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CatalysisScience &Technology

PAPER

Cite this: Catal. Sci. Technol., 2019,

9, 5339

Received 15th July 2019,Accepted 24th August 2019

DOI: 10.1039/c9cy01396b

rsc.li/catalysis

Selective electrochemical reduction of CO2 to COon CuO/In2O3 nanocomposites: role of oxygenvacancies†

Pinki Devi,a Karan Malik, b Ekta Arora,a Saswata Bhattacharya,a V. Kalendra,c

K. V. Lakshmi,c Anil Vermab and Jitendra P. Singh *a

Sequestration of carbon dioxide (CO2) through electrocatalytic reduction to produce high-value industrial

precursors, such as CO, is a promising avenue for sustainable development. Copper–indium is a non-noble

metal catalyst with high activity towards CO production. However, there is a lack of control of parameters

which enhance the selective CO production on copper–indium oxide. Here, the role of oxygen vacancy (VO)

defects in the electrochemical reduction of CO2 (ERC) to selective CO production is reported. CuO/In2O3nanocomposites with different VO concentrations were synthesized under different synthesis environments.

CuO/In2O3 nanocomposites prepared in an argon environment exhibited higher VO concentrations as

compared to nanocomposites prepared in an air environment. DFT calculations demonstrated that the

activation barrier decreases from 0.48 eV to 0.31 eV for the nanocomposite having a higher VOconcentration, thus exhibiting a CO yield with 85% faradaic efficiency at −0.895 V vs. RHE, suggesting thatVO defects serve as the active sites for CO2 adsorption. This study provides a novel route for the

enhancement in selective ERC to CO by inducing the optimum concentration of VO in CuO/In2O3nanocomposites.

1. Introduction

The production of clean, sustainable, and carbon-neutralenergy is the most important scientific and technical challengefacing the 21st century.1 An efficient means of converting CO2into industrial feedstock and chemicals is a sustainable andpromising approach to attain a carbon-neutral cycle that canpotentially mitigate impending environmental crises.2–5

Therefore, there is an urgent need to develop stable andenergetically feasible electrocatalysts for the reduction of CO2,which can minimize the hydrogen evolution reaction (HER).

The production of CO is important as it is widely used asa feedstock for industrial synthesis of hydrocarbons and

liquid fuels.6,7 There have been extensive studies on copper-based electrocatalysts owing to the low cost and higherselectivity of CO2 to CO conversion.

8–11 However, therelatively high overpotential requirements and incompleteunderstanding of the factors affecting product selectivity arethe key challenges for this approach.12 Numerous strategieshave been reported in the literature to enhance the faradaicefficiency of CO production (FECO), such as utilization ofbimetallic copper alloys with Pd and In,9,13,14 bimetallic Cowith Ag,15 different stoichiometric mixtures of Cu with Snand Au,16,17 composites of Cu with indium oxide (IO),18

carbon-coated indium oxide nanobelts,19 Cu–In co-dopedTiO2,

20 and the introduction of oxygen vacancy (VO) defectsin Ga2O3, Co3O4 and ZnO electrocatalysts.

21–23 Moreover,recent investigations have been focused on Cu–Inelectrocatalysts as the presence of In is known to suppressthe HER.24 Bimetallic Cu–In electrodes were found toimprove the FECO to 85% at the moderate potential of −0.6 V(vs. RHE) in comparison with an oxidized Cu electrode.25

Kink sites of Cu, which are active for HER, were covered byIn, thus suppressing the HER, and enhanced selectivity forCO on the bimetallic Cu–In electrodes was achieved.25 This issupported by the observation that an In alloy supported onCu nanowires displays CO formation with a FECO of 90% at−0.6 V (vs. RHE).26 It has also been observed that thedendritic morphology of the Cu–In bimetallic alloy further

Catal. Sci. Technol., 2019, 9, 5339–5349 | 5339This journal is © The Royal Society of Chemistry 2019

aDepartment of Physics, Indian Institute of Technology Delhi, Hauz Khas, New

Delhi 110016, India. E-mail: [email protected] of Chemical Engineering, Indian Institute of Technology Delhi, Hauz

Khas, New Delhi 110016, Indiac Department of Chemistry and Chemical Biology and The Baruch '60 Center for

Biochemical Solar Energy Research, Rensselaer Polytechnic Institute, Troy, NY

12180, USA

† Electronic supplementary information (ESI) available: XRD of CuO, HRTEM,EDX and elemental mapping of CuO/In2O3 nanocomposites, SEM/TEM/PL/N2adsorption desorption isotherm of CuO nanorods and CuO/In2O3nanocomposites, EPR of Cu saturation in CuO/In2O3 nanocomposites,electrochemical measurements of In2O3, cyclic voltammetry andchronoamperometry measurements of CuO/In2O3 nanocomposites, post reactionXRD, SEM and PL of CuO/In2O3 nanocomposites. See DOI: 10.1039/c9cy01396b

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5340 | Catal. Sci. Technol., 2019, 9, 5339–5349 This journal is © The Royal Society of Chemistry 2019

enhances the selectivity of the reaction.14 Syngas formationcan be tuned on the Cu/In2O3 catalyst maybe due toenhancement in binding strength of *COOH on its surfacebecause of lattice compression and Cu doping in the In2O3shell.27 Additionally, metal oxide–metal oxide surfaces arefound to contain more active sites for CO2 reduction ascompared to metal–metal surfaces, which further leads to theenhanced selectivity for CO formation.18

Recent investigations have suggested that introducing VOdefects in electrocatalysts could facilitate the activation of CO2and dissociation of the products by tailoring the properties ofthe electrode surface.23 For example, periodic densityfunctional theory (DFT) calculations of CO2 hydrogenation onan In2O3 (110) surface has also suggested that the presence ofVO could assist in CO2 activation and stabilization of keyreaction intermediates.21 A study of the VO defect sites onGa2O3 has shown that the oxygen atom of CO2 is adsorbed onthe VO sites of Ga2O3 which lowers the adsorption energy ofCO2 from −0.13 eV to −0.31 eV.21 The selectivity for theformation of formate was found to increase to 85% for VO-richCo3O4 in comparison with Co3O4 containing fewer VO defects(∼67.3%) due to a decrease in the rate-limiting activationenergy barrier from 0.51 to 0.40 eV, respectively.22 In a separatestudy, the efficiency for the formation of formate was enhancedto 83% in VO-rich ZnO nanosheets as the introduction of VOsites modified the surface properties and increased the bindingstrength of CO2 on the catalytic surface.

23 However,enhancement in selective product formation and highoverpotential requirements still remain a big challenge to theuse of these catalysts at the industrial level.

In the present study, we engineer different VOconcentrations in CuO/In2O3 nanocomposites which were usedas electrocatalysts for the selective electrochemical reduction ofCO2 to CO at low overpotential. To the best of our knowledge,previous studies have not demonstrated a direct correlationbetween the presence of VO defect concentration and ERC.Also, there is a lack of data on the effect of VO defectconcentration in CuO/In2O3 nanocomposites on ERC. Here, wereport an effective and economical chemical protocol tooptimize the concentration of VO defects in CuO/In2O3nanocomposites for ERC. Indium oxide was used as thesupporting material in the CuO/In2O3 nanocomposite, asindium requires a higher potential for the adsorption ofhydrogen as compared to Cu, which suppresses the hydrogenevolution reaction.24 Photoluminescence (PL), X-rayphotoelectron (XPS) and electron paramagnetic resonance(EPR) spectroscopy were utilized to demonstrate that thenanocomposites synthesized in an argon environment containa higher concentration of VO defects and thus exhibit higherfaradaic efficiency for CO formation (FECO) as compared tonanocomposites with lower VO defect concentration. Moreover,we conduct electrochemical measurements and DFT modelingto gain an in-depth understanding of the plausible correlationbetween VO defect concentration and the CO2 reductioncapabilities of the nanocomposites. We demonstrate that theconcentration of VO defects in CuO/In2O3 nanocomposites

controls the amount of selective CO production throughelectrochemical reduction of CO2 at low overpotential values.

2. Experimental section2.1. Materials

The chemicals used to synthesize the bare CuO nanorodsand CuO/In2O3 nanocomposites were obtained fromcommercial sources: copper nitrate trihydrate (CuĲNO3)2·3H2O, 95%, CDH, India), indiumĲIII) nitrate hexahydrate(InĲNO3)3·6H2O, 99.9%, Sigma Aldrich, India), andammonium hydroxide (NH4OH, Sigma Aldrich, India). Thechemicals for the electrochemical measurements, such asNafion® 117 and porous carbon paper (thickness of 180 μmand 77% porosity) were purchased from Fuel Cell Store, USA,and Cetech, Taiwan, respectively. All of the chemicals were ofanalytical grade and used without further purification.Deionized water (18 MΩ cm) was used during all of theexperiments.

2.2. Electrocatalyst synthesis

2.2.1. Preparation of CuO nanorods. The CuOnanostructures were prepared by a modified sol–gelmethod.28 For the preparation of CuO nanorods, 5 g ofCuĲNO3)2·3H2O was added to 100 ml distilled water to obtaina homogeneous solution, and the pH was adjusted to 9.0 bydropwise addition of NH4OH. The solution was stirred at 80°C for 2 hours to obtain a precipitate that was washed withdistilled water to remove unreacted species. The precipitatewas dried in an oven at 120 °C for 7–8 hours. The productwas powdered and calcined at 500 °C for 4 hours in a tubularfurnace for complete phase transformation of CuĲOH)2 tocopper oxide (CuO). Finally, two different CuO samples withdifferent concentrations of VO defects were obtained bychanging the environment (air and argon at a flow rate of100 sccm) in a tubular furnace during the calcination step ofthe synthesis process.

2.2.2. Preparation of CuO/In2O3 nanocomposites. Differentconcentrations of In2O3 (5%, 10% or 15%) were used forthe synthesis of the CuO/In2O3 nanocomposites. 5 g ofCuĲNO3)2·3H2O and 0.25, 0.50 or 0.75 g of InĲNO3)3·6H2Owere added to 100 ml distilled water for the preparation of5%, 10% or 15% CuO/In2O3 nanocomposites, respectively.Then NH4OH solution was added dropwise to thehomogenous Cu–In solution to adjust its pH to 9.0. Thesolution was stirred at 80 °C for 2 hours to obtain aprecipitate that was washed with distilled water to removeunreacted nitrate ions and dried in an oven at 120 °C for7–8 hours. The dried precipitate was powdered andcalcined at 500 °C for 4 hours in a tubular furnace forcomplete phase transformation of InĲOH)3 to In2O3 andCuĲOH)2 to CuO. The 5%, 10% and 15% CuO/In2O3nanocomposites with different VO defect concentrationswere obtained by changing the calcination environment (airor argon at a flow rate of 100 sccm) in a tubular furnace.

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https://doi.org/10.1039/c9cy01396b


2.3. Electrocatalyst characterization

The XRD patterns of bare CuO nanorods and CuO/In2O3nanocomposites were obtained by using a Rigaku Ultima IVX-ray diffractometer equipped with monochromatic Cu-Kαradiation (λ = 1.541 Å) as the X-ray source and scanned in the2θ range from 10° to 80°. The morphology of the samples wasdetermined by scanning electron microscopy (Zeiss SEM EVO18 Special Edition). The elemental composition of the sampleswas determined using a Swift ED 3000 instrument attached toa TM 3000 Tabletop Microscope. A JEOL JEM 1400 electronmicroscope with LaB6 filament was used for particle sizedetermination of the CuO nanorods and CuO/In2O3nanocomposites. High-resolution transmission electronmicroscopy (HRTEM) (Tecnai G20) was used to analyze thegrain size, crystallographic facets, and structures of CuO/In2O3nanocomposites. X-ray photoelectron spectroscopy (XPS) wasused to quantify the defects in the CuO/In2O3 nanocomposites.Spectra were obtained by XPS (Kα+ Thermo Fisher ScientificInstruments, UK, operating in ultrahigh vacuum with apressure of 2 × 10−9 mbar). The XPS spectrometer uses an Al KαX-ray source, operating at 6 mA beam current and 12 kV. Theinstrument was calibrated with Ag3d (352 eV). The XPS peakposition was referenced to the carbon C 1s peak (284.8 eV).

Photoluminescence spectroscopy (HORIBA LabRAM HREvolution) with an excitation wavelength of 514 nm (green laser)was used for the determination of the concentration of VO andcopper vacancy (VCu) defects in the electrocatalysts. Cryogeniccontinuous-wave (cw) electron paramagnetic resonance (EPR)spectroscopy measurements were performed to confirm the VOdefects in the CuO/In2O3 nanocomposites. The cw EPR spectrawere obtained on a custom-designed cw/pulsed X-band BrukerElexsys 580 EPR spectrometer at 5.5 K using a dual-mode ER4116-DM resonator (Bruker BioSpin Corporation) equipped witha continuous-flow E900 cryostat (Oxfordshire, UK). The cw EPRspectra were acquired at an operating microwave frequency of9.64 GHz with a modulation frequency of 100 kHz, modulationamplitude of 3 G, and microwave power of 2 mW. A Brunauer–Emmett–Teller (BET) (BELSORP-MINI) surface area analyzer at77 K in a N2 atmosphere was used to determine the porosityand surface area for each CuO/In2O3 nanocomposite. Forsurface area measurement, 0.15 g of each electrocatalyst wasused. The electrochemical measurements were performed usinga PGstat-320N, Autolab potentiostat/galvanostat.

2.4. Electrode preparation

A catalyst ink solution was prepared by ultrasonication of amixture of 10 mg of synthesized samples, 5 μl Nafionsolution (5 wt%) and 5 ml ethanol for 60 minutes. To obtainthe working electrode, the ink solution was uniformly coatedon the surface of porous carbon paper (2 cm2) by using anair spray technique with nitrogen as the carrier gas.8,15

2.5. Electrochemical setup

A two-chamber electrochemical cell with a three-electrodeassembly was used to perform the electrochemical reduction

of CO2.8 The cathodic and anodic chambers of the cell were

separated by a pre-treated Nafion-117 membrane. Bothchambers contained 20 ml of 0.5 M KHCO3 solution, and areference electrode (Ag/AgCl/(1 M KCl), CH Instruments) andthe working electrode (catalyst coated on carbon paper) wereplaced in the cathodic chamber. A platinum rod was used asa counter electrode in the anodic chamber. The electrolyte inthe cathodic chamber was saturated with CO2 gas that wasalso continuously purged through the chamber at a flow rateof 15 ml min−1. The electrochemical reduction of CO2 wasperformed at various potentials using a potentiostat/galvanostat. All of the potentials were converted to referencehydrogen electrode (RHE) by using the relation ERHE (V) =EAg/AgCl (V) + 0.23 (V) + 0.059 pH (V). The gaseous productsfrom the cathodic chamber were analyzed using a gaschromatograph (GC, NUCON 5765) equipped with a thermalconductivity detector (TCD) for H2 gas and a flame ionizationdetector (FID) for CO and hydrocarbons. The presence of COwas analyzed in the FID after conversion to CH4 using amethanizer.4 All electrochemical experiments were conductedat 25 °C temperature and 1 atm pressure.

2.6. Density functional theory calculations

Our theoretical methodology involves first principles calculationsunder the framework of density functional theory (DFT) using aplane-wave basis set as implemented in Vienna Ab initioSimulation Package (VASP).29 PAW potentials were used withingeneralized gradient approximations (GGAs) and a Perdew–Burke–Ernzerhof (PBE) functional.30,31 Initially, we determinedthe global minimum configurations of a wide range of CuxInyOzclusters using a cascade genetic algorithm.31 Here, the x and ystoichiometry was varied to match the composition of theexperimentally synthesized nanocomposites (viz. 5% In and 95%Cu [Cu19In1], 10% In and 90% Cu [Cu18In2] or 15% In and 85%Cu [Cu17In3]). Following this, the oxygen content (z) was variedin the range from zero to the maximum number of atoms thatcan be adsorbed onto the cluster to determine the most stablephase under the oxygen environment. For each stoichiometry,the total free energy was minimized with respect to the chemicalpotential of oxygen to determine the minimum free energyconfiguration that is likely representative of the model systems[Cu17In3O15] to mimic the experimentally determinednanostructures. All of the DFT calculations were performed withenergy and force tolerance values that converged at less than10−5 eV and 10−3 eV Å−1, respectively. A plane wave cut-off valueof 600 eV was used in all of the calculations with a Γ-centeredK-mesh.

3. Results and discussions3.1. Physical characterization

The structure and phase data of the bare CuO nanorods andCuO/In2O3 nanocomposites, synthesized in both air andargon environments, were confirmed by X-ray diffraction(XRD). The peaks at 32.50°, 35.46°, 38.73°, 48.74°, 53.46°,58.31°, 60.67°, 66.27°, 68.14°, 72.43° and 75.26° (2θ)

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corresponding to the (110), (1̄11), (111), (2̄02), (020), (202),(1̄13), (3̄11), (220), (311) and (2̄22) plane with the monoclinicphase of CuO (JCPDS: 41-0254) are shown in Fig. S1.† Therewere no extra peaks observed in the XRD pattern of CuO. Thediffraction peaks of the 5%, 10% and 15% CuO/In2O3nanocomposites synthesized in an air and argon environmentare shown in Fig. 1a and b, respectively. The peaks at 21.49°,30.57° and 51.03° correspond to the (211), (222) and (440)plane, respectively, that are well indexed to the body centeredcubic phase of In2O3 (JCPDS: 06-0416). The peaks at 32.50°,38.73°, 48.74°, 53.46°, 58.31°, 66.27°, 68.14°, 72.43° and 75.26°(2θ) correspond to the (110), (111), (2̄02), (020), (202), (3̄11),(220), (311) and (2̄22) plane, respectively, that are in agreementwith the monoclinic phase of CuO. However, some of the peakswere associated with both the materials. The peaks at 35.46°and 60.67° match well with the (400) and (622) plane of In2O3and the (1̄11) and (1̄13) planes of CuO in the nanocomposites.No significant shift in peak positions of the nanocompositeswas observed revealing that the nanocomposites with differentconcentrations prepared in different environments have thesame crystal structure. The crystallite sizes of the catalysts areshown in Table S1.†

The morphologies of all of the electrocatalysts are shownin Fig. S2.† Energy dispersive X-ray spectroscopy (EDX)measurements were used to determine the elementalcomposition of the nanocomposites. These measurementsrevealed the presence of In, O, and Cu in the samples and analuminium peak in each EDX spectrum appeared due to the

instrument stub which was used to mount the samples formeasurements (Fig. S3†).

To further examine the nanocomposites in real andreciprocal space, we performed TEM and HRTEMmeasurements, which confirmed that the nanoparticles werearranged in a 1D morphology (Fig. S4a–f†). Thenanocomposites were highly agglomerated when prepared inargon (Fig. S4d–f†) in comparison to air (Fig. S4a–c†).Moreover, it is interesting that some of the nanoparticleswere deposited on the fiber-like morphology of thenanocomposites as highlighted by yellow circles in Fig. S4f.†The HRTEM images (Fig. S4g and h†) reveal the crystallinenature of the nanocomposites, as we observed well-organizedlattice fringes of interplanar spacing, d = 0.29 nm and d =0.25 nm, corresponding to the (222) plane of In2O3

32 and the(1̄11) plane of CuO,33 respectively. The planes were obtainedby relating the corresponding d-spacing with JCPDS datamarked on the electron diffraction (SAED) pattern (inset ofFig. S4g and h†). The elemental mapping images revealed thepresence of Cu, In and O elements (Fig. S4i–l†).

In order to determine the relative concentration of VO inthe CuO and CuO/In2O3 nanostructures, we performed aseries of measurements using various techniques.Photoluminescence (PL) emission spectra with excitationwavelength λex = 514 nm of the CuO/In2O3 nanocompositesand CuO nanostructures are shown in Fig. 2a–d. For the PLmeasurement, a uniform film of each electrocatalyst wasobtained by dropcasting of the solution on an n-type siliconwafer; this solution was prepared by ultrasonicating a mixtureof 3 mg of each electrocatalyst in 5 ml of ethanol for 30minutes. In the PL spectra (Fig. 2a–c), the peak at 781 nm wasattributed to singly ionized VO of CuO/In2O3 nanocompositesarising from the recombination of a photogenerated hole witha singly ionized electron in the valence band and the peak at906 nm corresponds to VCu defects in bare CuO nanorods andCuO/In2O3 nanocomposites (Fig. 2a–d).

34,35 There were noshifts in the wavelength of the PL spectra of the CuO/In2O3nanocomposite and CuO nanostructures; however, theconcentration of defects was different for nanocompositesand nanostructures synthesized in air and argonenvironment. The PL spectra revealed the presence of higherconcentrations of both singly charged VO as well as VCudefects in the nanocomposites synthesized in argon incomparison to nanocomposites that were synthesized in air(Fig. 2a–c).

The higher concentration of VO in CuO/In2O3 synthesizedin an argon environment might be due to the inert nature ofthe gas. Moreover, in the case of the bare CuO nanorods thatwere synthesized in argon, there was a significant increase inthe VCu defects as compared to nanorods synthesized in air(Fig. 2d). The PL intensity curve (Fig. S5†) revealed that the10% CuO/In2O3 nanocomposite that was synthesized in argonexhibited the highest number of VO and VCu defects amongstall of the nanocomposites. Thus, the samples prepared inargon are termed as VO-rich, while those that were preparedin air are referred to as VO-poor samples.

Fig. 1 XRD spectra of 5% (black trace), 10% (red trace) and 15% (bluetrace) CuO/In2O3 nanocomposites synthesized in an (a) air and (b)argon environment.


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The density of defects and the chemical state of copperand indium in both the VO-rich and the VO-poor 10% CuO/In2O3 nanocomposites were determined by XPSmeasurements (Fig. 3). The purity and crystallinity of theCuO/In2O3 nanocomposite were confirmed by an XPS surveyscan which revealed that there were no impurities present(Fig. S6†). In the XPS spectra of In 3d, the peaks at 452.0 eV

and 444.8 eV correspond to the 3d3/2 and 3d5/2 transition,respectively (Fig. 3a).32 This suggests that indium was presentas In3+ in the nanocomposites. There were no horizontalshifts in the XPS pattern of In 3d in VO-rich and VO-poorCuO/In2O3 nanocomposites. However, the intensity values ofthe In 3d spectra of VO-rich samples were more as comparedto those of VO-poor CuO/In2O3 nanocomposites. This factor

Fig. 2 PL spectra of (a) 5%, (b) 10%, (c) and 15% CuO/In2O3 nanocomposites and (d) CuO nanorods synthesized in air and argon environments.

Fig. 3 Core level XPS spectra of 10% CuO/In2O3 nanocomposite synthesized in air and argon environments. (a) In 3d scan, (b) Cu 2p scan, and (cand d) O 1s scan.


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had significantly affected the hydrogen evolution reactionduring ERC which is discussed in a later section. In the Cu2p XPS spectra (Fig. 3b), the peaks at 953.8 ± 0.2 eV and933.8 ± 0.2 eV correspond to 2p1/2 and 2p3/2, respectively, andthe strong satellite peak leads to the formation of CuO.36 TheO 1s XPS spectra (Fig. 3c and d) displayed a peak at 529.9 ±0.2 eV that corresponded to lattice oxygen (OL) enclosed byeither copper or indium, and the highest binding energypeak at 532.2 ± 0.2 eV was related to the near-surface oxygen(ON) atom or OH group attached or H2O in the CuO/In2O3nanocomposite.36,37 The peak arising at 531.1 ± 0.2 eV (OV)was attributed to VO defects in CuO of CuO/In2O3nanocomposites.36 There was no significant shift in peakpositions for both 10% VO-rich and VO-poor nanocomposites.The presence of relative VO concentrations in both VO-richand VO-poor nanocomposites was analyzed by calculatingtheir relative peak area ratio of the oxygen vacancy peak (OV)using the relation OV/(OL + OV + ON). Fig. S7† shows that therelative peak area for the O-vacancy peak in the VO-richnanocomposite was 35.9%, while for the VO-poor CuO/In2O3nanocomposite, it was 20.7%. Thus, the VO-richnanocomposite had a relatively higher VO concentration incomparison to the VO-poor CuO/In2O3 nanocomposite.

We conducted electron paramagnetic resonance (EPR)spectroscopy measurements to gain further insight into thenumber of defects and the oxidation state of the dopant ionin the CuO/In2O3 nanocomposites. The cw EPR spectra of the5%, 10% and 15% CuO/In2O3 nanocomposites shown inFig. 4a–c, respectively, display an intense peak at 3200 G (gvalue of ∼2.15) that could arise due to either VO or Cu2+defects in each sample.38 There was no shift in the g value ofthe EPR signal in VO-rich and VO-poor nanocomposites.

However, there was an increase in the intensity of the signalthat was accompanied by line broadening in VO-rich CuO/In2O3 in comparison with the analogous VO-poor sample. Thebroadening of the EPR signal was due to spin–spin couplingbetween the VO defects in VO-rich CuO/In2O3. Additionally,the EPR spectra also revealed that the 10% VO-rich CuO/In2O3nanocomposite had the highest concentration of defectsamongst all of the nanocomposites. This was in agreementwith the PL (Fig. S5†) and XPS (Fig. 3) observations. Incomparison with the nanocomposites, the EPR spectrum ofCuO displayed a negligible concentration of VO defects(Fig. 4d). It is important to note that the signals arising fromthe Cu2+ and VO defects overlap in the g ∼2 region of theEPR spectra which makes it difficult to distinguish betweentheir respective contributions.

We conducted cryogenic EPR measurements on aninorganic CuĲII) model complex, Cu2+(Phen)2ĲH2O)ĲNO3)2, todetermine the dominant defects in the CuO/In2O3nanocomposite, which indicated that the microwave powersaturation behaviors of the Cu2+ and VO signals are indeedvery different. As shown in Fig. S8a–d,† the microwave powersaturation curves of Cu2+(Phen)2ĲH2O)ĲNO3)2 at 20 K and 7 Kconfirm that Cu2+ EPR signals saturate at 4 mW and 1 mW,respectively. This confirmed that the EPR spectra of the 5%,10% and 15% CuO/In2O3 that are shown in Fig. 4a–c weredominated by the spectral contribution of the VO defects inall of the nanocomposites.

The pore size and surface area of the CuO/In2O3nanocomposites and CuO nanorods were determined by theirN2 adsorption and desorption behavior at 77 K (Fig. S9†).The N2 adsorption–desorption isotherm was classified as atype IV isotherm with hysteresis, where the desorption energy

Fig. 4 EPR spectra of (a) 5% CuO/In2O3, (b) 10% CuO/In2O3, and (c) 15% CuO/In2O3 nanocomposites and (d) CuO nanorods.


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was higher than the adsorption energy.39 The CuO/In2O3nanocomposites that were synthesized in argon exhibitedlarger hysteresis and higher N2 adsorption capacity atrelatively higher pressures, indicating a larger surface area incomparison with the analogous CuO/In2O3 nanocompositesthat were synthesized in air. The insets of Fig. S9† representthe slope of the respective N2 adsorption and desorptionwhich was used to calculate the surface area of theelectrocatalyst. The BET surface area, mean pore diameterand pore volume for all of the nanocomposites are shown inTable S2.† The VO-rich nanocomposites display a highersurface area as compared to the VO-poor analogs (Table S2†).Moreover, the VO-rich 10% CuO/In2O3 nanocompositeexhibits the highest surface area amongst all the samples.

3.2. Electrochemical measurements for ERC onelectrocatalysts

The cyclic voltammetry (CV) measurements of bare CuOnanorods and all of the CuO/In2O3 nanocomposites wereconducted in N2 saturated and CO2 saturated 0.5 M KHCO3solutions. The 10th CV cycles of all the electrocatalysts areshown in Fig. 5. Multiple cycling led to the reduction/oxidation along with the dissolution/re-deposition of theelectrocatalyst that aided in achieving a steady state,40 whichwas evidenced by the superimposable CV scans as shown inFig. S10.† During the negative scan from 0.5 V to −1.25 V,some differences in the reduction current densities of thetwo solutions can be seen for all the electrocatalysts(Fig. 5a, b and d). This difference in current densities wasmaximized for the VO-rich 10% CuO/In2O3 nanocomposite(Fig. 5c), which indicates that it should have the lowest FE

towards the HER and the highest FE towards the ERC,suggesting that the presence of VO defects favors the ERC.

The reduction peak observed in the region from −0.5 V to0.0 V for all of the electrocatalysts in both N2 and CO2 saturatedsolutions corresponds to the copper oxide reduction (Fig. 5a). Aminor shift in the reduction peak was observed with theincreased indium concentration in the nanocomposites, whichwas due to a change in the electronic properties with theaddition of indium. Moreover, a shift in this peak was alsoobserved in N2 and CO2 saturated environments, which couldbe attributed to the difference in the pH of the N2 and CO2saturated solutions. The corresponding re-oxidation peak forCu was observed at around 1.0 V. However; there were nodistinct reduction/oxidation peaks for indium (Fig. S11†). Theconsistency of the CV measurements on the electrocatalystsdescribed earlier was independently confirmed bychronoamperometry, as shown in Fig. S12.†

The electrocatalytic performance of all the electrocatalyststowards the ERC was observed for one hour in the potentialrange of −0.4 V to −1.4 V vs. RHE. During the ERC, currentand faradaic efficiencies remained constant, indicating thestable nature of the electrocatalyst. Moreover, CO was formedas the sole ERC product along with H2. The selectivity of theelectrocatalysts towards the HER and ERC products arepresented in terms of the faradaic efficiency of H2 (FEH) andCO (FECO) as a function of the applied potential (Fig. 6). Allof the electrocatalysts displayed a similar behavior with abell-shaped profile for FECO as the potential was changedfrom −0.4 to −1.2 V. Among all the electrocatalysts, bare CuOnanorods synthesized in both the environments (air andargon) had the minimum FECO (∼16.6%) and the highestHER (Fig. 6a1 and a2). The CuO nanorods synthesized in

Fig. 5 10th cyclic voltammetry cycles of (a) CuO nanorods and (b) 5%, (c) 10% and (d) 15% CuO/In2O3 nanocomposites in 0.5 M KHCO3 with ascan rate of 50 mV s−1.


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argon contained a higher concentration of VCu defects incomparison with those synthesized in the air environment asseen in their PL spectra (Fig. 2d). However, the FECO wasfound to be similar for both CuO nanorod electrocatalysts.This suggests that the concentration of VCu sites in CuO didnot play a significant role in the enhancement of FECO. Asdiscussed earlier, in the Cu–In system, indium covers most ofthe kink sites of Cu (which are HER active).25 Moreover, ahigher potential is required for the adsorption of hydrogenon indium, which suppresses the HER.24 Thus, the CuO/In2O3 nanocomposite formed by the addition of indium intoCu led to the suppression of the parasitic parallel reaction,the HER (Fig. 6b1–d2).

Additionally, the XPS spectra (Fig. 3a) revealed that ahigher amount of indium was present on the surface(covering the HER sites) of VO-rich as compared to VO-poor

CuO/In2O3 nanocomposites. Thus, VO-rich catalysts exhibitlower HER activity as compared to VO-poor CuO/In2O3nanocomposites (Fig. 6b1–d2). Among all VO-poor CuO/In2O3nanocomposites, the 10% CuO/In2O3 nanocompositedisplayed a maximum FECO of ∼63% at −0.645 V vs. RHE.Moreover, FECO was approximately similar with an increasein the indium concentration from 5% to 15% in thenanocomposites (Fig. 6). As mentioned earlier, theconcentration of VO defects was higher in the CuO/In2O3nanocomposites that were synthesized in argon (Fig. 2).

Thus, the electrocatalysts synthesized in argon had higherFECO, with the VO-rich 10% CuO/In2O3 nanocompositedisplaying the highest FECO of ∼85% at −0.895 V vs. RHE.However, it is interesting that minimal variation wasobserved for FECO upon varying the indium oxideconcentration from 5% to 15% in electrocatalysts synthesized

Fig. 6 Effect of applied potential on the FECO and FEH for (a1) CuO-argon, (a2) CuO-air, (b1) 5% CuO/In2O3-argon, (b2) 5% CuO/In2O3-air, (c1) 10%CuO/In2O3-argon, (c2) 10% CuO/In2O3-air, (d1) 15% CuO/In2O3-argon, and (d2) 15% CuO/In2O3-air nanocomposites.


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in either air or argon environment. In the case of pure In2O3,the maximum FECO observed was only 23.5% at −0.645 V vs.RHE (Table S3†), whereas at −0.895 V, FECO was 8.9% butFEH was only 34.4%, indicating that some liquid productswere forming. In the reported literature, formates wereobtained by electrochemical reduction of CO2 on indiumoxide.32 This means that the sole role of indium in thenanocomposite was to suppress the HER. This means that VOdefects were the primary contributors to the increase in theFECO during the ERC.

Moreover, post reaction analysis revealed that copperindium oxide was reduced to its metallic state as shown inFig. S13.† This reduction occurs during the initial fewminutes of the ERC.8,41 The morphology of VO-rich andVO-poor CuO/In2O3 nanocomposites did not change after thereaction, indicating that no segregation has occurred (Fig.S14†). PL spectra confirm the presence of defects in VO-richand VO-poor nanocomposites after the CO2 reductionreaction (Fig. S15†). Thus, the VO defects were stable in bothVO-rich and VO-poor CuO/In2O3 nanocomposites (Fig. S15†).Hence, under the applied potential, VO defects were notcompletely removed. Thus, the concentration of VO defects inCuO/In2O3 nanocomposites is a dominant factor in theenhancement of FECO, while the indium oxide content (5%,10% and 15%) does not have a significant influence on theenhancement of the ERC. Thus, it can be concluded that thepresence of higher concentrations of VO defects enhances theERC and improves the FECO by modifying the chargetransport and surface properties of catalyst.

3.3. DFT modeling

The effect of introducing oxygen vacancy defects to theelectronic and geometric structure of the CuO/In2O3nanocomposites was studied by DFT.29–31 Initially, weinduced different point defects, namely, single O-vacancy,single Cu-vacancy and simultaneous Cu and O vacancies inthe Cu17In3O15 cluster and adsorbed a *COOH group alongwith one H+ ion at various sites, viz. Cu, In, O and VO todetermine the favorable sites for reduction of the COOHgroup to CO according to the reaction: *COOH + H+ + e− → *+ CO + H2O.

23,26,27

We found that among the various sites, the reduction ofthe COOH group was possible only on sites proximal to theO-vacancy defects. Additionally, the bare Cu17In3O15 and thecluster with VCu defects did not catalyze the formation of CO.Thus, we may infer that the catalytic activity of thenanocomposites is mainly due to the presence of the VOdefects in the system. It is important to note that thereduction of CO2 to CO takes place in two steps. In the firststep, CO2 adsorbed on the surface of the nanocompositereacts with H+ ions to yield *COOH. Following which, *COOHreacts with a second H+ ion to produce CO and H2O. The firststep is endothermic while the second one is exothermic. Theenthalpy of the first step determines the ease with which thereduction reaction takes place in a particular configuration.

In the second step, protons are transferred to the system and*COOH reacts with H+ to yield CO with the formation of H2Oand the release of energy.

In order to confirm the role of the VO defects in thecatalytic activity of the nanocomposite, we calculated theGibbs free energy (ΔG) for the two-step reduction process ofCO2 on the cluster with one and two VO defects. Fig. 7 showsthe two-step mechanism of the reduction process of CO2 onthe Cu17In3O15 composite having one and two VO defects. Wefind that in case of a single VO defect, the ΔG of first step was0.48 eV, while in the presence of two VO defects, it was 0.31eV. Hence, we conclude that an increase in the concentrationof VO defects in the system leads to a decrease in theactivation barrier that is required for the reduction of CO2 toCO. This could be because of the presence of VO defects thatprovide additional electrons to the system, which facilitatesthe reduction of CO2 to COOH. In addition to this, energyevolved during the second step was more in case of two VOdefects in comparison to one VO defect in the system. Thus,the concentration of the VO defects at an optimum levelfavors the ERC. Thus, the number of oxygen vacancy defectsthat are required depends upon the number of electrons thatare needed for the completion of the ERC.

4. Conclusion

In summary, oxygen vacancies engineered in non-noble metaloxide catalysts were a proficient and cost-effective approachfor enhancement of selective ERC. The controlledconcentrations of oxygen vacancy defects were induced in

Fig. 7 Gibbs free energy (ΔG) of the two steps involved in reductionof CO2 to CO on top of VO-poor (dotted line) and VO-rich (solid line)Cu17–In3–O15 composite systems.


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CuO/In2O3 nanocomposites by a novel chemical syntheticroute for the efficient and selective electrochemical reductionof CO2 to CO. PL, XPS and EPR measurements revealed thatthe samples synthesized in an argon environment containhigher concentrations of VO defects as compared to thosesynthesized in air. The VO-rich 10% CuO/In2O3nanocomposite exhibited the maximum FECO of ∼85% at−0.895 V vs. RHE, while the VO-poor 10% CuO/In2O3nanocomposite showed a maximum FECO of ∼63% at −0.645V. The BET results indicated that the VO-rich samples possessa higher surface area than the VO-poor samples, whichdemonstrated that the former samples contained more activesites for CO2 adsorption. Finally, DFT calculations revealedthat the activation barrier decreased from 0.48 eV to 0.31 eVas the concentration of VO increased in the composite systemfor completion of the ERC. Thus, we have successfullydemonstrated that VO defects in highly active, stable, andcost-effective CuO/In2O3 nanocomposites is a dominant factorfor selective and efficient ERC of CO2 to CO.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

PD kindly acknowledges the University Grant Commission(UGC), India, for providing a junior research fellowship. Theauthors are thankful to the Nanoscale Research Facility andCentral Research Facility, IIT Delhi, for electrocatalystscharacterization. PD is thankful to Dr. Rajni Verma forconsistent support in manuscript preparation. The authorsare thankful to Dr. Pankaj Poddar, NCL Pune, for XPSmeasurements. This research work was supported by grantnumber EMR/2015/001477 from SERB-DST, India.

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