron trap has been reported to
83
improve PC performance dramatically.16-19. Among different metals, Cu is unique in
84
its ability toward CO2 reduction with products distribution dependent on applied
85
potentials.20,21 The work function of Cu (4.65 eV22) is lower than the Fermi level of
86
Co3O4 (-6.1eV23). When in contact, the band edges of Co3O4 bend downward and
87
photo-induced electrons can transfer facilely from Co3O4 to Cu to drive the PEC
88
process. Therefore, the Co3O4 NTs and Cu NPs composite and their interface may
89
serve as an outstanding PEC platform for CO2 reduction.
90
We report here highly efficient and selective PEC CO2 reduction to formate at a
91
metallic Cu decorated Co3O4 NTs under visible-light irradiation. A well-aligned,
92
upright 1D Co3O4 NTs has successfully grown at a Co foil substrate. The NTs layer
93
has a high specific surface area and allows for facile electron transfer, making it an
94
excellent substrate for Cu NPs loading and CO2 reduction. Moreover, the nanotube
95
structure can also trap the incident light efficiently by internal multi-scattering. The
96
yield of formate was comparatively high related to those in the literature and its
97
selectivity was close to 100%. Possible mechanism of the selective PEC CO2
98
reduction to formate at the Cu-Co3O4 NTs was explored.
99 100
2. EXPERIMENTAL SECTION
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2.1. Materials. Cobalt (99.5%) was purchased from Aladdin Industrial Inc., and
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ammonium fluoride (NH4F), glycerol, ethylene glycol (HOCH2CH2OH), Cu2SO4
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were all analytical reagent and purchased from Sinopharm Chemical Reagent Co.,
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Ltd., SCRC, China.
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2.2. Preparation of Cu-Co3O4 NTs Electrode. Cobalt foils were polished to a
106
mirror finish before use. Before anodization, the foils were cleaned via ultrasonication
107
in acetone and ethanol successively, followed by drying the samples in the flow of N2.
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Electrochemical anodization was performed using a DC power supply (Sovotek,
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E5200-3 75v/2A DC Power Supply, DaHua Electronic, China) in a two-electrode
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configuration with a platinum foil as counter electrode. Anodization was carried out at
111
0 oC and 50 V for 8 h to grow Co3O4 NTs layers in electrolytes including ethylene
112
glycol and glycerol (1/3, v/v) with 3 M H2O and 0.54 M NH4F.14 After the
113
anodization process, the samples were rinsed with ethanol and then dried in air,
114
followed by heat-treating at 350 oC for 30 min in air.
115
Cu NPs filling Co3O4 NTs were decorated by pulsed galvanostat method under
116
high current conditions.22 The electrodepositions were carried out in a conventional
117
three electrodes system using CHI 660C with the electrolytes solution containing 0.4
118
M CuSO4 and 3 M lactic acid at room temperature. The concentrated lactic acid acted
119
as a complex agent for stabilizing the copper ions.24 And then the pH of the solution
120
was adjusted with NaOH to 7.00. Pulsed approach was rationally designed with 1 s
121
negative current pulse time and 7 s delay time with a constant current density of 50
122
mA·cm‒2. To obtain a uniform deposition of Cu NPs into the nanotubes, the pulse
123
cycles performed at 10, 20 and 30 cycles were investigated.
124
2.3. Characterization. The morphology of the as-prepared electrodes was
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characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi
126
S-4800, Japan) and transmission electron microscopy (TEM, JEM-2100, JEOL,
127
Japan). X-ray diffractometer (XRD, D/max2550VB3+/PC, Rigaku International
128
Corporation, Japan) was carried out to determine the crystalline structures of Co3O4
129
and Cu-Co3O4. Surface elemental analysis of Co3O4 and Cu-Co3O4 were performed
130
on an X-ray photoelectron spectroscopy (XPS, AXIS Ultra HSA, Kratos Analytical
131
Ltd., UK). The binding energies obtained in the XPS were all corrected by referencing
132
the carbon 1s peak to 284.7 eV. The optical absorption properties were investigated by
133
AvalightDHS UV-Vis absorbance measurements (UV-DRS, Avantes, Netherlands).
134
All PEC measurements were performed on a CHI 660C electrochemical
135
workstation (CH Instruments, Inc.) using a conventional three-electrode system with
136
the as-prepared electrode as working electrode, saturated calomel electrode (SCE) as
137
reference electrode, and a platinum foil as counter electrode in 0.1 M Na2SO4 aqueous
138
solution. All the potentials were referred to SCE unless stated otherwise. Linear
139
sweep voltammetry (LSV) was performed from the potential range of -0.2 to -1.1 V at
140
a scan rate of 0.05 V·s-1 in order to avoid the oxidation of Cu NPs. The amperometric
141
i−t curve was recorded at a constant potential of −0.9 V with an interval of 200 s for
142
light on/off under the light intensity of 20 mW·cm−2 (light source: LA-410UV with
143
UV cutoff, Hayashi, Japan).
144
2.4. PEC Reduction of CO2. The PEC reduction of CO2 was performed in a
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homemade 250 mL gastight chamber with 100 mL 0.1 M Na2SO4 as electrolyte
146
solution, as seen in Scheme 1b. Before reduction, high purity CO2 (99.99%) gas was
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bubbled for 30 min at a flow rate of 20 mL·min-1 into the solution until the
148
concentration of CO2 reached saturation and the dissolved oxygen was removed
149
completely. The concentration of free CO2 measured by acid-base titration was
150
0.0331 M and the pH of the solution was 4.11. The potential during the PEC reduction
151
was kept constant at −0.9 V under 10 mW·cm−2 irradiation (light source:
152
PLS-SXE300 xenon lamp, Beijing PerfectLight Co., Ltd., China, with UV cutoff).
153
The formate and methanol in the aqueous phase were determined by high
154
performance liquid chromatography (HPLC). For the detection of formaldehyde, 2.0
155
mL liquid product was directly mixed with 2.0 mL Nash’s reagent (25 g ammonium
156
acetate, 2.1 mL glacial acetic acid, and 0.2 mL acetylacetone dissolved in 100 mL
157
ultrapure water), and the mixture was shaken for 1 h. The final solution was analyzed
158
by UV−vis spectroscopy (8453, Agilent). The products in the gas phase were
159
determined by gas chromatograph equipped with thermal couple detector (Techcomp,
160
China).
161
3. RESULTS AND DISCUSSION
162
3.1. Fabrication and Physicochemical Properties. Scheme 1a illustrates the
163
schematic representation of the construction of 1D aligned Cu-Co3O4 NTs. Briefly,
164
Co3O4 NTs was prepared by one-step anodization of a Co foil, which was then
165
followed by electrochemical deposition to load Cu NPs.
166
Figure 1a shows the top view and side view (inset) of SEM images of the
167
nanotube arrays layer formed on a cobalt foil after 8 h anodization. As can be seen, a
168
well-aligned, upright 1D nanotube arrays layer grew successfully on the substrate.
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The thickness of Co3O4 NTs layer was ca. 2.5 μm and the diameter was ca. 100 nm
170
(Figure 1b, inset). Such a structure of nanotube arrays can not only provide a large
171
specific surface area for deposition of Cu NPs, but also trap the incident light
172
efficiently by internal multi-scattering.25 Additionally, the inside wall of nanotubes
173
layer was composed of numerous Co3O4 NPs that could contribute further to an
174
increasing specific surface area. Figure 1c shows the microstructure of Co3O4 NTs
175
layer loaded with Cu NPs by 20 cycles of pulsed electrodeposition. Cu NPs were
176
dispersed uniformly both inside and outside nanotubes. The high-resolution
177
transmission electron microscopy (HRTEM) in Figure 1d shows that the diameters of
178
Cu NPs were 12 - 15 nm. Under the electrodeposition condition, the original Co3O4
179
nanotube structure was well retained.
180
Figure 2a compares the XRD patterns of Co3O4 NTs before and after Cu NPs
181
loading. Before Cu NPs loading, a set of diffraction peaks indexed to cubic phase
182
Co3O4 was observed with the lattice constant a = 8.084 Å, consistent with that of
183
JCPDS Card No. 42-1467. Note that the diffraction peaks at 42.0°, 44.7°and 47.6°
184
arise from the underlying Co foil substrate. After Cu NPs loading onto the Co3O4 NTs,
185
highly (111) and (200) preferred orientation diffraction peaks appeared, corresponding
186
to the crystal planes of metallic copper. The Cu (111) crystal plane could also be
187
observed from the HRTEM image of Cu-Co3O4 NTs (Figure 1d). Figure 2b shows the
188
XPS spectrum of Cu-Co3O4 NTs. Two distinctive peaks without any shaken-up
189
satellite peaks were observed at 932.5 eV and 952.5 eV, corresponding to the 2p3/2 and
190
2p1/2 spin orbits of Cu (0).26 In addition, the Auger LMM peak at 918.8 eV in the inset
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of Figure 2b is also consistent with the presence of Cu (0).26 These results indicated
192
that Cu NPs deposited onto Co3O4 NTs in the form of metallic copper, instead of CuO
193
or Cu2O.
194
3.2. Photoelectrocatalytic Activity toward CO2 Reduction. UV diffraction
195
reflection spectroscopy (UV-DRS) was applied to investigate the light absorption
196
property of Co3O4 NPs, Co3O4 NTs and Cu-Co3O4 NTs. Two broad absorption bands
197
were observed at wavelength of 300 - 550 and 600 - 800 nm, respectively, for all three
198
samples (Figure 3a). Compared with Co3O4 NPs, Co3O4 NTs exhibit stronger
199
absorption in the visible spectral range indicative of the enhanced light harvest
200
efficiency of nanotube arrays structure, note Scheme 1b. Although the absorption
201
intensity of ultraviolet region was weakened slightly with Cu NPs deposited, the
202
absorption in the visible spectral range became stronger and broader than the
203
individual Co3O4 NTs. The increase in roughness after Cu deposition increased the
204
lighted area, resulting in stronger absorption in the range of 600 - 800 nm. Further
205
increase in the amount of the Cu NPs deposited results in a less porous structure
206
(Figure S1). Accordingly, the absorption in the visible spectral range decreased
207
(Figure S3), presumably due to a less penetration for the incident light into the
208
framework. The broad and slightly enhanced absorption in the range of 510 - 550 nm
209
could be attributed to the local surface plasmon resonance (LSPR) effect of Cu NPs.
210
In an earlier report by Liu et al,27 Cu NPs with a diameter of 2 - 16 nm exhibit an
211
LSPR absorption peak at ca. 570.0 nm.
212
The band gaps of Co3O4 calculated from Tauc plot (Figure S4a) were 1.44 eV
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and 1.77 eV, which could be assigned to Co3+-O2− transition and Co2+-O2− transition,
214
respectively.28 The flat-band potential of Co3O4 NTs was estimated to be -0.23 V (vs
215
SCE) through Mott-Schottky plot (Figure S5a). Based on the assumption that the
216
valence band position is 0.1~0.2 V lower than the flat-band potential for a p-type
217
semiconductor, the valence band edge of Co3O4 NTs could be estimated to be -0.19 to
218
-0.09 V vs NHE. Thus, the conduction band edge of Co3O4 NTs was located at -1.96
219
to -1.86 V vs NHE. After Cu NPs loading, no obvious change on the band gap of
220
Co3O4 NTs was observed. Compared to other semiconductors reported for CO2
221
reduction, such as GaP (-1.18 V vs NHE),29 Zn2GeO4 (-0.70 V vs NHE),30 and
222
ZnGa2O4 (-1.32 V vs NHE),31 the conduction band edge of Co3O4 or Cu-Co3O4 NTs
223
is located at a comparatively negative energy level. This provides a larger driving
224
force to reduce CO2 than other semiconductor photocathodes on thermodynamic
225
feasibility.
226
The as-prepared Cu-Co3O4 NTs exhibit excellent PEC activity toward CO2
227
reduction. Figure 3b shows that, whether with illumination or not, the current profile
228
of a Cu-Co3O4 NTs electrode in N2-saturated solution is relatively flat with a slight
229
current increase appeared after -0.75 V, arising from background water/proton
230
reduction. By contrast, in CO2-saturated solution, a dramatic current enhancement
231
related to CO2 reduction was observed at the Cu-Co3O4 NTs electrode from -0.72 V
232
with a current peak appearing at -0.85 V.15 It is worth noting that the onset potential
233
observed here was approximately 350 mV more positive than that at a copper
234
electrode20 or a copper foam electrode,32 indicating a superior electrocatalytic
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performance of Cu-Co3O4 NTs towards CO2 reduction. On the Co3O4 NTs electrode,
236
the onset potential of CO2 reduction at the Co3O4 NTs electrode was -0.74 V, close to
237
that at a Cu-Co3O4 NTs electrode, but the catalytic current density was only half of
238
that at a Cu-Co3O4 NTs electrode. The catalytic performance is relevant to the charge
239
carrier concentration of the nanotube electrodes. As determined by Mott−Schottky
240
measurements (Figure S5), the charge carrier concentration of Cu-Co3O4 NTs was
241
1.80 × 1024 cm-3, which was 2 orders of magnitude higher than that of Co3O4 NTs
242
(4.10 × 1022 cm-3). Under visible light irradiation, the onset potential of the Cu-Co3O4
243
NTs and Co3O4 NTs electrodes both shifted positively by 40 mV concomitant with
244
increase in catalytic current. These results suggested that the deposition of Cu NPs
245
and the introduction of visible light were in favor of CO2 reduction.
246
In a more accurate way, the current transformation efficiencies (η) of PEC CO2
247
reduction were calculated from LSVs of Co3O4 NTs and Cu-Co3O4 NTs according to
248
the following formula: 33
249
𝜂=
𝐽𝐶𝑂2 − 𝐽𝑁2 × 100% 𝐽𝐶𝑂2
250
Where JCO2 is the current density of CO2 reduction, and JN2 is the current density of
251
background in solution saturated with N2 that arises from hydrogen evolution.
252
Because the reactions of CO2 reduction and water/proton reduction are in competition
253
on the surface of these electrodes, the value of JCO2-JN2 can reflect the net current
254
density of CO2 reduction. As shown in Figure 3c, the value of η increased initially
255
with the applied potential reaching a maximum one. Beyond that, the value of η
256
decreased because the hydrogen evolution reaction gradually became dominant. For 13
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the EC process at the Cu-Co3O4 NTs electrode, the maximum η value was 62.9% at a
258
potential of -0.89 V. With the visible light applied to the EC system, it increased by
259
23% to 77.5% appearing at a slightly less negative potential of -0.87 V, indicating that
260
the PC process can further promote the EC CO2 reduction process. Without Cu NPs
261
loading, the η value of PEC process was only 51.6%, which was just 2/3 fold than that
262
at the Cu-Co3O4 NTs electrode. It indicates that deposition of Cu NPs facilitates the
263
photoelectron transfer from Co3O4 to CO2.
264
The amperometric i-t curves of these electrodes under chopped light irradiation
265
(Figure 4) also complied with the above results. Under N2 or CO2 atmosphere, the i-t
266
curve of the Co3O4 NTs electrode reached a steady state slowly by at least 80 s
267
irradiation duration, attributable to the slow separation of photo-generated
268
electron-hole pair within Co3O4 NTs. By contrast, at the Cu-Co3O4 NTs electrode, the
269
photocurrent reached a steady state at a significantly reduced time of 50 s. The
270
photocurrent density of the Cu-Co3O4 NTs electrode reached a steady level of -0.122
271
mA·cm-2, which also outperformed that of the Co3O4 NTs electrode (-0.105 mA·cm-2),
272
confirming that the deposited Cu NPs expedites the electron transfer leading to the
273
enhanced CO2 reduction performance.
274
To better understand the Cu NPs-promoted photoelectron-hole separation,
275
Mott-Schottky measurements on the flat-band potentials of both electrodes were
276
performed. As shown in Figure S5, the flat-band potential of Co3O4 NTs was
277
estimated to be -0.232 V, and that of Cu-Co3O4 NTs 0.055 V. It is well known that
278
when semiconductor and metal are in contact, the free electrons will flow between
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metal and semiconductor due to their different work functions.34 As a p-type
280
semiconductor, the Fermi level of Co3O4 (-6.1 eV)23 is more negative than the work
281
function of Cu (4.65 eV)22. When in contact, the electrons will flow spontaneously
282
from Cu to Co3O4 until their Fermi levels are aligned (Figure S6). In the reaction
283
solution, the flat-band potential of Co3O4 NTs raised from -0.232 V to 0.055 V. Upon
284
equilibrium, Co3O4 was negatively charged and Cu was positively charged at the
285
Cu/Co3O4 interface. The band edge of Co3O4 bent downward because of electrostatic
286
induction effect as shown in Figure S6. Such band bending can serve as an electron
287
trap preventing electron–hole recombination in PC process, which resulted in an
288
enhanced PEC performance.
289
3.3. PEC Reduction of CO2. Sustained PEC CO2 reduction was conducted in
290
100 mL CO2-saturated 0.1 M Na2SO4 solution under visible light irradiation at −0.9 V.
291
At both electrodes, formate anion, HCOO−, in the liquid phase, was detected as the
292
dominant product, with no evidence for other C1 products-HCHO, CH3OH and
293
gaseous CO. With the Cu-Co3O4 NTs electrode, the amount of formate reached 6.75
294
mmol·L-1·cm-2 in 8 h, 56% higher than that at the Co3O4 NTs electrode (4.34
295
mmol·L-1·cm-2) (Figure 5). This yield is much higher than that at a Cu NPs modified
296
glassy carbon electrode (0.065 mmol·L-1·cm-2) in a control experiment. It was also
297
comparatively high among those reported in the literature, where the main product
298
was formate/formic acid either in EC or PC process.13,35 For instance, the formate
299
yield is comparable to that of Ru (II) molecular catalyst attached on semiconductor
300
(InP/[MCE2-A+MCE4]).36
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There is evidence for the photoelectric synergy effect of PEC process on the
302
yield enhancement. At the Cu-Co3O4 NTs electrode, the yield of formate was 4.14
303
mmol·L-1·cm-2 in EC process, accounting for 3/5 that of the PEC process, while PC
304
reduction yielded 1.26 mmol·L-1·cm-2 of formate, which was only 1/5 that of PEC
305
process. Noticeably, the yield of formate in the PEC process was higher than the sum
306
of the PC and EC processes (PC + EC). The greatly enhanced yield of PEC CO2
307
reduction could be attributed to the following points: i) The growth of the Co3O4 NTs
308
from the underlying Co foil substrate could minimize the contact resistance between
309
the substrate and nanotube layer. ii) The well-defined 1D vertical arrays structure of
310
Co3O4 NTs possesses more active sites and is favor of light harvesting. In addition,
311
such a structure could also shorten the electron transfer distance, thus greatly reducing
312
the probability of the recombination of the photo-induced electron and hole. iii) The
313
interface band structure between Co3O4 NTs and deposited Cu NPs drives the
314
photo-electrons flow to Cu NPs which also prevents the recombination of electrons
315
and holes.
316
The possible mechanism of selective formate production was explored. As
317
mentioned above, copper is unique in its ability to electrochemically reduce CO2. At
318
potentials less than -0.5 V vs NHE, only CO and HCOO‒ can be produced from CO2
319
reduction at a Cu electrode.37,38 The selectivity toward CO or HCOOH depends on the
320
stability of some important reaction intermediates, such as *CO2, *CO, *OCHO, and
321
*HCOOH, and the surface desorption energy of *CO and *HCOOH species (*
322
denotes activated state).39 At the Cu-Co3O4 NTs electrode, Cu NPs were positively
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charged near the Cu/Co3O4 NTs interface due to the flow of free electron from Cu to
324
Co3O4, which made the adsorption of CO2 on Cu NPs in the form of O-Cu, as shown
325
in Figure S7b, c. PEC Tafel plot measurement was used to analyze the electron
326
transfer during CO2 reduction at the Cu-Co3O4 NTs electrode. Figure S8 shows a
327
Tafel plot with a slope of 122 mV/dec under light irradiation. It indicates that the
328
rate-determining step of CO2 reduction was likely the first electron reduction to form
329
CO 2ads
330
photo-electrons generated from Co3O4 NTs flowed to Cu NPs due to the band bending
331
between the interface of Co3O4 NTs and Cu NPs, and captured by the chemisorbed
332
CO2. The suspending carbon atom of chemisorbed CO2 could attract a proton in
333
solution, leading to chemisorbed HCOOads . DFT calculation shows that the formation
334
of a stable intermediate, *OCHO, was favorable to generate HCOO‒.38 In the
335
following step, a second electron rapidly transfers to the chemisorbed HCOOads ,
336
resulting in desorption of HCOO‒ from the electrode surface. Alternatively, the
337
anion radical may react immediately with the chemisorbed H at the CO 2 ads
338
neighboring Cu or Co3O4 NTs to form HCOO‒. In both schemes, the photogenerated
339
electron in PEC process is driven to the electrode surface to react with the carboxylic
340
acid radical, resulting in enhanced yield of formate production.
anion radical at the Cu-Co3O4 NTs. Driven by visible light, the
341
In conclusion, CO2 was effectively reduced to formate at a metallic Cu decorated
342
Co3O4 NTs electrode by PEC method. The self-supported 1D Co3O4 NTs on a Co foil
343
can minimize the contact resistance between the nanotube layer and substrate, and
344
possess more active sites. The upright nanotube structure is also in favor of light
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harvesting, allowing outstanding PEC response toward CO2 reduction. With the
346
assistance of Cu NPs, the yield of formate was further increased compared to the
347
unsophisticated Co3O4 NTs. The positively charged Cu NPs induced by its interaction
348
with Co3O4 makes the intermediate, CO , adsorb on Cu by its oxygen atom with 2 ads
349
carbon atom suspended in solution that ensures protonation of reduction intermediates,
350
resulting in a high yield and high selective synthesis of formate from CO2. This study
351
provides a new approach for CO2 fixation and transformation with low-cost materials
352
under benign conditions in environmental field.
353 354
AUTHOR INFORMATION
355
Corresponding Author
356
*Phone: (+86)21-65981180. Fax: (+86)21-65982287. E-mail: g. [email protected]
357
Notes
358 359
The authors declare no competing financial interest.
360
ACKNOWLEDGMENT
361
This work was financially supported by the National Natural Science Foundation of
362
China (NSFC, No. 21477085, 21405114, and 21277099).
363 364
SUPPORTING INFORMATION AVAILABLE
365
Some supplementary data and correlation relationships related to this article. This
366
information is available free of charge via the Internet at http://pubs.acs.org. 18
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Electrochemical Reduction of Carbon Dioxide on a Gold Electrode in Phosphate
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23. Zhang, H. Y.; Suman, P., Ji, Z. X., Meng, H., Wang, X., Lin, S. J., PdO Doping
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Z., Single-Crystalline, Ultrathin ZnGa2O4 Nanosheet Scaffolds To Promote
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FIGURE CAPTIONS
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Scheme 1. Schematic representation of the Cu-Co3O4 NTs fabrication (a) and
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schematic mechanism of PEC reduction of CO2 on Cu-Co3O4 NTs (b).
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Figure 1. SEM images (top view) of anodic layer of the Co3O4 NTs (a) and Cu-Co3O4
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NTs by 30 cycles of pulsed current deposition (c). Inset shows the corresponding
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SEM images of side views. HRTEM images of the Co3O4 NTs (b) and the Cu-Co3O4
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NTs by 30 cycles of pulsed current deposition (d). Inset shows the corresponding
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TEM images.
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Figure 2. (a) XRD patterns of Co3O4 NTs and Cu-Co3O4 NTs. (b) XPS of Cu 2p in
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Cu-Co3O4 NTs. Inset: Auger electron spectroscopy of Cu LMM peak.
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Figure 3. (a) UV-DRS of Co3O4 NPs, Co3O4 NTs and Cu-Co3O4 NTs; inset:
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magnified area of black frame. (b) LSV of Co3O4 NTs and Cu-Co3O4 NTs electrode
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in N2-saturated and CO2-saturated 0.1 M Na2SO4 solution with the scan rate of 0.05
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V/s under light on/off; (c) Current transformation efficiency (η) of Co3O4 NTs and
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Cu-Co3O4 NTs electrode
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Figure 4. Amperometric i-t curves of Co3O4 NTs and Cu-Co3O4 NTs in N2 and
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CO2-saturated 0.1 M Na2SO4 at -0.9 V with light on/off.
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Figure 5. Formate yields under photoelectrocatalytic (PEC, solid square),
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electrocatalytic (EC, hollow circle) and photocatalytic (PC, hollow triangle) condition
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with the reduction time on Co3O4 NTs electrode (a) and Cu-Co3O4 NTs electrode (b).
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And the sums of yields by PC and EC (hollow square).
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