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Uddin, M. R.; Khan, M. R.; Rahman, M. W.; Yousuf, A.; Cheng, C. K. Photocatalytic reduction of CO2 into methanol over CuFe2O4/TiO2 under visible light...
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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Photoelectrocatalytic Reduction of Carbon Dioxide to Methanol Using CuFe2O4 Modified with Graphene Oxide under Visible Light Irradiation Kaykobad Md. Rezaul Karim,† Mostafa Tarek,†,‡ Huei Ruey Ong,†,§ Hamidah Abdullah,† Abu Yousuf,∥ Chin Kui Cheng,† and Md. Maksudur Rahman Khan*,†,∥

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†

Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Kuantan, Pahang, Malaysia ‡ Centre of Excellence for Advanced Research in Fluid Flow (CARIFF), Universiti Malaysia Pahang, 26300 Kuantan, Pahang, Malaysia § Faculty of Engineering and Technology, DRB-HICOM University of Automotive Malaysia, 26607 Pekan, Pahang, Malaysia ∥ Department of Chemical Engineering and Polymer Science, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh S Supporting Information *

ABSTRACT: Photoelectrocatalytic reduction of CO2 into valuable products can provide energy in a sustainable way with leveling off of the concentration of CO2 in our environment. In this study, graphene oxide (GO) incorporated with copper ferrite (CuFe2O4) has been employed to enhance photoelectrocatalytic CO2 reduction under visible light. The TEM and XPS characterization indicated a strong interaction between the CuFe2O4 and GO in the hybrid catalyst. The GO incorporation reduced the e−/h+ recombination in the hybrid catalyst by trapping the photoexcited electrons from CuFe2O4 leading to high methanol yield of 28.8 μmol L−1 cm−2 at 20.5% quantum efficiency. The incident photon current efficiency (IPCE) and Faradaic efficiency for methanol formation were observed as 8.02% and 87%, respectively. The results showed that the photoelectrocatalytic activity for CO2 reduction can be improved by incorporating GO with CuFe2O4, and it provides a universal platform to fabricate GO−CuFe2O4 based hybrid photocatalyst with promising applications in CO2 reduction.

1. INTRODUCTION

used as photocatalysts for CO2 photoreduction to produce methanol but suffer from slow kinetics and low products yields due to high probability of charge (e−/h+) recombination rate and mismatch between the solar spectrum and band gap of the used semiconductors during CO2 reduction. The photocatalytic performance of a semiconductor depends on the ability of the catalyst to create e−/h+ pairs, which generate secondary reaction for CO2 reduction. Different approaches like single electron reduction and proton-coupled multielectron reduction scheme have been applied to transform CO2 into fuels and commodity chemicals.5 In electroreduction of CO2, the overpotential for single electron transfer reaction of CO2 is very high (−1.90 V vs NHE) whereas protoncoupled multielectron reduction of CO2 producing valuable

The ongoing increase of greenhouse gas CO2 and the depletion of fossil fuels have received much attention recently from various points of view especially in the context of global warming and energy shortage.1 Recycling of atmospheric CO2 for use as fuel can provide energy in a sustainable way with solving of two major problems: global warming and crisis of energy. The idea of converting CO2 to value added fuel and chemicals comes from nature where photosynthesis occurs in plants by using H2O and CO2 to produce carbohydrate food source in the presence of light. Electrocatalysis (EC) and photocatalysis (PC) are attractive ways to produce hydrocarbons/oxygenated hydrocarbons from water and CO2 reduction.2,3 The electrocatalytic reduction of CO2 usually requires high overpotentials and suffers from hydrogen evolution leading to low Faradaic efficiency. In PC, photocatalysts with appropriate band gap energies are required which can absorb visible light efficiently, yielding high-energy photoinduced electrons capable of reducing CO2 to produce fuel. To date, various semiconductors and hybrids materials like TiO2,4 CuFe2O4−TiO2,2 GO,3 Cu2O/rGO1 have been © XXXX American Chemical Society

Special Issue: 2018 International Conference of Chemical Engineering & Industrial Biotechnology Received: Revised: Accepted: Published: A

July 30, 2018 November 24, 2018 November 27, 2018 November 27, 2018 DOI: 10.1021/acs.iecr.8b03569 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

drawbacks of CuFe2O4 in PEC reduction of CO2 for methanol production with high selectivity. In the present work, visible light active hybrid catalysts GO− CuFe2O4 were synthesized by ultrasound assisted wet impregnation method and characterized by using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet spectroscopy (UV− vis), photoluminescence (PL), and Mott−Schottky analysis. Methanol was formed as a major product, whereas a small amount of CO was found in the gas phase. The methanol yield was increased by ∼2 times with 5% GO−CuFe2O4 photocathode.

products, such as carbon monoxide, methane, ethane, formate, formaldehyde, ethanol, methanol, etc., requires less potential (−0.61 to −0.24 V vs NHE).6 During PEC CO2 reduction, the role of applied potential is significant to control the selectivity of the products.7 Photoelectrocatalytic approach integrates the electrocatalytic and the photocatalytic methods where solar energy can act as the driving force to affect the reaction thermodynamics pushing it to lower potentials compared to the standards ones, thus decreasing the electricity consumption.5 The efficiency of CO2 reduction in an aqueous solution can be increased by the application of light along with suitable applied potential where light produces photogenerated electrons in the photocathode surface and the applied potential inhibits the e−/h+ recombination rate simultaneously during CO2 reduction. Moreover, the external electric field can provide the additional electrons to the photocathode surface which can also contribute to enhance the photocelectroatalytic efficiency.8 p-Type semiconductor and their composites are used in PEC CO2 reduction.6−10 Formation of CO from PEC reduction of CO2 is kinetically feasible compared to the formation of CH3OH due to the fact that the former reaction requires 2 H+/e− transfer in contrast with the later requiring 6 H+/e− transfer to form one molecule.11 However, the formation of methanol (−0.38 V vs NHE) is thermodynamically more favorable compared to the CO formation (−0.51 V vs NHE).5 Kumagai et al.12 reported the formation of CO and H2 in the PEC CO2 reduction under visible light using p-type RuRe/CuGaO2 as hybrid photocathode at applied potential of −0.1 V vs NHE where 81% Faradaic efficiency was achieved. In our recent work p-type CuFe2O4 was synthesized for PEC reduction of CO2 in aqueous solution where methanol was formed as sole product at an applied potential of −0.5 V vs NHE.13 The quantum efficiency for PEC reduction of CO2 was found to be low (14.4%) due to the high e−/h+ recombination rate. However, the important feature of this metal ferrite is its inverse spinel structure containing Cu2+ ions in octahedral position while Fe3+ ions occupy both octahedral and tetrahedral sites of the unit cell.14,15 The bulk electron transfer in CuFe2O4 can occur by electron hopping mechanism.16 The large abundance of Cu and Fe in nature, structural stability, and low band gap make it attractive to be used as a platform to develop the photocathode for CO2 reduction.13,15,17 In our previous work, it was found that the e−/h+ recombination rate of CuFe2O4 could be reduced by incorporating with TiO2 in PC reduction of CO2 leading to high methanol yield.2 In order to enhance the light harvesting capacity, low band gap semiconductors are preferable and their incorporation with wide band gap photocatalyst like graphene or graphene based materials can be useful to enhance the efficiency of charge separation. In recent years, metal ferrites (MnFe 2O 4 , CoFe2O4) decorated with 15−55% graphene content have been used as an efficient light harvesting photocatalyst where graphene could act as cocatalyst preventing the charge recombination rate.18,19 Graphene oxide (GO) is also reported as a visible light active photocatalyst for water splitting20 and CO2 reduction.3 GO decorated with metal/metal oxides nanoparticles was used as an efficient photocatalyst for CO2 reduction under visible light where GO could act as photogenerated electron acceptor, as well as CO2 reduction reaction site.21 Moreover, the presence of GO in the photcathode can form a conductive network to facilitate electron and proton transport as well as supporting material.22 In this context, GO is an ideal candidate to improve the

2. EXPERIMENTAL SECTION 2.1. Materials. Cu(NO3)2·3H2O, Fe(NO3)3·9H2O, HNO3 (65%), HCl (37%), NaHCO3, C3H8O, N2H4·H2O, H2SO4 (98%), K2S2O8, KMnO4, P2O5, H2O2, C2H5OH, Nafion solution, and agar were in precise condition and used with no extra purification. 10 wt% Pt/C and Toray carbon paper were procured from Sigma-Aldrich and Kuantan Sunny Scientific Collaboration Sdn. Bhd. Malaysia, respectively. 2.2. Catalyst Preparation. CuFe2O4 was prepared followed by a method reported in our earlier work.13 In brief, all precursors (Cu(NO3)2·3H2O and Fe(NO3)3·9H2O in a stiochiometric ratio 1:2, HNO3, and agar) were dissolved in distilled water and stirred for about 3 h at room temperature and then again ∼3 h at 80 °C where a green gel was formed. The gel was dried at 120 °C overnight followed by calcination at 700 °C for 8 h. GO was synthesized from graphite flakes using modified Hummers method.23 For this, graphite flakes (4.0 g, 325 mesh) was mixed with H2SO4 (60 mL), P2O5 (6.0 g), and K2S2O8 (6.0 g) under constant stirring at 80 °C for 6 h. After cooling, KMnO4 (35 g) was added into the solution slowly under stirring by maintaining the solution temperature at 20 °C by ice bath. After that, the mixture was heated with continuous stirring at 35 °C for 5 h until a dark green color was observed. To reduce the excess KMnO4, 30% H2O2 (∼100 mL) was injected into the mixture dropwise with stirring. After that, 1 M HCl (∼10 mL) and deionized water (DIW) (∼100 mL) were added to the mixture and centrifuged using Eppendorf centrifuge 5810R at 10 000 rpm for 10 min. The supernatant was removed, and the residue was washed with DIW 3 times. Finally the precipitate was dried using vacuum oven at 90 °C for 24 h to produce the GO powder. GO− CuFe2O4 hybrid catalysts with different GO content (1, 5, and 10 wt %) were prepared by wet impregnation method.24 For the preparation of 5 wt % GO−CuFe2O4, 30 mg of GO was mixed with 0.6 g of CuFe2O4 in 60 mL of ethanol (99.9%) and then sonication was done for 6 h (first 3 h at room temperature and then the temperature was raised at 80 °C). After that, the mixture was dried at 85−90 °C in a vacuum oven. Finally, the dried solid was calcined at 250 °C for 3 h, maintaining the heating rate at 10 °C per minute. 2.3. Preparation of Electrode. The electrodes were prepared following our previous method.13 Briefly, for 5% GO−CuFe2O4 catalyst ink, 22 mg of catalyst powder, 140 μL of Nafion solution, and 280 μL of isopropanol were mixed in together and sonicated overnight. The ink was brushed evenly in the Toray carbon paper (loading area 1 cm2) and then dried at 60 °C for 6 h. 2.4. Characterization. XPS measurements were carried out by using a Quantum 2000 scanning ESCA microprobe B

DOI: 10.1021/acs.iecr.8b03569 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. XPS spectra of (a) C 1s for GO and (b) C 1s, (c) Fe 2p, and (d) Cu 2p for as-prepared 5% GO−CuFe2O4.

chamber containing 0.1 M NaHCO3) and the reference electrode (Ag/AgCl) was placed near the working electrode using Luggin capillary. Pt/C electrodes (10% Pt) were used as anode placed in anode chamber. The anode chamber contains the same electrolyte. The two chambers of the PEC cell were separated by a Nafion 117 membrane. The liquid products during PEC CO2 reduction were collected by a syringe at different time intervals and analyzed using GC-FID (6890 series GC System, Agilent Technologies Co.). For the analysis of the gas products, the cathode chamber was connected to a gas bag from the top. The accumulated gas products were analyzed by gas chromatography (GC) using TCD (thermal conductivity detector) and FID (6890 series GC System, Agilent Technologies Co.).

XPS spectrometer. FTIR was performed by PerkinElmer, 670 FTIR. XRD patterns of the sample were collected using Rigaku MiniFlex2 counter in the range of 2θ value 10−70° with Cu Kα radiation. TEM images of the sample were collected by using transmission electron microscope (Phillips Technai G220) maintaining operating voltage 200 kV, and the UV− vis spectra were obtained using UV 2600, Shimadzu spectrophotometer, and the PL spectra of the samples were acquired by using PerkinElmer LS55 luminescence spectrophotometer. The electrochemical and photoelectrochemical characterization of the as-prepared electrodes was performed in double chamber photelectrochemical cell using three-electrode configuration, where the as-prepared photocathode was used as working electrode and Ag/AgCl and platinum foil were used as reference and counter electrode, respectively. The cell was connected to a potentiostat (Autolab Compact PGSTAT 204, The Netherlands). All potentials are presented as V vs NHE until stated otherwise. Motts−Schottky analysis was done in 0.1 M NaHCO3 with constant CO2 gas pursing at −0.4 V in the range of 0.5−2 kHz. Linear sweep voltammetry (LSV) was run in both N2 and CO2 saturated 0.1 M NaHCO3 solution in the potential range of 0.2 to −1.0 V. Light was irradiated (source: XD 300 high brightness cold light) using different cutoff filter in the range of 420−730 nm through the quartz window to the photocathode surface during light on condition. Chronoamperometry experiments were performed by using the same electrolyte and light source used in LSV with continuous CO2 gas purging at constant potential (−0.4 V). 2.5. Photoelectrochemical CO2 Reduction. The photoelectrochemical reduction of CO2 was carried out at an applied potential of −0.4 V and a cutoff filter of 470 nm using the double chamber PEC cell reactor used in our previous work.13 In brief, during CO2 reduction activity test, the as-prepared photocathode was used as working electrode (in cathode

3. RESULTS AND DISCUSSION 3.1. Characterization of 5% GO−CuFe2O4 and GO. XPS analyses of the hybrid 5% GO−CuFe2O4 and GO were done to elucidate the composition and electronic structure of the as-prepared catalysts, and the deconvoluted XPS spectra are illustrated in Figure 1. The presence of compositional elements (C, Fe, and Cu) in 5% GO−CuFe2O4 was confirmed from the survey spectrum of 5% GO−CuFe2O4 (Figure S1a). The existenece of carbon (C) in the synthesized GO sheets was also confirmed from the XPS survey spectrum of GO (Figure S1b). The asymmetric deconvoluted C 1s spectra of GO and 5% GO−CuFe2O4 are presented in Figure 1a and Figure 1b, respectively. In Figure 1a, the C 1s spectra of GO were deconvoluted into four peaks at 284.24, 285.50, 286.98, and 289.2 eV for sp2-hybridized CC in basal plane of aromatic ring, carbonyl group (C−O), epoxy group (C−O− C), and carboxylic group (O−CO) respectively. For 5% GO−CuFe2O4 hybrid, the deconvoluted of C 1s showed the presence of sp2-hybridized CC in basal plane of aromatic C

DOI: 10.1021/acs.iecr.8b03569 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research ring, carbonyl group (C−O) and carboxylic (O−CO) group at 284.03, 285.02, and 288.86 eV respectively. The results clearly showed the downshift of the oxygenated functional groups (carbonyl group (C−O) and O−CO group) in the composite. Moreover, the presence of epoxy group was not found in the XPS C 1s spectra of 5% GO−CuFe2O4 hybrid catalyst indicating a strong interaction of CuFe2O4 with GO functional groups. The deconvoluted Fe peaks are presented in Figure 1c. The peaks at 711.93 and 710.41 eV represented Fe 2p3/2, whereas peaks at 724.28 eV were for Fe 2p1/2 of Fe3+ and a shakeup peak at 719.9 eV was attributed for Fe 2p1/2 for Fe3+.14,25−27 The Cu 2p spectra were fitted into five peaks presented in Figure 1d. The peaks at 954.79 and 933.36 eV correspond to the binding energy of Cu 2p1/2 and Cu 2p3/2, respectively, for Cu2+ positioned in the octahedral sites of the spinel CuFe2O4.1,28 The shakeup peaks accompanied by main peaks at 960.50 eV (Cu 2p1/2), 942.02 eV (Cu 2p3/2), and 939.59 eV (Cu 2p3/2) represented the position of Cu2+ in tetrahedral coordination.1,6,14,28 Binding energies of Cu2+ and Fe3+ have been slightly shifted positively in the 5% GO− CuFe2O4 compared to CuFe2O4 reported in our previous work.13 The results indicate the anchoring of CuFe2O4 onto GO where the functional groups of GO facilitate the interaction between GO and CuFe2O4. FTIR spectra of 5% GO−CuFe2O4 and GO are shown in the Figure S2. The characteristics bands a of GO were found at 3445, 1728, 1628, 1392, 1211, 1051, and 979 cm−1 corresponding to the stretching vibration of −OH group associated with water, CO stretching vibration of carbonyl groups, CC skeleton vibrations of basal plane, carboxyl O−H stretching, phenolic C−OH stretching, alokoxy C−O stretching, and stretching of C−O−C of epoxy group, respectively.29−32 In 5% GO− CuFe2O4, the epoxy and COOH groups were observed indicating the reduction of GO surface by CuFe2O4 during the preparation of the hybrid catalyst. Moreover, blue-shift of C−O stretching (from 1051 to 1060 cm−1) was observed in the hybrid catalyst. These results indicate the interaction between GO and CuFe2O4 and partial reduction of GO during the preparation of 5% GO−CuFe2O4 hybrid catalyst which is consistent with the XPS results. The XRD patterns of GO and 5% GO−CuFe2O4 are shown in Figure 2a. In Figure 2a (inset), GO showed a sharp diffraction peak at 2θ = 10.60° representing (002) plane (DB card number 9012590), and the interlayer distance of ∼0.8 nm indicates the presence of functional group of GO.33 In XRD pattern of 5% GO−CuFe2O4 hybrid catalyst, the diffraction peaks at 2θ = 18.39°, 30.18°, 35.49°, 37.02°, 43.08°, 53.40°, 56.86°, 62.51°, and 66.46° represent the crystal planes of (111), (220), (311), (222), (400), (422), (511), (440), and (531) of spinel CuFe2O4 (DB-9012841).34−45 Besides this, weak diffraction peaks at 2θ = 38.93°, 48.94°, and 68.31° for the crystal planes of (111), (202), and (113), respectively, of tenorite phase (DB-1011148) appeared due to the presence of trace amount of CuO. Similar crystal plane and angles of diffraction values were observed in our previous study for CuFe2O4 catalyst.13 No characteristic diffraction peak of GO (002) was found in hybrid catalyst which might be due to the low amount of GO.43,46 The crystallite size for (311) plane for CuFe2O4, 1% GO−CuFe2O4, 5% GO−CuFe2O4, and 10% GO−CuFe2O4 from Figure 2a and Figure S3 using Scherer equation2,47 was found to be 53.47, 44.6, 42.99, and 44.85 nm, respectively. The crystallite size was found to be decreased with the increase in GO loading. Furthermore, the microstrain

Figure 2. (a) XRD of 5% GO−CuFe2O4 and GO (inset). (b) and (c) are the low and high resolution TEM images of 5% GO−CuFe2O4.

and crystallite size were calculated using Williamson−Hall equation47,48 as follows: kλ + 4ε sin θ (1) D where ε is the microstrain, k is the shape factor (0.9), and B, θ, λ are the full width at half-maximum (fwhm) in rad, Bragg angle of diffraction peak, and wavelength of the X-ray radiation source (0.154 118 nm), respectively. Average crystallite size and microstrain were obtained from the intercept and slope of the plot of B cos θ vs sin θ for all samples presented in Figure S4a−d. The absence of strain broadening would result in a B cos θ =

D

DOI: 10.1021/acs.iecr.8b03569 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. (a) UV−vis spectra of CuFe2O4 and 5% GO−CuFe2O4 hybrid catalyst. (b) PL Emission spectra of CuFe2O4 and 5% GO−CuFe2O4 at excitation wavelength 565 nm.

Figure 4. Mott−Schottky plot of (a) GO and (b) 5% GO−CuFe2O4 in CO2 saturated 0.1 M NaHCO3 solution at 0.4 V.

respectively. The normalized UV−visible spectra of GO is presented in Figure S5b. From Figure S5b, GO shows the absorption at about 230 nm which is mainly ascribed to the π → π* transition of CC, whereas the small shoulder at 275 nm indicated the n → π* transition of CO in GO structure.53 The typical absorption peak of GO disappeared for the 5% GO−CuFe2O4 (Figure 3a) hybrid catalyst, and the coverage of visible light absorption was shifted toward a lower region compared to the CuFe2O4, indicating the increase in band gap. The band gaps for CuFe2O4, GO, and 5% GO− CuFe2O4 were found to be 1.35, 2.5, and 2.0 eV, respectively (Figure S5a and Figure S5b inset) suggesting that the 5% GO− CuFe2O4 catalyst could harvest visible light (up to 580 nm) effectively.54,55 PL was carried out to elucidate the effect of GO incorporation in 5% GO−CuFe2O4 on the e−/h+ recombination process as shown in Figure 3b. A sharp emission peak at about 566 nm in the PL spectrum of CuFe2O4 is produced by the recombination of the photogenerated e−/h+ pairs and the intensity was drastically reduced for 5% GO−CuFe2O4 sample. The low intensity in PL spectrum of 5% GO−CuFe2O4 indicates the low recombination rate of the photoinduced charge carriers due to the well interaction of GO and CuFe2O4 nanoparticle.56,57 Mott−Schottky analysis for GO and 5% GO−CuFe2O4 electrodes was performed to determine the flat band potential and to confirm the type of the semiconductor. The following linearized equation6 was used to for the Mott−Schottky analysis:

horizontal line. However, in our all samples nonzero and positive slopes were observed indicating the presence microstrain of all samples. The resulting crystalline size and microstrain are presented in Figure S4e. It can be seen that microstrain was initially increased with 1% GO loading and thereafter reduced with the increase in GO loading. The variations in microstrain indicate that there is a variation in the distribution of atomic level strain in the samples along the scattering vector. The crystallite size was dropped with GO loading; however for 5−10% GO loading the crystallite size was almost unchanged. Compared to CuFe2O4, a 20% drop in crystallite size was observed in the 5% GO−CuFe2O4 sample. TEM was carried out to investigate of morphology of the hybrid catalyst shown in Figure 2 b,c. Figure 2b (low resolution of TEM) showed the CuFe2O4 nanoparticles (mostly cubic shaped) anchored on GO sheets. The particle sizes of hybrid catalyst were found in the range of 45−65 nm (Figure 2b) which is slightly higher than the XRD results. However, agglomerations of CuFe2O4 nanoparticles were seen in GO sheets which might be due to the magnetic interaction of CuFe2O4 nanoparticles.49 The interplanar spacing (dhkl) of the spinel CuFe2O4 measured from adjoining CuFe2O4 lattice fringes (Figure 2c) was found to be 0.25 nm, indicating the presence of crystal (311) plane of CuFe2O4.50,51 The results indicate that GO could act as a surfactant for segregation and dispersion of CuFe2O4 nanoparticles during the synthesis process.52 UV−vis and PL spectroscopy of CuFe2O4 and 5% GO− CuFe2O4 were conducted to explore the optical property of the photocatalysts and are presented in Figure 3a and Figure 3b, E

DOI: 10.1021/acs.iecr.8b03569 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. LSV of as prepared catalysts, (a) N2 condition and (b) CO2 condition, in 0.1 M NaHCO3 aqueous solution under light off/on condition using 470 nm cutoff filter at 10 mV s−1 scan rate.

Figure 6. (a) Chronoamperometry of CuFe2O4, GO, and 5% GO−CuFe2O4 at −0.4 V using 470 nm cutoff filter. (b) UV−vis spectra along with IPCE of 5% GO−CuFe2O4 photocathode in different wavelength (420−730 nm).

1 2 ij kT yz jjE − Efb − zz = e ϵϵ0N k e { C2

3.2. Photoelectrochemical Activity. LSV was carried out to find out the effect of light irradiation for CuFe2O4, GO, and 5% GO−CuFe2O4 photocathodes, and the results are shown in Figure 5a and Figure 5b. In N2 condition (Figure 5a), the CuFe2O4 and GO produce almost the same reducing current in both light off and light on state, whereas for GO−CuFe2O4 electrode, the current density for light on condition is slightly higher at all potentials from −0.2 to −1.0 V than the dark system which may be due to the water/proton reduction.7 The reducing current showed a drastic increase when the system was bubbled with CO2 under dark condition for all electrodes indicating the occurrence of electrocatalytic CO2 reduction, and under light on condition, the reducing current further increased revealing the synergistic effect of PEC CO2 reduction (Figure 5b). The current density for 5% GO−CuFe2O4 electrode was approximately 12.30 mA/cm2 at −1.0 V, which was higher than individual CuF2O4 and GO electrodes (8.05 and 9.1 mA/cm2, respectively) under light on condition. The enrichment of the current density under light irradiation in 5% GO−CuFe2O4 photocathode surface occurs for the additional light induced electron which can be transferred to the CO2 molecules.18,59 The enrichment of the cathodic current density in CO2 under light irradiation is the line with the findings of previous studies.57 These results indicate that 5% GO− CuFe2O4 catalyst possesses more PEC activity compared to the individual components (CuFe2O4 and GO) in photoelectrocatalytic reduction of CO2.

(2)

where C, N, E, Efb, T, ϵ, e, ϵ0, and k are the capacitance, acceptor density, electrode potential, flat-band potential, temperature, dielectric constant, electron charge, permittivity of vacuum, and Boltzmann constant, respectively. Figure 4 represents the Mott−Schottky plot where negative slopes were observed for all samples indicating the p-type of semiconductor. As shown in Figure 4a and Figure 4b, the x-axis intercepts were 1.98 and 0.81 V vs NHE representing the Efb positions for GO and 5% GO−CuFe2O4, respectively. For ptype materials, the valence band (VB) position is located near the Efb. The VB position of the each photocathode was determined as 2.0 and 0.83 V for GO and 5% GO−CuFe2O4, respectively, after the pH correction.6 Conduction bands (CB) of GO and 5% GO−CuFe2O4 were determined on the basis of VB potential and were found to be −0.50 and −1.17 V, respectively. From our earlier work, the VB for CuFe2O4 was found to be 0.85 V,13 revealing that the VB position was not significantly shifted due to the GO inclusion in the hybrid catalyst. The CB potential of hybrid photcatalyst is found to be more negative than the thermodynamic potential of protoncoupled multielectron transfer CO2 redox reactions to produce CH3OH (Eo = −0.38 V), CO (Eo = −0.53 V), HCOOH (Eo = −0.61 V), HCHO (Eo = −0.48 V), suggesting that the abovementioned products could be formed by the PEC reduction of CO2 over GO−CuFe2O4 photcathode.58 F

DOI: 10.1021/acs.iecr.8b03569 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 7. (a) Products of GO and CuFe2O4 with differing wt % of GO in CuFe2O4 at −0.4 V in 0.1 M NaHCO3 solution. (b) Methanol production over 5% GO−CuFe2O4 photocathode at applied potential of −0.4 V with reaction time.

transfer CO2 reduction reactions of CO2 in GO−CuFe2O4 hybrid catalyst.58 Furthermore, the PC, PEC, and EC of CO2 reduction were conducted for 4 h over 5% GO−CuFe2O4, and the product yields obtained in the liquid phase are shown in Figure 7b. The methanol yield was increased significantly until 2 h, and after that the product yields were increased very slowly with the reaction time. The methanol yield for PC (3.88 μmol/(L· cm2)) was significantly lower than the EC (21.9 μmol/(L· cm2)) which is synergistically increased in the PEC system (28.8 μmol/(L·cm2)). Control experiments without CO2 were run for all processes maintaining the same conditions, and the samples were collected and analyzed where methanol was not detected indicating that the only source of carbon for methanol formation could be from the CO2 purging. The quantum efficiency and the Faradaic efficiency (FE) for the methanol formation in PEC system were calculated, and the results are shown in Supporting Information (Figure S6). From Figure S6, it is revealed that the quantum efficiency was increased from 10.68% to 21.10% and the FE was increased from 48% to 89% with the increase in GO content from 0% to 10%. Moreover, the quantum efficiency and FE at a GO content higher than 5 wt % showed almost similar results during PEC CO2 reduction. To elucidate the role of GO for the photocatalytic activity enhancement, the possible mechanism (Figure 8) is presented as follows:

Figure 6a shows the cathodic photocurrent density with time via several light on−off cycles of CuFe2O4, GO, and 5% GO− CuFe2O4 electrodes at an applied potential of −0.40 V. It was found that the cathodic photocurrent density rapidly increased under light irradiation and then declined gradually with time and reached a steady-state for all electrodes. The photocurrent of the 5% GO−CuFe2O4 (29.8 μA) is ∼7 times higher than the CuFe2O4 electrode (4.17 μA) and ∼6 times higher than GO (5.10 μA), indicating an enhanced efficiency of photoinduced e−/h+ separation due to the incorporation of GO. Figure 6b represents the combined IPCE and UV−visible absorption spectrum of 5% GO−CuFe2O4. The IPCE for each wavelength cutoff filter ranging from 420−730 nm was calculated as shown in Supporting Information. The IPCE value for 5% GO−CuFe2O4 was increased from 7.23% to 8.02% with the increase in cutoff filter wavelength from 420 to 470 nm and thereafter decreased following the UV−visible absorption spectrum. From our previous work, the IPCE value for CuFe2O4 was found to be 5.11%,13 which was increased by 56% with the inclusion of GO indicating that 5% GO− CuFe2O4 can utilize higher number of incident photons driving the PEC CO2 reduction more efficiently. The PEC activity of the CuFe2O4, GO, and GO−CuFe2O4 (with GO content of 1, 5, and 10 wt %) photocatalysts toward CO2 reduction under visible light irradiation was experimentally investigated and shown in Figure 7a. Both the liquid and gas products were analyzed, and methanol and carbon monoxide were found as the major products in the liquid and gas samples, respectively. The methanol formation for CuFe2O4, GO, 1% GO−CuFe2O4, 5% GO−CuFe2O4, and 10% GO−CuFe2O4 was found to be 15.02, 9.08, 26.59, 28.8, and 27.6 μmol/(L cm2), respectively, whereas the CO yield was 0, 2.8, 1.5, 3.7, and 3.5 μmol/(L cm2) for CuFe2O4, GO, 1% GO−CuFe2O4, 5% GO−CuFe2O4, and 10% GO− CuFe2O4, respectively. The results showed that the overall product yield was increased significantly due to the incorporation of GO with CuFe2O4. The highest methanol yield was achieved in 5% GO−CuFe2O4 hybrid photocatalyst which was increased by ∼90% compared with the CuFe2O4, and with further increase in GO content, the methanol yield was insignificantly reduced; however the CO yield was slightly increased. GO possessing a larger surface area and electron transport capacity than CuFe2O4 provided the available reaction sites and facilitated the proton coupled multielectron

CuFe2O4 + hν → CuFe2O4 (h +vb + e−cb)

(3)

CuFe2O4 (e−cb) + GO → CuFe2O4 + GO (e−cb)

(4)

H 2O + 2h+ →

1 O2 + 2H+ + 2e− 2

(5)

During light irradiation, the narrow band gap CuFe2O4 (1.35 eV) can effectively utilize the visible light to produce the e−/h+ on the surface of CuFe2O4 (reaction 3), and the photoinduced electrons can be trapped by the π-conjugated structure of GO sheets following the percolation mechanism (reaction 4) where the GO could act as an active site for the reduction of CO2. Besides these, the electrons generated by the applied potential using potentiostat deplete into the photogenerated holes on the hybrid catalyst suppressing the e−/h+ recombination rate. Additionally, the holes could take part in water splitting reaction producing H+ and electrons (reaction 5). At the same time water oxidation reactions also occur in the anode provide G

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quantum efficiency of GO−CuFe2O4 for methanol formation were increased by 30 and 41%, respectively, compared to CuFe2O4. A drastic increase in methanol yield during PEC reduction of CO2 compared to the PC and EC suggests the synergistic effect of photogenerated electron along with applied potential of the PEC system.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b03569. XPS survey spectrum of 5% GO−CuFe2O4 and GO, FTIR spectrum of GO and 5% GO−CuFe2O4, XRD pattern of CuFe2O4, 1% GO−CuFe2O4, and 10% GO− CuFe2O4, plot of B cos θ vs sin θ for CuFe2O4, 1% GO− CuFe2O4, 5% GO−CuFe2O4, and 10% GO−CuFe2O4 for calculation of average crystallite size and microstrain, Tauc plot of CuFe2O4 and 5% GO−CuFe2O4, UV− visible spectra of GO along with Tauc plot (inset), quantum efficiency and Faradaic efficiency of methanol formation of CuFe2O4 and GO−CuFe2O4 at different wt % GO loading, calculation of incident photon current efficiency (IPCE) and quantum efficiency (PDF)

Figure 8. Mechanism of PEC CO2 reduction in GO−CuFe2O4 electrode surface.

protons to the cathode through the nafion membrane. Finally, in photocathode the CO2 reduction takes place by the following reactions:58 CO2 + 6H+ + 6e− → CH3OH + H 2O

(6)

CO2 + 2H+ + 2e− → CO + H 2O

(7)

■

To investigate the stability of the GO−CuFe2O4 hybrid catalyst, the PEC experiment was conducted for four cycles and after each cycle (4 h) the electrode was rinsed with DI water very gently and carefully several times. As shown in Figure 9, no obvious decreases in catalytic activity were

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chin Kui Cheng: 0000-0002-2984-7606 Md. Maksudur Rahman Khan: 0000-0001-6594-5361 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We highly acknowledge the Ministry of Higher Education, Malaysia, for FRGS (Grant RDU 150118) and Universiti Malaysia Pahang for Leap3 grant (Grant RDU 172202) and Postgraduate Research Scheme (Grant PGRS 170353) grants to carry out the research work.

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REFERENCES

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Figure 9. Stability test of the 5% GO−CuFe2O4 hybrid photocatalyst in four cycles for PEC CO2 reduction in 0.1 M NaHCO3 solution at −0.4 V.

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4. CONCLUSION In this study, the importance of GO in the photoelectrochemical reduction of CO2 is demonstrated in GO− CuFe2O4 photocathode with different GO content. The improved catalytic activity of GO−CuFe2O4 is explored on the base of the tailored band gap and chemical interaction between CuFe2O4 and GO leading to the fast charge transport through the interfacial layers, inhibiting the charge recombination (e−/h+ pairs) and ensuring the accessibility of free charge carriers to support the catalytic activity. The FE and H

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J

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