High-Yield and Selective Photoelectrocatalytic Reduction of CO2 to

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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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Environmental Science & Technology

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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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ACS Paragon Plus Environment


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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

27

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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water

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

249

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

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

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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

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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.

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

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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

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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

298

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

300

(InP/[MCE2-A+MCE4]).36

15



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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

303

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

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.

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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 (*

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

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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

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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

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CO 2ads

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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 HCOOads . 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 HCOOads ,

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

17



345

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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FIGURE CAPTIONS

496

Scheme 1. Schematic representation of the Cu-Co3O4 NTs fabrication (a) and

497

schematic mechanism of PEC reduction of CO2 on Cu-Co3O4 NTs (b).

498

Figure 1. SEM images (top view) of anodic layer of the Co3O4 NTs (a) and Cu-Co3O4

499

NTs by 30 cycles of pulsed current deposition (c). Inset shows the corresponding

500

SEM images of side views. HRTEM images of the Co3O4 NTs (b) and the Cu-Co3O4

501

NTs by 30 cycles of pulsed current deposition (d). Inset shows the corresponding

502

TEM images.

503

Figure 2. (a) XRD patterns of Co3O4 NTs and Cu-Co3O4 NTs. (b) XPS of Cu 2p in

504

Cu-Co3O4 NTs. Inset: Auger electron spectroscopy of Cu LMM peak.

505

Figure 3. (a) UV-DRS of Co3O4 NPs, Co3O4 NTs and Cu-Co3O4 NTs; inset:

506

magnified area of black frame. (b) LSV of Co3O4 NTs and Cu-Co3O4 NTs electrode

507

in N2-saturated and CO2-saturated 0.1 M Na2SO4 solution with the scan rate of 0.05

508

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).

25



515

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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537


Scheme 1

538

27



539

FIGURE 1

540 541 542 543

544

545

546

547

548

549 28


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Page 29 of 32

550


FIGURE 2

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552 553

554 555

556

557

29



558

FIGURE 3

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560

561 562

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Page 31 of 32

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FIGURE 4

564 565

566

567

568

569

570

571

572

573

574

31



575

FIGURE 5

576

577 578

579

580 581

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High-Yield and Selective Photoelectrocatalytic Reduction of CO2 to

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