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NiWO4@Au photocatalyst for efficient CO2 reduction under visible light
Journal Pre-proof Effective charge separation in rGO/NiWO4 @Au photocatalyst for efficient CO2 reduction under visible light Jongmin Shin, Jun Neoung Heo, Jeong Yeon Do, Young-Il Kim, Seog Joon Yoon, Yang Soo Kim, Misook Kang
PII:
S1226-086X(19)30511-8
DOI:
https://doi.org/10.1016/j.jiec.2019.09.033
Reference:
JIEC 4792
To appear in:
Journal of Industrial and Engineering Chemistry
Received Date:
14 July 2019
Revised Date:
16 September 2019
Accepted Date:
19 September 2019
Please cite this article as: Shin J, Heo JN, Do JY, Kim Y-Il, Yoon SJ, Kim YS, Kang M, Effective charge separation in rGO/NiWO4 @Au photocatalyst for efficient CO2 reduction under visible light, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.09.033
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Effective charge separation in rGO/NiWO4@Au photocatalyst for efficient CO2 reduction under visible light
Jongmin Shina, Jun Neoung Heoa, Jeong Yeon Doa, *, Young-Il Kima, Seog Joon Yoona,
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Yang Soo Kimb, Misook Kanga, *
Department of Chemistry, College of Natural Sciences, Yeungnam University, Gyeongsan,
Gyeongbuk 38541, Republic of Korea
Korea Basic Science Institute, Gwahak-ro, Yuseong-gu, Daejeon 34133, Republic of Korea
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*Corresponding author: [email protected] (Jeong Yeon Do), +81-53-810-3798;
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Graphical Abstract
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[email protected] (Misook Kang), +81-53-810-2363
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Highlights
Introducing electron donor onto both the VB and CB of the main catalyst.
Facilitating charge separation and suppressing recombination between electrons and
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holes.
Electrons are filled from the π-electron rich rGO into the VB of NiWO4.
Electrons on Au surfaces are amplified by its SPR effect.
Carbon dioxide reduction capability under visible light
ABSTRACT
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Catalyst performance can be improved by introducing an electron donor into both the valence band (VB) and conduction band (CB) to facilitate charge separation and suppress electron-
hole recombination. Herein, Au nanoparticles served as CB electron donors in NiWO4 core
particles which were evenly dispersed on a reduced graphene oxide (rGO) sheet that served
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as a VB electron donor. The resulting rGO/NiWO4@Au photocatalyst was applied to
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reducing CO2. The particles exhibited broadband absorbance from the ultraviolet to nearinfrared, with a specific Au surface plasmon resonance (SPR) absorption peak at 600 nm.
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Moreover, the catalyst exhibited low photoluminescence (PL) and a high photocurrent density, indicating that photo-excited electron-hole recombination was suppressed and the
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charges effectively separated. Photocatalytic reduction of CO2 on rGO/NiWO4@Au was significantly enhanced as evidenced by the total amounts of reduction products (CO and
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CH4), which were 15 times those for NiWO4 and six times those for rGO/NiWO4 and NiWO4@Au. The expected electron-transfer mechanism on rGO/NiWO4@Au involves
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electron donation into the VB from the π-electron rich rGO, combined with photo-excited electrons from the NiWO4 and Au particles where electrons on the Au surfaces were amplified by the SPR and then moved to the CB of NiWO4. Intensity-modulated photovoltage spectroscopy of rGO/NiWO4@Au indicated a significantly reduced electronhole recombination rate.
Keywords: Charge separation; Electron donor; Carbon dioxide photoreduction; rGO/NiWO4@Au
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Introduction
The photochemical conversion of carbon dioxide (CO2) using solar energy has recently attracted attention for ecofriendly future technologies. However, to use such technologies
industrially, it is necessary to develop a novel catalyst. Sunlight reaching Earth comprises
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4%, 53%, and 43% ultraviolet (UV) rays, visible rays, and infrared (IR) rays, respectively.
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Until now, no photocatalyst having an energy band capable of efficiently utilizing the various wavelengths of sunlight has been developed. In particular, to improve the performance of the
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photocatalyst, it is necessary to spatially separate electrons and holes generated by light, thereby slowing the recombination of electrons and holes [1, 2]. Additionally, it is necessary
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to increase the frequency of the oxidation–reduction reaction occurring at the interface between the semiconductor and the reactant so that the photoactivity can be maintained
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continuously [3, 4]. A photosensitized semiconductor system [5], a two-step excitation semiconductor heterojunction system [6], and a Z-scheme system [7] have been proposed to
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slow the electron/hole recombination. Recently, it has been reported that when precious metals such as Au [8], Ag [9], and Cu [10] are grafted together, the photocatalytic performance of CO2 reduction in the visible-light region is significantly enhanced by their surface plasmon resonance (SPR) effect. In particular, because these metals have a high reduction potential, they can act as electron donors or acceptors, promoting the movement of electrons. Additionally, many researchers have recently attempted to improve the
performance of photocatalysts via efficient light harvesting using not only visible light but the entire wavelength range, including UV and IR rays. One method for this involves forming a heterojunction comprising graphene [11-13], C nanotubes [14], C quantum dots [15], or C3N4 [16] and the main photocatalyst. Owing to the added C material, the catalyst can absorb light in the entire wavelength range of sunlight. C, which contains π-electrons, can also act as an electron donor to continuously supply electrons to the empty holes of the main
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photocatalyst, facilitating the photoreaction. The objective of this study was to exploit basic chemistry to improve the catalyst
performance. All the chemical reactions were redox reactions (NiWO4 electron acceptor and Au or reduced graphene oxide (rGO) electron donor). If the redox reaction can be sustained
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in the optical system, the photocatalytic activity can be increased and maintained. In addition,
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we designed a photocatalyst capable of maximizing the plasmonic effect and absorbing the entire wavelength range of sunlight, thereby promoting the CO2 photoreduction reaction. In
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particular, instead of TiO2, which is widely used as a main catalyst, NiWO4, which contains Ni as a methanation-promoting component and WO3 with a band-gap of 2.6–3.0 eV, was
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employed as a main photocatalyst [17]. Recently, metal oxygenates such as metal vanadate [18], metal borate [19], metal tungstate [20], and metal phosphate [21] have been reported as
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photocatalysts. Because they have a smaller band-gap than metal oxides, they can be activated under visible light, but their use alone has not yielded remarkable results, owing to
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the rapid recombination of electrons and holes. Therefore, we first attempted to incorporate an electron-transport mediator, i.e., rGO, as an electron donor, into the electron acceptor NiWO4 to absorb light over a broad wavelength range. Because rGO and NiWO4 have a twodimensional structure, it was expected that the electron transport from the rGO to the valence band (VB) of NiWO4 would be easy, because the orbitals overlap on a wide plane [22]. Second, we doped the NiWO4 catalyst with Au nanoparticles having a high reduction
potential, which can act as electron acceptors or donors (here, they acted as electron donors). Subsequently, NiWO4@Au particles were uniformly dispersed on rGO to obtain an rGO/NiWO4@Au catalyst. When the rGO/NiWO4@Au catalyst was placed under visible light, the excited electrons from the VB of NiWO4 were promoted to the conduction band (CB). The electrons amplified by the SPR effect on the surface of the Au particles also moved into the CB of NiWO4 and enhanced the electron density of the CB. The rGO
transfer cycle was expected to be maintained constantly.
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Experimental
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continuously supplied π-electrons to the vacant VB of NiWO4; thus, the redox electron-
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Synthesis of NiWO4, NiWO4@Au, rGO/NiWO4, and rGO/NiWO4@Au particles
Graphene oxide (GO) was produced via the following method. While the temperature was
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maintained at 0 oC, 6.0 g of graphite was added to a beaker containing 300.0 mL of sulfuric acid, followed by stirring for 2 h and sonication for 3 h. Then, 4.0 g of sodium nitrate
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(NaNO3, ≥99.0%, Sigma–Aldrich) was added, with stirring for 1 h. The temperature was increased to 10 oC, 25.0 g of potassium permanganate (KMnO4, ≥99.0%, Sigma–Aldrich)
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was added, and the mixture was stirred for 4 h. Here, heat was generated. The temperature was increased to 35 oC, and the mixture was stirred for an additional hour before being allowed to cool. Then, 300.0 mL of distilled water was added, and after stirring for 1 h, another 900.0 mL of distilled water was added. Subsequently, 80.0 mL of a 30 wt.% hydrogen peroxide solution (H2O2, ≥30%, Sigma–Aldrich) was added to obtain a precipitate. This precipitate was washed with 1.0 L of 5.0% hydrochloric acid (HCl, 37%, Sigma–
Aldrich), washed again with 4.0 L of distilled water, and dried at 50 °C for 24 h. To prepare the rGO, 2.0 g of GO was added to 1200.0 mL of ethylene glycol (C38H70O4, >99%, Sigma–Aldrich), followed by stirring for 1 h. Then, 2.0 mL of a 35 wt.% hydrazine solution in H2O (N2H4 65%, 99.99%, Sigma–Aldrich) was added, followed by stirring for 1 h. The solution was placed in an autoclave and heat-treated at 180 °C for 16 h to obtain a precipitate. The resulting precipitate was washed three times with distilled water and twice with 96% ethanol and then dried at 50 °C for 24 h.
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To prepare the Au solution, 1.0 wt.% gold(III) chloride trihydrate (HAuCl4•3H2O,
≥99.9%, Sigma–Aldrich) was added to 100.0 mL of distilled water, followed by sonication
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for 1 h. After the solution was boiled at ≥100 oC, 2.0 mL of a 10.0 wt.% sodium citrate
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solution (Na3C3H5O7, ≥99%, Sigma–Aldrich) was added, followed by stirring for 15 min and then cooling.
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NiWO4 particles were synthesized via a typical hydrothermal method, as shown in Scheme 1a. Nickel nitrate (Ni(NO3)2•6H2O, 99.99%, Sigma–Aldrich) and sodium tungstate
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dihydrate (Na2WO4•2H2O, 99.99%, Sigma–Aldrich) were used as starting materials for Ni and W, respectively. First, 0.1 mol of sodium tungstate dihydrate was dissolved in 1.0 L of
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distilled water, 0.1 mol of nickel nitrate was added dropwise, and the mixture was sonicated for 10 min. After 1 h of stirring, the final solution was transferred to a 2.0 L autoclave. The
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temperature was increased to 180 °C at a heating rate of 10 °C per minute, and the solution was heat-treated at this temperature for 5 h. The resulting light-green precipitate was washed twice with distilled water and three times with ethanol. Subsequently, it was dried at 80 oC for 24 h and transferred to an electric furnace. It was then heated to 600 °C for 2.5 h and crystallized via sintering at this temperature for 6 h (Scheme 1a). To synthesize NiWO4@Au particles, 2.0 g of the prepared NiWO4 powder was added to
400.0 mL of distilled water, followed by stirring for 1 h. Then, 10.0 mL of the prepared Au solution was added to the solution from the previous step, followed by stirring for 30 min. Next, 4.0 mg of sodium borohydride (NaBH4, 99%, Sigma–Aldrich) was added, followed by stirring for 3 h to obtain a precipitate. The precipitate was washed three times with distilled water and twice with ethanol and then dried at 50 °C for 24 h. The powder was sintered for 2 h in an electric furnace set at 450 ℃ (Scheme 1b).
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To prepare the rGO/NiWO4 particles, 0.2 g of NiWO4 powder was added to 400.0 mL of distilled water, followed by sonication for 1 h. Next, 2.0 g of rGO powder was added, and the mixture was stirred for 1 h. Subsequently, 40.0 mL (1.0 mol) of sulfuric acid (H2SO4 ≥98%,
Sigma–Aldrich) was added, followed by stirring for 3 h and then ultrasonic treatment for 10
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min. The final solution was placed in an autoclave and treated at 170 °C for 24 h to obtain a
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precipitate. The precipitate was washed with distilled water three times, washed with ethanol, and dried (Scheme 1c).
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Finally, rGO/NiWO4@Au particles were synthesized via the following steps. First, 0.2 g of NiWO4 powder was added to 400.0 mL of distilled water, followed by ultrasonic treatment
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for 1 h. Next, 2.0 g of rGO powder was added, and the mixture was stirred for 1 h. Then, 10.0 mL of an Au solution was added, followed by stirring for another 1 h. After 40.0 mL (1.0
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mol) of H2SO4 and 4.0 mg of NaBH4 were added, the mixture was stirred for 3 h and sonicated for 10 min. The final solution was transferred to an autoclave and heat-treated at
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170 °C for 24 h. The resulting precipitate was washed three times with distilled water and twice with ethanol and then dried (Scheme 1d).
Characterizations of particles
The crystal structures and shapes of the synthesized NiWO4, NiWO4@Au, rGO/NiWO4,
and rGO/NiWO4@Au particles were examined via X-ray diffraction (XRD) analysis (X’Pert Pro MPD PANalytical, Ni-filtered Cu Kα (λ = 1.5406 Å), 30 kV, 15 mA, 2θ angle of 10°– 80°) and transmission electron microscopy (TEM, H-7600, Hitachi, Japan). The crystal lattice structure of the particles was analyzed using high-resolution TEM (HRTEM, JEM-2100 F, JEOL), and the images were obtained at 200 kV. The X-ray photoelectron spectra of the particles were obtained using a Kratos Axis Nova instrument with monochromatic Al Kα
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radiation (225 W, 15 mA, 15 kV). To determine the optical properties, a diffuse-reflectance UV–visible (DIR-UV–vis) spectrometer (Neosys-2000, Scinco Co., Korea, wavelength of 200~800 nm), photoluminescence (PL) spectroscopy (PerkinElmer, He–Cd laser source, wavelength of 320 nm), and photocurrent measurements (2000 solar simulator, ABET
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Technologies) were used. Raman spectroscopy was performed using a Horiba Jobin-Yvon
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LabRam HR instrument with laser-beam excitation at = 633 nm. Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy
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(IMPS) were performed under open-circuit conditions to measure the electron and hole recombination properties and the electron-transport time using a Sun 2000 Solar Simulator
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(IVIUM STAT, ABET Technologies) under illumination with a power density of AM1.5 (100 mW/cm2). Time constants were calculated using the frequency at which a minimum occurred
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in the imaginary response, via a nonlinear fitting procedure. To confirm the amounts of CO2 adsorbed on the surfaces of the catalysts, CO2-TPD experiments (BEL Japan Inc., Japan)
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were conducted.
CO2 photoreduction on powdered catalysts
The CO2 photoreduction activities of the synthesized NiWO4, NiWO4@Au, rGO/NiWO4, and rGO/NiWO4@Au particles with H2O were examined in a closed cylinder-type quartz
vessel (length, 15.0 cm; diameter, 1.0 cm; total volume, 12.5 mL) that was employed in our previous study [23]. Here, 0.2 g of the catalyst and 40.0 mL of distilled water were placed in the photoreactor. CO2 gas with 99.999% purity was used as the reactant. The CO2 gas was flowed into the chamber to purge air from the chamber before irradiation. The reactor chamber was then closed, and a Sun 2000 Solar Simulator (IVIUM STAT, ABET Technologies) with a power density of AM1.5 (100 mW/cm2) was used as a light source. The
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distance between the reactor and the light source was fixed at 1.0 cm. The photoreduction was conducted at room temperature and the atmospheric pressure. The gases inside the
reactor were sampled at intervals of 1 h using a nanoliter syringe, and the product gases were analyzed using a gas chromatography instrument (Master GC, Scinco, Korea) equipped with
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thermal conductivity and flame ionization detectors to separate the C1–C3 light hydrocarbons
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and oxygenated compounds, such as CH4, CH3OH, HCHO, HCOOH, and CO. The product selectivity was calculated using the following equation:
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Results and discussion
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Ci (%) = Ci moles of the product/total moles of Cproduced × 100%. (1)
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Physical properties of catalysts
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Fig. 1A shows the XRD patterns of NiWO4, NiWO4@Au, rGO/NiWO4, and
rGO/NiWO4@Au particles. In general, the NiWO4 particles exhibited XRD peaks in the same positions, which corresponded to the monoclinic crystal system [24]. The obtained patterns also clearly indicated the absence of phase impurities. For the NiWO4@Au particles, the peak intensity was only slightly reduced, and there was no change in the monoclinic crystal structure compared with NiWO4. The (111) and (200) diffraction planes corresponding
to the Au cubic structure were observed at 2θ = 38.27° and 44.6°, respectively [25]. These results indicate that the Au nanoparticles were well loaded on the surface of the NiWO4 particles. In contrast, for the rGO/NiWO4 particles, the presence of C was expected, as the baseline of the 2θ = 25° peak was broad and excited, which is attributed to the C in rGO [26]. Finally, for the rGO/NiWO4@Au particles, the characteristic peaks of Au exhibited higher intensities; thus, the presence of Au was more clearly detected. In conclusion, the presence of
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Au loaded on NiWO4 and the presence of NiWO4 loaded on rGO were confirmed by the XRD analysis. However, the presence of rGO was not observed in the XRD patterns.
Therefore, its presence was confirmed via Raman spectroscopy, as shown in Fig. 1B. The
presence of rGO in the rGO/NiWO4@Au catalyst was confirmed by the peaks at 1300 and
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1600 cm-1 [27]. A sharp peak at <1000 cm-1 corresponding to the NiWO4 was also observed
the rGO/NiWO4@Au catalyst [29].
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[28]. Moreover, the broad peak near 2700 cm-1 was strong evidence for the presence of Au in
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The shapes of the particles were examined using TEM, as shown in Fig. 2. The NiWO4 particles exhibited a rhomboidal shape, and their sizes ranged from 100 to 200 nm. The
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image of NiWO4@Au particles indicated that Au particles approximately 50 nm in size were loaded on the surface of a large NiWO4 core particle with a width of 200 nm and a length of
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300 nm. The image of the rGO/NiWO4 particles indicated that NiWO4 particles 50–100 nm in size were embedded on the thin rGO sheet. Finally, the images of the rGO/NiWO4@Au
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particles indicated that the Au-loaded NiWO4 particles were uniformly embedded on the rGO. Thus, the TEM image well matched the XRD results. The crystal lattices of the rGO/NiWO4@Au particles were identified, and the selected-
area electron diffraction (SAED) pattern was obtained to confirm the crystallinity and to confirm that the elements present in the sample were uniformly distributed. Fig. 3 shows HRTEM images (a), specific lattice planes (b), SAED patterns (c), and high-angle annular
dark-field imaging (HAADF) element-mapping images (d). As indicated by the HRTEM images, the NiWO4@Au particles were uniformly dispersed in the rGO, and the Au was loaded not on the rGO but only on the NiWO4 particles. Lattice-plane calculations indicated that the lattice widths of the Au (111) and NiWO4 (110) planes were 1.43 and 3.47 Å, respectively. The SAED patterns exhibited a uniform circular shape, suggesting that the particles were uniform single crystals. The distribution of elements in the rGO/NiWO4@Au
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particles was very uniform, with the C from the rGO having the highest density, followed by O, W, and Ni. In particular, Au exhibited a dense electron density, with a shape similar to that of red beads approximately 20 nm in size. These results confirmed that the rGO/NiWO4@Au catalyst prepared in this study was made of NiWO4@Au particles uniformly loaded with Au
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on NiWO4 particles well dispersed on rGO.
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Fig. 4 shows the X-ray photoelectron spectroscopy (XPS) measurements for the oxidation states of the constituent elements in each sample. XPS was performed to monitor the
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characteristic binding energy of the C 1s peaks, corresponding to each functional group on the rGO/NiWO4@Au and rGO/NiWO4 particles. The C 1s XPS spectra were deconvoluted
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into two peaks corresponding to C=C (284.5 eV) and C-C (285.6 eV) [30]. These peaks were separated in almost the same area in the two samples, and the binding-energy positions were
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almost the same. For the NiWO4 and NiWO4@Au samples, the O 1s regions exhibited two main contributions: the O2− state of lattice O (M−O) at 530.6 eV and the O2− state of O
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defects/vacancies at 531.8 eV [33]. For the rGO/NiWO4 and rGO/NiWO4@Au samples, the O 1s peaks were divided into two peaks at 530.5–530.6 and 532.5–532.6 eV via Gaussian fitting. Notably, the second peak was clearly observed and was due to the C-O in rGO [34]. For the NiWO4 and NiWO4@Au samples, the fitted Ni 2p peaks at binding energies of 854.2–855.9 and 872.7–873.5 eV are ascribed to the Ni2+, whereas the fitted peaks at binding energies of 856.9–857.6 and 874.3–875.3 eV correspond to the Ni3+ [31]. The Ni 2p3/2 peaks
were located at 855.1–856.2 and 856.5–857.7 eV for the rGO/NiWO4 and rGO/NiWO4@Au samples. The Ni 2p peaks for the Au-loaded catalysts were observed at lower binding energies that those for NiWO4. The W 4f profile was fitted by two Gaussian peaks centered around 37 and 35 eV, which were the binding energies of electrons in the 4f5/2 and 4f7/2 levels of W in the W6+ valence state in WO3, respectively [34]. For the rGO/NiWO4 and NiWO4@Au samples, the two peaks shifted to lower binding energies compared with
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NiWO4, indicating that the W ions were reduced. The shift was larger for the NiWO4@Au sample. This indicates that the electrons were more enriched in the Au-loaded catalyst
compared with the NiWO4 catalyst. Thus, in the Au-loaded catalyst, the electrons were easily excited by light. Finally, the Au in the NiWO4@Au sample existed in metallic states, as
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indicated by the Au 4f spectra, which were composed of Au 4f7/2 and Au 4f5/2 peaks at 83.6
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and 87.3 eV, respectively [35]. For the rGO/NiWO4@Au sample, the Au 4f7/2 peak was slightly shifted toward the lower-binding energy region (maybe bulk Au), indicating a strong
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interaction between the Au and the NiWO4 support. The strong interaction between the Au nanoparticles and the NiWO4 support was expected to enhance hot-electron transfer from the
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Au nanoparticle surface to the NiWO4 CB through the Schottky energy barriers [36].
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Catalytic performance of catalysts for CO2 photoreduction
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Fig. 5 shows the yields of the primary product of CO and the final product of CH4 when
the following reaction was conducted under visible light using four types of particles: NiWO4, NiWO4@Au, rGO/NiWO4, and rGO/NiWO4@Au catalysts: CO2 + 4H2O + hv → CH4 + 2H2O (2) For the NiWO4 catalyst, very small amounts of the CO and CH4 gases were accumulated after 10 h of reaction, and their ratio was almost 1:1. For the NiWO4@Au catalyst (Au loaded on
NiWO4), the amount of CO generated increased sharply, and the CO production was nearly 10 times that for CH4. Many studies have indicated that the amount of CO2 is significantly reduced by the effect of Au on the SPR [37], and the selectivity to CH4 is increased [38]. However, for NiWO4@Au, the selectivity of CH4 hardly changed, and the fact that only the production of CO increased significantly is unusual. However, for the rGO/NiWO4 catalyst, the amount of CH4 increased by more than 4 times compared with that for the NiWO4@Au
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catalyst. This result implies that the CO on the rGO in the rGO/NiWO4 catalyst reacted with H and was converted into CH4. For the final catalyst, i.e., the rGO/NiWO4@Au particles, the total product amount increased by a factor of 6 compared with that for the rGO/NiWO4 catalyst and by a factor of 15 compared with that for the NiWO4 catalyst. However, the
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amount of CH4 hardly increased. According to these results, we predicted that the addition of
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Au in rGO/NiWO4@Au increased the amount of the primary product, CO, because the system did not contain redox to decompose water.
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For clarifying the CO2 photoreduction mechanism, Fig. 6A shows the mass spectra of the gas species formed on rGO/NiWO4@Au during 10 h of CO2 photoreduction. In our
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mass analysis, •C, •CH2, and •CH3 intermediates were observed. Therefore, we predicted that CO2 was reduced through the carbene pathway for the rGO/NiWO4@Au catalyst. There were
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two pathways, and both preferred the two-electron mechanism, where two photogenerated electrons participated in concert in one elemental step [39]. One pathway is called the
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formaldehyde pathway (fast hydrogenation pathway), in which the reaction follows the path CO2 → HCOOH → HCHO → CH3OH → CH4 [40]. This pathway is thermodynamically feasible; however, in our mass analysis, HCOOH and CH3OH were not observed. The other pathway is the carbene pathway (fast deoxygenation pathway), in which CO2 is reduced along the path CO2 → CO→ •CHO→ •CH2 →•CH3 → CH4 [41]. Fig. 6B shows the IR spectra of the surface of rGO/NiWO4@Au before and after CO2 adsorption and after CO2
reduction. In the rGO/NiWO4@Au before CO2 adsorption, six peaks at 706, 832, 1180, 1390, 1555, and 3413 cm−1 were observed, corresponding to Ni-O, W-O, C-OH, C-O, C=C, and OH configurable stretching and bend vibrations in the NiWO4 and rGO [42, 43]. After CO2 adsorption, the vibration mode for isolated CO2 gas was observed at 2450 cm−1. This indicates that CO2 was well adsorbed onto the rGO/NiWO4@Au catalyst. After the CO2 photoreduction reaction, the C=O vibrational mode was observed at 2030 cm−1, and three large peaks corresponding to the C-H bending mode in CH4 were observed at 1076, 1210,
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and 1302 cm−1 [44]. The O-H stretching mode at 3413 cm−1 exhibited the largest peak,
indicating that the reaction intermediate (CO) adsorbed on the catalyst became Cat-CO, and
the following three mechanisms in this study.
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then H+ was bound to Cat-COH. According to the report of W. Zhang et al. [45], we predicted
(a) + e- Cat-CO (b) + OH- (4)
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Cat + CO2 Cat-CO2 + H2O Cat-COOH (a) + OH- (3)
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(a) + H+ + e- Cat-COH + 3H+ + 3e- Cat-CH2 + H+ + e- Cat-CH3 + H+ + e- Cat-
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CH4 (c) + OH- (5)
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Optical properties of catalysts
Fig. 7A shows the DIR-UV–vis spectra of the NiWO4, NiWO4@Au, rGO/NiWO4, and
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rGO/NiWO4@Au particles, which were used to determine their bandgaps. According to de Oliveira et al. [46], the absorption bands in NiWO4 above 750 (IR range), 470, and 380 nm (UV range) can transition from 3A2g to the excited states 3T2g, 3T1g, and 3T1g, respectively. Because a 1-sun light source was used in this study, the absorbance of NiWO4 was reasonable at 470 nm; thus, the bandgap was 2.63 eV. This is consistent with previously reported results [47]. The absorption peak of Au, which indicated the effect of the SPR on the NiWO4@Au
particles, was clearly observed at 600 nm [48]. The rGO/NiWO4 and rGO/NiWO4@Au particles containing rGO absorbed light in the wavelength range of 300–1000 nm (from the UV to the near-IR region). This result implies that a catalyst that absorbs light over the entire wavelength range can be fabricated by uniformly applying nanoparticles to rGO. The position of the VB in the NiWO4 obtained via XPS was 2.27 eV, as shown in Fig. 7B. By subtracting this VB position from the bandgap obtained in Fig. 7A, we obtained the desired CB, which
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corresponded to -0.36 eV. The electrons excited into the CB of the particles were relaxed again and recombined with the holes. The analytical method for measuring these relaxed photoelectrons is PL, and the PL spectroscopy results are shown in Fig. 8A. The electrons excited by 300 nm wavelength light
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were emitted through the relaxation process at 410 nm. For the NiWO4 with pure crystals, a
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large phosphorescence peak appeared at 410 nm, and small fluorescence peaks were observed at 470 and 540 nm. For NiWO4@Au, the intensities of the PL peaks were significantly lower.
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In particular, the PL peaks for the rGO/NiWO4 and rGO/NiWO4@Au particles containing rGO were small. This is attributed to electron capture caused by defects in the rGO/NiWO4
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and rGO/NiWO4@Au crystals or the intercalation phenomenon between the adjacent particles [49, 50]. Eventually, in any case, the number of excited electrons recombined with
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the holes will be reduced, which ultimately increases the activity of the photocatalyst. However, in the NiWO4 and NiWO4@Au films, photocurrent hardly flowed when the light
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was on, as shown in Fig. 8B. However, in the rGO-added rGO/NiWO4, the current increased 2
in the 1st cycle of the light being turned on, tended to stabilize until the 3rd
cycle, and then decreased in the 4th cycle. Thus, the initial activity of the catalyst was good, but catalyst deterioration occurred owing to the recombination of electrons and holes during the cycling. The rGO/NiWO4@Au catalyst exhibited the largest photocurrent increase, and the photocurrent did not decrease until the 5th cycle. These results indicate that the separation
of electrons and holes was easy in this catalyst. To calculate the electron-transport time and the recombination lifetime for NiWO4 and rGO/NiWO4@Au, IMPS and IMVS were performed, and the typical responses are presented in Fig. 9. Generally, it is reasonable to use IMPS to determine the electron-transport time [51], and the transport time constant affects the measured IMPS signals at high frequencies, as shown in Fig. 9A. A higher voltage across both catalysts yielded a larger movement of the
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circle to the left and a smaller circle. This indicates that a higher voltage led to faster electron transport. However, the spacing of the circles for the rGO/NiWO4@Au catalyst was narrower and smaller than that for the NiWO4 catalyst. This was due to the faster transfer of electrons on the rGO/NiWO4@Au catalyst. As shown in Fig. 9B, as the applied voltage increased, the
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IMVS circles moved to the right and became larger. This indicates that when a high voltage
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was applied, the recombination rate of the electrons and holes decreased. The IMVS circles were more shifted to the right for the rGO/NiWO4@Au catalyst than for the NiWO4 catalyst,
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and the circle spacing was wider. This shift indicated slower recombination of electrons and holes in rGO/NiWO4@Au compared with NiWO4.
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According to the IMPS and IMVS curves, the electron-transport times (s, y-axis) and recombination lifetimes were estimated using the expression s = 1/2pft (or fr), where ft or fr
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represents the characteristic frequency at the minimum value of the imaginary components of IMPS or IMVS [52]. The results are presented in Fig. 10. The electron-transfer rate for the
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rGO/NiWO4@Au catalyst (Fig. 10A) was significantly higher (approximately 30%) than that for the NiWO4 catalyst. This indicates that the electrons generated in the Au moved quickly to the NiWO4, and the charge separation between the electrons and holes was useful because the rGO quickly filled the electrons with the empty orbit of Au and NiWO4. It is believed to be accelerated. Additionally, as shown in Fig. 10B, the rate of recombination of electrons and holes for the rGO/NiWO4@Au catalyst was significantly lower than that for the NiWO4
catalyst, by a factor of approximately 10. This was due to the continuous supply of electrons from the electron-rich rGO to the empty valence shell of NiWO4. According to the foregoing catalyst properties and performance, the expected CO2 photoreduction mechanism on the rGO/NiWO4@Au catalyst is depicted in Scheme 2. In the CO2 photocatalytic reduction, the enhanced photocatalysis of the ternary nanocomposites is attributed to the synergistic effect of the three components. First, both the NiWO4 and the
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rGO nanosheets provided adsorption to CO2 molecules. Then, the photogenerated carriers were formed through the excitations of both the bandgap transition of NiWO4 and the SPR of Au nanoparticles upon the visible-light irradiation. The Au nanoparticles enhanced the
visible-light harvesting owing to their SPR properties, and the amplified electrons on the
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surface of Au moved to the CB of NiWO4. Additionally, in the electron-rich CB of NiWO4,
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the reduction of the adsorbed CO2 molecules into CO occurred. Alternatively, holes (h+) could have oxidized the surface H2O, forming reactive •OH and H+. The rGO nanosheets
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offered a path for rapid transport of the photogenerated electrons, yielding efficient charge separation during the photocatalytic process. The injected electrons in the rGO sheets were
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subsequently captured by the adsorbed H+, producing the active CHO, CH2, and CH3 radicals during the photocatalysis. Finally, they combined with H+ on the electron-rich rGO,
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producing CH4.
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Conclusions
In this study, rGO/NiWO4@Au catalysts with electron donors of rGO and Au grafted to
an NiWO4 main catalyst were synthesized. We intended to promote the reduction of CO2 by efficiently inducing oxidation and reduction reactions at two bands through the continuous supply of electrons to both the VB and CB of NiWO4. The synthesized rGO/NiWO4@Au
three-component catalyst exhibited synergy between the constituents, in contrast to NiWO4 and bimetallic catalysts, yielding better catalytic activity in the CO2 reduction reaction. The rGO/NiWO4@Au three-component catalyst had several advantages, exhibiting improved photocatalytic and photoelectric performance. (1) The Au nanoparticles of rGO/NiWO4@Au acted as electron donors, promoting the electron transfer and improving the quantum efficiency. (2) The excellent conductivity and electron-transfer capability of rGO contributed
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to the sustained maintenance of the catalytic reaction cycle, as the rGO supplied electrons to the VB of NiWO4. (3) The irregular interfaces of the rGO/NiWO4@Au three-component catalyst not only provided many reactive sites but also absorbed and scattered light,
increasing the light utilization. These three advantages contributed to the enhancement of the
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catalytic activity through the effective separation of the charges formed on the catalyst
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Acknowledgements
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surface.
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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2018R1A2B6004746), and the authors
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appreciate these supports.
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Fig. 1. (A) XRD patterns and (B) Raman spectra of the synthesized particles.
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Fig. 2. TEM images of synthesized particles.
Fig. 3. (a) HRTEM image, (b) specific lattice planes, (c) SAED pattern, and (d) HAADF
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element-mapping images of rGO/NiWO4@Au particles.
Fig. 4. C1s, O1s, Ni2p, W4f, and Au4f X-ray photoelectron spectra of the synthesized
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particles.
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Fig. 5. Catalytic performances of the synthesized catalysts for CO2 photoreduction.
Fig. 6. (A) Mass spectra of gas species formed on rGO/NiWO4@Au during 10 h of CO2 photoreduction; (B) IR spectra of the surface of rGO/NiWO4@Au before and after CO2
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adsorption and after CO2 reduction.
Fig. 7. (A) DIR-UV–vis spectra of the synthesized particles; (B) XPS curve for
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determining the position of the VB in the NiWO4.
Fig. 8. (A) PL curves and (B) photocurrent density cycles of the synthesized particle
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films.
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Fig. 9. (A) IMPS and (B) IMVS results for NiWO4 and rGO/NiWO4@Au.
Fig. 10. (A) Recombination rate of electrons and holes and (B) electron-transfer rate for
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NiWO4 and rGO/NiWO4@Au.
Scheme 1. Synthesis sequences for the NiWO4, NiWO4@Au, rGO/NiWO4, and
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rGO/NiWO4@Au particles.
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Scheme 2. CO2 photoreduction mechanism on the rGO/NiWO4@Au catalyst.
PII:
S1226-086X(19)30511-8
DOI:
https://doi.org/10.1016/j.jiec.2019.09.033
Reference:
JIEC 4792
To appear in:
Journal of Industrial and Engineering Chemistry
Received Date:
14 July 2019
Revised Date:
16 September 2019
Accepted Date:
19 September 2019
Please cite this article as: Shin J, Heo JN, Do JY, Kim Y-Il, Yoon SJ, Kim YS, Kang M, Effective charge separation in rGO/NiWO4 @Au photocatalyst for efficient CO2 reduction under visible light, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.09.033
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Effective charge separation in rGO/NiWO4@Au photocatalyst for efficient CO2 reduction under visible light
Jongmin Shina, Jun Neoung Heoa, Jeong Yeon Doa, *, Young-Il Kima, Seog Joon Yoona,
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Yang Soo Kimb, Misook Kanga, *
Department of Chemistry, College of Natural Sciences, Yeungnam University, Gyeongsan,
Gyeongbuk 38541, Republic of Korea
Korea Basic Science Institute, Gwahak-ro, Yuseong-gu, Daejeon 34133, Republic of Korea
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*Corresponding author: [email protected] (Jeong Yeon Do), +81-53-810-3798;
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Graphical Abstract
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[email protected] (Misook Kang), +81-53-810-2363
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Highlights
Introducing electron donor onto both the VB and CB of the main catalyst.
Facilitating charge separation and suppressing recombination between electrons and
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holes.
Electrons are filled from the π-electron rich rGO into the VB of NiWO4.
Electrons on Au surfaces are amplified by its SPR effect.
Carbon dioxide reduction capability under visible light
ABSTRACT
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Catalyst performance can be improved by introducing an electron donor into both the valence band (VB) and conduction band (CB) to facilitate charge separation and suppress electron-
hole recombination. Herein, Au nanoparticles served as CB electron donors in NiWO4 core
particles which were evenly dispersed on a reduced graphene oxide (rGO) sheet that served
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as a VB electron donor. The resulting rGO/NiWO4@Au photocatalyst was applied to
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reducing CO2. The particles exhibited broadband absorbance from the ultraviolet to nearinfrared, with a specific Au surface plasmon resonance (SPR) absorption peak at 600 nm.
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Moreover, the catalyst exhibited low photoluminescence (PL) and a high photocurrent density, indicating that photo-excited electron-hole recombination was suppressed and the
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charges effectively separated. Photocatalytic reduction of CO2 on rGO/NiWO4@Au was significantly enhanced as evidenced by the total amounts of reduction products (CO and
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CH4), which were 15 times those for NiWO4 and six times those for rGO/NiWO4 and NiWO4@Au. The expected electron-transfer mechanism on rGO/NiWO4@Au involves
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electron donation into the VB from the π-electron rich rGO, combined with photo-excited electrons from the NiWO4 and Au particles where electrons on the Au surfaces were amplified by the SPR and then moved to the CB of NiWO4. Intensity-modulated photovoltage spectroscopy of rGO/NiWO4@Au indicated a significantly reduced electronhole recombination rate.
Keywords: Charge separation; Electron donor; Carbon dioxide photoreduction; rGO/NiWO4@Au
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Introduction
The photochemical conversion of carbon dioxide (CO2) using solar energy has recently attracted attention for ecofriendly future technologies. However, to use such technologies
industrially, it is necessary to develop a novel catalyst. Sunlight reaching Earth comprises
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4%, 53%, and 43% ultraviolet (UV) rays, visible rays, and infrared (IR) rays, respectively.
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Until now, no photocatalyst having an energy band capable of efficiently utilizing the various wavelengths of sunlight has been developed. In particular, to improve the performance of the
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photocatalyst, it is necessary to spatially separate electrons and holes generated by light, thereby slowing the recombination of electrons and holes [1, 2]. Additionally, it is necessary
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to increase the frequency of the oxidation–reduction reaction occurring at the interface between the semiconductor and the reactant so that the photoactivity can be maintained
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continuously [3, 4]. A photosensitized semiconductor system [5], a two-step excitation semiconductor heterojunction system [6], and a Z-scheme system [7] have been proposed to
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slow the electron/hole recombination. Recently, it has been reported that when precious metals such as Au [8], Ag [9], and Cu [10] are grafted together, the photocatalytic performance of CO2 reduction in the visible-light region is significantly enhanced by their surface plasmon resonance (SPR) effect. In particular, because these metals have a high reduction potential, they can act as electron donors or acceptors, promoting the movement of electrons. Additionally, many researchers have recently attempted to improve the
performance of photocatalysts via efficient light harvesting using not only visible light but the entire wavelength range, including UV and IR rays. One method for this involves forming a heterojunction comprising graphene [11-13], C nanotubes [14], C quantum dots [15], or C3N4 [16] and the main photocatalyst. Owing to the added C material, the catalyst can absorb light in the entire wavelength range of sunlight. C, which contains π-electrons, can also act as an electron donor to continuously supply electrons to the empty holes of the main
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photocatalyst, facilitating the photoreaction. The objective of this study was to exploit basic chemistry to improve the catalyst
performance. All the chemical reactions were redox reactions (NiWO4 electron acceptor and Au or reduced graphene oxide (rGO) electron donor). If the redox reaction can be sustained
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in the optical system, the photocatalytic activity can be increased and maintained. In addition,
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we designed a photocatalyst capable of maximizing the plasmonic effect and absorbing the entire wavelength range of sunlight, thereby promoting the CO2 photoreduction reaction. In
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particular, instead of TiO2, which is widely used as a main catalyst, NiWO4, which contains Ni as a methanation-promoting component and WO3 with a band-gap of 2.6–3.0 eV, was
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employed as a main photocatalyst [17]. Recently, metal oxygenates such as metal vanadate [18], metal borate [19], metal tungstate [20], and metal phosphate [21] have been reported as
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photocatalysts. Because they have a smaller band-gap than metal oxides, they can be activated under visible light, but their use alone has not yielded remarkable results, owing to
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the rapid recombination of electrons and holes. Therefore, we first attempted to incorporate an electron-transport mediator, i.e., rGO, as an electron donor, into the electron acceptor NiWO4 to absorb light over a broad wavelength range. Because rGO and NiWO4 have a twodimensional structure, it was expected that the electron transport from the rGO to the valence band (VB) of NiWO4 would be easy, because the orbitals overlap on a wide plane [22]. Second, we doped the NiWO4 catalyst with Au nanoparticles having a high reduction
potential, which can act as electron acceptors or donors (here, they acted as electron donors). Subsequently, NiWO4@Au particles were uniformly dispersed on rGO to obtain an rGO/NiWO4@Au catalyst. When the rGO/NiWO4@Au catalyst was placed under visible light, the excited electrons from the VB of NiWO4 were promoted to the conduction band (CB). The electrons amplified by the SPR effect on the surface of the Au particles also moved into the CB of NiWO4 and enhanced the electron density of the CB. The rGO
transfer cycle was expected to be maintained constantly.
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Experimental
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continuously supplied π-electrons to the vacant VB of NiWO4; thus, the redox electron-
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Synthesis of NiWO4, NiWO4@Au, rGO/NiWO4, and rGO/NiWO4@Au particles
Graphene oxide (GO) was produced via the following method. While the temperature was
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maintained at 0 oC, 6.0 g of graphite was added to a beaker containing 300.0 mL of sulfuric acid, followed by stirring for 2 h and sonication for 3 h. Then, 4.0 g of sodium nitrate
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(NaNO3, ≥99.0%, Sigma–Aldrich) was added, with stirring for 1 h. The temperature was increased to 10 oC, 25.0 g of potassium permanganate (KMnO4, ≥99.0%, Sigma–Aldrich)
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was added, and the mixture was stirred for 4 h. Here, heat was generated. The temperature was increased to 35 oC, and the mixture was stirred for an additional hour before being allowed to cool. Then, 300.0 mL of distilled water was added, and after stirring for 1 h, another 900.0 mL of distilled water was added. Subsequently, 80.0 mL of a 30 wt.% hydrogen peroxide solution (H2O2, ≥30%, Sigma–Aldrich) was added to obtain a precipitate. This precipitate was washed with 1.0 L of 5.0% hydrochloric acid (HCl, 37%, Sigma–
Aldrich), washed again with 4.0 L of distilled water, and dried at 50 °C for 24 h. To prepare the rGO, 2.0 g of GO was added to 1200.0 mL of ethylene glycol (C38H70O4, >99%, Sigma–Aldrich), followed by stirring for 1 h. Then, 2.0 mL of a 35 wt.% hydrazine solution in H2O (N2H4 65%, 99.99%, Sigma–Aldrich) was added, followed by stirring for 1 h. The solution was placed in an autoclave and heat-treated at 180 °C for 16 h to obtain a precipitate. The resulting precipitate was washed three times with distilled water and twice with 96% ethanol and then dried at 50 °C for 24 h.
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To prepare the Au solution, 1.0 wt.% gold(III) chloride trihydrate (HAuCl4•3H2O,
≥99.9%, Sigma–Aldrich) was added to 100.0 mL of distilled water, followed by sonication
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for 1 h. After the solution was boiled at ≥100 oC, 2.0 mL of a 10.0 wt.% sodium citrate
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solution (Na3C3H5O7, ≥99%, Sigma–Aldrich) was added, followed by stirring for 15 min and then cooling.
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NiWO4 particles were synthesized via a typical hydrothermal method, as shown in Scheme 1a. Nickel nitrate (Ni(NO3)2•6H2O, 99.99%, Sigma–Aldrich) and sodium tungstate
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dihydrate (Na2WO4•2H2O, 99.99%, Sigma–Aldrich) were used as starting materials for Ni and W, respectively. First, 0.1 mol of sodium tungstate dihydrate was dissolved in 1.0 L of
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distilled water, 0.1 mol of nickel nitrate was added dropwise, and the mixture was sonicated for 10 min. After 1 h of stirring, the final solution was transferred to a 2.0 L autoclave. The
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temperature was increased to 180 °C at a heating rate of 10 °C per minute, and the solution was heat-treated at this temperature for 5 h. The resulting light-green precipitate was washed twice with distilled water and three times with ethanol. Subsequently, it was dried at 80 oC for 24 h and transferred to an electric furnace. It was then heated to 600 °C for 2.5 h and crystallized via sintering at this temperature for 6 h (Scheme 1a). To synthesize NiWO4@Au particles, 2.0 g of the prepared NiWO4 powder was added to
400.0 mL of distilled water, followed by stirring for 1 h. Then, 10.0 mL of the prepared Au solution was added to the solution from the previous step, followed by stirring for 30 min. Next, 4.0 mg of sodium borohydride (NaBH4, 99%, Sigma–Aldrich) was added, followed by stirring for 3 h to obtain a precipitate. The precipitate was washed three times with distilled water and twice with ethanol and then dried at 50 °C for 24 h. The powder was sintered for 2 h in an electric furnace set at 450 ℃ (Scheme 1b).
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To prepare the rGO/NiWO4 particles, 0.2 g of NiWO4 powder was added to 400.0 mL of distilled water, followed by sonication for 1 h. Next, 2.0 g of rGO powder was added, and the mixture was stirred for 1 h. Subsequently, 40.0 mL (1.0 mol) of sulfuric acid (H2SO4 ≥98%,
Sigma–Aldrich) was added, followed by stirring for 3 h and then ultrasonic treatment for 10
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min. The final solution was placed in an autoclave and treated at 170 °C for 24 h to obtain a
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precipitate. The precipitate was washed with distilled water three times, washed with ethanol, and dried (Scheme 1c).
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Finally, rGO/NiWO4@Au particles were synthesized via the following steps. First, 0.2 g of NiWO4 powder was added to 400.0 mL of distilled water, followed by ultrasonic treatment
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for 1 h. Next, 2.0 g of rGO powder was added, and the mixture was stirred for 1 h. Then, 10.0 mL of an Au solution was added, followed by stirring for another 1 h. After 40.0 mL (1.0
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mol) of H2SO4 and 4.0 mg of NaBH4 were added, the mixture was stirred for 3 h and sonicated for 10 min. The final solution was transferred to an autoclave and heat-treated at
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170 °C for 24 h. The resulting precipitate was washed three times with distilled water and twice with ethanol and then dried (Scheme 1d).
Characterizations of particles
The crystal structures and shapes of the synthesized NiWO4, NiWO4@Au, rGO/NiWO4,
and rGO/NiWO4@Au particles were examined via X-ray diffraction (XRD) analysis (X’Pert Pro MPD PANalytical, Ni-filtered Cu Kα (λ = 1.5406 Å), 30 kV, 15 mA, 2θ angle of 10°– 80°) and transmission electron microscopy (TEM, H-7600, Hitachi, Japan). The crystal lattice structure of the particles was analyzed using high-resolution TEM (HRTEM, JEM-2100 F, JEOL), and the images were obtained at 200 kV. The X-ray photoelectron spectra of the particles were obtained using a Kratos Axis Nova instrument with monochromatic Al Kα
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radiation (225 W, 15 mA, 15 kV). To determine the optical properties, a diffuse-reflectance UV–visible (DIR-UV–vis) spectrometer (Neosys-2000, Scinco Co., Korea, wavelength of 200~800 nm), photoluminescence (PL) spectroscopy (PerkinElmer, He–Cd laser source, wavelength of 320 nm), and photocurrent measurements (2000 solar simulator, ABET
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Technologies) were used. Raman spectroscopy was performed using a Horiba Jobin-Yvon
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LabRam HR instrument with laser-beam excitation at = 633 nm. Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy
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(IMPS) were performed under open-circuit conditions to measure the electron and hole recombination properties and the electron-transport time using a Sun 2000 Solar Simulator
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(IVIUM STAT, ABET Technologies) under illumination with a power density of AM1.5 (100 mW/cm2). Time constants were calculated using the frequency at which a minimum occurred
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in the imaginary response, via a nonlinear fitting procedure. To confirm the amounts of CO2 adsorbed on the surfaces of the catalysts, CO2-TPD experiments (BEL Japan Inc., Japan)
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were conducted.
CO2 photoreduction on powdered catalysts
The CO2 photoreduction activities of the synthesized NiWO4, NiWO4@Au, rGO/NiWO4, and rGO/NiWO4@Au particles with H2O were examined in a closed cylinder-type quartz
vessel (length, 15.0 cm; diameter, 1.0 cm; total volume, 12.5 mL) that was employed in our previous study [23]. Here, 0.2 g of the catalyst and 40.0 mL of distilled water were placed in the photoreactor. CO2 gas with 99.999% purity was used as the reactant. The CO2 gas was flowed into the chamber to purge air from the chamber before irradiation. The reactor chamber was then closed, and a Sun 2000 Solar Simulator (IVIUM STAT, ABET Technologies) with a power density of AM1.5 (100 mW/cm2) was used as a light source. The
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distance between the reactor and the light source was fixed at 1.0 cm. The photoreduction was conducted at room temperature and the atmospheric pressure. The gases inside the
reactor were sampled at intervals of 1 h using a nanoliter syringe, and the product gases were analyzed using a gas chromatography instrument (Master GC, Scinco, Korea) equipped with
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thermal conductivity and flame ionization detectors to separate the C1–C3 light hydrocarbons
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and oxygenated compounds, such as CH4, CH3OH, HCHO, HCOOH, and CO. The product selectivity was calculated using the following equation:
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Results and discussion
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Ci (%) = Ci moles of the product/total moles of Cproduced × 100%. (1)
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Physical properties of catalysts
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Fig. 1A shows the XRD patterns of NiWO4, NiWO4@Au, rGO/NiWO4, and
rGO/NiWO4@Au particles. In general, the NiWO4 particles exhibited XRD peaks in the same positions, which corresponded to the monoclinic crystal system [24]. The obtained patterns also clearly indicated the absence of phase impurities. For the NiWO4@Au particles, the peak intensity was only slightly reduced, and there was no change in the monoclinic crystal structure compared with NiWO4. The (111) and (200) diffraction planes corresponding
to the Au cubic structure were observed at 2θ = 38.27° and 44.6°, respectively [25]. These results indicate that the Au nanoparticles were well loaded on the surface of the NiWO4 particles. In contrast, for the rGO/NiWO4 particles, the presence of C was expected, as the baseline of the 2θ = 25° peak was broad and excited, which is attributed to the C in rGO [26]. Finally, for the rGO/NiWO4@Au particles, the characteristic peaks of Au exhibited higher intensities; thus, the presence of Au was more clearly detected. In conclusion, the presence of
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Au loaded on NiWO4 and the presence of NiWO4 loaded on rGO were confirmed by the XRD analysis. However, the presence of rGO was not observed in the XRD patterns.
Therefore, its presence was confirmed via Raman spectroscopy, as shown in Fig. 1B. The
presence of rGO in the rGO/NiWO4@Au catalyst was confirmed by the peaks at 1300 and
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1600 cm-1 [27]. A sharp peak at <1000 cm-1 corresponding to the NiWO4 was also observed
the rGO/NiWO4@Au catalyst [29].
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[28]. Moreover, the broad peak near 2700 cm-1 was strong evidence for the presence of Au in
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The shapes of the particles were examined using TEM, as shown in Fig. 2. The NiWO4 particles exhibited a rhomboidal shape, and their sizes ranged from 100 to 200 nm. The
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image of NiWO4@Au particles indicated that Au particles approximately 50 nm in size were loaded on the surface of a large NiWO4 core particle with a width of 200 nm and a length of
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300 nm. The image of the rGO/NiWO4 particles indicated that NiWO4 particles 50–100 nm in size were embedded on the thin rGO sheet. Finally, the images of the rGO/NiWO4@Au
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particles indicated that the Au-loaded NiWO4 particles were uniformly embedded on the rGO. Thus, the TEM image well matched the XRD results. The crystal lattices of the rGO/NiWO4@Au particles were identified, and the selected-
area electron diffraction (SAED) pattern was obtained to confirm the crystallinity and to confirm that the elements present in the sample were uniformly distributed. Fig. 3 shows HRTEM images (a), specific lattice planes (b), SAED patterns (c), and high-angle annular
dark-field imaging (HAADF) element-mapping images (d). As indicated by the HRTEM images, the NiWO4@Au particles were uniformly dispersed in the rGO, and the Au was loaded not on the rGO but only on the NiWO4 particles. Lattice-plane calculations indicated that the lattice widths of the Au (111) and NiWO4 (110) planes were 1.43 and 3.47 Å, respectively. The SAED patterns exhibited a uniform circular shape, suggesting that the particles were uniform single crystals. The distribution of elements in the rGO/NiWO4@Au
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particles was very uniform, with the C from the rGO having the highest density, followed by O, W, and Ni. In particular, Au exhibited a dense electron density, with a shape similar to that of red beads approximately 20 nm in size. These results confirmed that the rGO/NiWO4@Au catalyst prepared in this study was made of NiWO4@Au particles uniformly loaded with Au
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on NiWO4 particles well dispersed on rGO.
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Fig. 4 shows the X-ray photoelectron spectroscopy (XPS) measurements for the oxidation states of the constituent elements in each sample. XPS was performed to monitor the
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characteristic binding energy of the C 1s peaks, corresponding to each functional group on the rGO/NiWO4@Au and rGO/NiWO4 particles. The C 1s XPS spectra were deconvoluted
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into two peaks corresponding to C=C (284.5 eV) and C-C (285.6 eV) [30]. These peaks were separated in almost the same area in the two samples, and the binding-energy positions were
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almost the same. For the NiWO4 and NiWO4@Au samples, the O 1s regions exhibited two main contributions: the O2− state of lattice O (M−O) at 530.6 eV and the O2− state of O
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defects/vacancies at 531.8 eV [33]. For the rGO/NiWO4 and rGO/NiWO4@Au samples, the O 1s peaks were divided into two peaks at 530.5–530.6 and 532.5–532.6 eV via Gaussian fitting. Notably, the second peak was clearly observed and was due to the C-O in rGO [34]. For the NiWO4 and NiWO4@Au samples, the fitted Ni 2p peaks at binding energies of 854.2–855.9 and 872.7–873.5 eV are ascribed to the Ni2+, whereas the fitted peaks at binding energies of 856.9–857.6 and 874.3–875.3 eV correspond to the Ni3+ [31]. The Ni 2p3/2 peaks
were located at 855.1–856.2 and 856.5–857.7 eV for the rGO/NiWO4 and rGO/NiWO4@Au samples. The Ni 2p peaks for the Au-loaded catalysts were observed at lower binding energies that those for NiWO4. The W 4f profile was fitted by two Gaussian peaks centered around 37 and 35 eV, which were the binding energies of electrons in the 4f5/2 and 4f7/2 levels of W in the W6+ valence state in WO3, respectively [34]. For the rGO/NiWO4 and NiWO4@Au samples, the two peaks shifted to lower binding energies compared with
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NiWO4, indicating that the W ions were reduced. The shift was larger for the NiWO4@Au sample. This indicates that the electrons were more enriched in the Au-loaded catalyst
compared with the NiWO4 catalyst. Thus, in the Au-loaded catalyst, the electrons were easily excited by light. Finally, the Au in the NiWO4@Au sample existed in metallic states, as
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indicated by the Au 4f spectra, which were composed of Au 4f7/2 and Au 4f5/2 peaks at 83.6
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and 87.3 eV, respectively [35]. For the rGO/NiWO4@Au sample, the Au 4f7/2 peak was slightly shifted toward the lower-binding energy region (maybe bulk Au), indicating a strong
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interaction between the Au and the NiWO4 support. The strong interaction between the Au nanoparticles and the NiWO4 support was expected to enhance hot-electron transfer from the
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Au nanoparticle surface to the NiWO4 CB through the Schottky energy barriers [36].
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Catalytic performance of catalysts for CO2 photoreduction
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Fig. 5 shows the yields of the primary product of CO and the final product of CH4 when
the following reaction was conducted under visible light using four types of particles: NiWO4, NiWO4@Au, rGO/NiWO4, and rGO/NiWO4@Au catalysts: CO2 + 4H2O + hv → CH4 + 2H2O (2) For the NiWO4 catalyst, very small amounts of the CO and CH4 gases were accumulated after 10 h of reaction, and their ratio was almost 1:1. For the NiWO4@Au catalyst (Au loaded on
NiWO4), the amount of CO generated increased sharply, and the CO production was nearly 10 times that for CH4. Many studies have indicated that the amount of CO2 is significantly reduced by the effect of Au on the SPR [37], and the selectivity to CH4 is increased [38]. However, for NiWO4@Au, the selectivity of CH4 hardly changed, and the fact that only the production of CO increased significantly is unusual. However, for the rGO/NiWO4 catalyst, the amount of CH4 increased by more than 4 times compared with that for the NiWO4@Au
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catalyst. This result implies that the CO on the rGO in the rGO/NiWO4 catalyst reacted with H and was converted into CH4. For the final catalyst, i.e., the rGO/NiWO4@Au particles, the total product amount increased by a factor of 6 compared with that for the rGO/NiWO4 catalyst and by a factor of 15 compared with that for the NiWO4 catalyst. However, the
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amount of CH4 hardly increased. According to these results, we predicted that the addition of
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Au in rGO/NiWO4@Au increased the amount of the primary product, CO, because the system did not contain redox to decompose water.
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For clarifying the CO2 photoreduction mechanism, Fig. 6A shows the mass spectra of the gas species formed on rGO/NiWO4@Au during 10 h of CO2 photoreduction. In our
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mass analysis, •C, •CH2, and •CH3 intermediates were observed. Therefore, we predicted that CO2 was reduced through the carbene pathway for the rGO/NiWO4@Au catalyst. There were
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two pathways, and both preferred the two-electron mechanism, where two photogenerated electrons participated in concert in one elemental step [39]. One pathway is called the
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formaldehyde pathway (fast hydrogenation pathway), in which the reaction follows the path CO2 → HCOOH → HCHO → CH3OH → CH4 [40]. This pathway is thermodynamically feasible; however, in our mass analysis, HCOOH and CH3OH were not observed. The other pathway is the carbene pathway (fast deoxygenation pathway), in which CO2 is reduced along the path CO2 → CO→ •CHO→ •CH2 →•CH3 → CH4 [41]. Fig. 6B shows the IR spectra of the surface of rGO/NiWO4@Au before and after CO2 adsorption and after CO2
reduction. In the rGO/NiWO4@Au before CO2 adsorption, six peaks at 706, 832, 1180, 1390, 1555, and 3413 cm−1 were observed, corresponding to Ni-O, W-O, C-OH, C-O, C=C, and OH configurable stretching and bend vibrations in the NiWO4 and rGO [42, 43]. After CO2 adsorption, the vibration mode for isolated CO2 gas was observed at 2450 cm−1. This indicates that CO2 was well adsorbed onto the rGO/NiWO4@Au catalyst. After the CO2 photoreduction reaction, the C=O vibrational mode was observed at 2030 cm−1, and three large peaks corresponding to the C-H bending mode in CH4 were observed at 1076, 1210,
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and 1302 cm−1 [44]. The O-H stretching mode at 3413 cm−1 exhibited the largest peak,
indicating that the reaction intermediate (CO) adsorbed on the catalyst became Cat-CO, and
the following three mechanisms in this study.
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then H+ was bound to Cat-COH. According to the report of W. Zhang et al. [45], we predicted
(a) + e- Cat-CO (b) + OH- (4)
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Cat + CO2 Cat-CO2 + H2O Cat-COOH (a) + OH- (3)
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(a) + H+ + e- Cat-COH + 3H+ + 3e- Cat-CH2 + H+ + e- Cat-CH3 + H+ + e- Cat-
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CH4 (c) + OH- (5)
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Optical properties of catalysts
Fig. 7A shows the DIR-UV–vis spectra of the NiWO4, NiWO4@Au, rGO/NiWO4, and
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rGO/NiWO4@Au particles, which were used to determine their bandgaps. According to de Oliveira et al. [46], the absorption bands in NiWO4 above 750 (IR range), 470, and 380 nm (UV range) can transition from 3A2g to the excited states 3T2g, 3T1g, and 3T1g, respectively. Because a 1-sun light source was used in this study, the absorbance of NiWO4 was reasonable at 470 nm; thus, the bandgap was 2.63 eV. This is consistent with previously reported results [47]. The absorption peak of Au, which indicated the effect of the SPR on the NiWO4@Au
particles, was clearly observed at 600 nm [48]. The rGO/NiWO4 and rGO/NiWO4@Au particles containing rGO absorbed light in the wavelength range of 300–1000 nm (from the UV to the near-IR region). This result implies that a catalyst that absorbs light over the entire wavelength range can be fabricated by uniformly applying nanoparticles to rGO. The position of the VB in the NiWO4 obtained via XPS was 2.27 eV, as shown in Fig. 7B. By subtracting this VB position from the bandgap obtained in Fig. 7A, we obtained the desired CB, which
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corresponded to -0.36 eV. The electrons excited into the CB of the particles were relaxed again and recombined with the holes. The analytical method for measuring these relaxed photoelectrons is PL, and the PL spectroscopy results are shown in Fig. 8A. The electrons excited by 300 nm wavelength light
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were emitted through the relaxation process at 410 nm. For the NiWO4 with pure crystals, a
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large phosphorescence peak appeared at 410 nm, and small fluorescence peaks were observed at 470 and 540 nm. For NiWO4@Au, the intensities of the PL peaks were significantly lower.
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In particular, the PL peaks for the rGO/NiWO4 and rGO/NiWO4@Au particles containing rGO were small. This is attributed to electron capture caused by defects in the rGO/NiWO4
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and rGO/NiWO4@Au crystals or the intercalation phenomenon between the adjacent particles [49, 50]. Eventually, in any case, the number of excited electrons recombined with
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the holes will be reduced, which ultimately increases the activity of the photocatalyst. However, in the NiWO4 and NiWO4@Au films, photocurrent hardly flowed when the light
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was on, as shown in Fig. 8B. However, in the rGO-added rGO/NiWO4, the current increased 2
in the 1st cycle of the light being turned on, tended to stabilize until the 3rd
cycle, and then decreased in the 4th cycle. Thus, the initial activity of the catalyst was good, but catalyst deterioration occurred owing to the recombination of electrons and holes during the cycling. The rGO/NiWO4@Au catalyst exhibited the largest photocurrent increase, and the photocurrent did not decrease until the 5th cycle. These results indicate that the separation
of electrons and holes was easy in this catalyst. To calculate the electron-transport time and the recombination lifetime for NiWO4 and rGO/NiWO4@Au, IMPS and IMVS were performed, and the typical responses are presented in Fig. 9. Generally, it is reasonable to use IMPS to determine the electron-transport time [51], and the transport time constant affects the measured IMPS signals at high frequencies, as shown in Fig. 9A. A higher voltage across both catalysts yielded a larger movement of the
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circle to the left and a smaller circle. This indicates that a higher voltage led to faster electron transport. However, the spacing of the circles for the rGO/NiWO4@Au catalyst was narrower and smaller than that for the NiWO4 catalyst. This was due to the faster transfer of electrons on the rGO/NiWO4@Au catalyst. As shown in Fig. 9B, as the applied voltage increased, the
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IMVS circles moved to the right and became larger. This indicates that when a high voltage
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was applied, the recombination rate of the electrons and holes decreased. The IMVS circles were more shifted to the right for the rGO/NiWO4@Au catalyst than for the NiWO4 catalyst,
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and the circle spacing was wider. This shift indicated slower recombination of electrons and holes in rGO/NiWO4@Au compared with NiWO4.
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According to the IMPS and IMVS curves, the electron-transport times (s, y-axis) and recombination lifetimes were estimated using the expression s = 1/2pft (or fr), where ft or fr
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represents the characteristic frequency at the minimum value of the imaginary components of IMPS or IMVS [52]. The results are presented in Fig. 10. The electron-transfer rate for the
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rGO/NiWO4@Au catalyst (Fig. 10A) was significantly higher (approximately 30%) than that for the NiWO4 catalyst. This indicates that the electrons generated in the Au moved quickly to the NiWO4, and the charge separation between the electrons and holes was useful because the rGO quickly filled the electrons with the empty orbit of Au and NiWO4. It is believed to be accelerated. Additionally, as shown in Fig. 10B, the rate of recombination of electrons and holes for the rGO/NiWO4@Au catalyst was significantly lower than that for the NiWO4
catalyst, by a factor of approximately 10. This was due to the continuous supply of electrons from the electron-rich rGO to the empty valence shell of NiWO4. According to the foregoing catalyst properties and performance, the expected CO2 photoreduction mechanism on the rGO/NiWO4@Au catalyst is depicted in Scheme 2. In the CO2 photocatalytic reduction, the enhanced photocatalysis of the ternary nanocomposites is attributed to the synergistic effect of the three components. First, both the NiWO4 and the
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rGO nanosheets provided adsorption to CO2 molecules. Then, the photogenerated carriers were formed through the excitations of both the bandgap transition of NiWO4 and the SPR of Au nanoparticles upon the visible-light irradiation. The Au nanoparticles enhanced the
visible-light harvesting owing to their SPR properties, and the amplified electrons on the
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surface of Au moved to the CB of NiWO4. Additionally, in the electron-rich CB of NiWO4,
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the reduction of the adsorbed CO2 molecules into CO occurred. Alternatively, holes (h+) could have oxidized the surface H2O, forming reactive •OH and H+. The rGO nanosheets
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offered a path for rapid transport of the photogenerated electrons, yielding efficient charge separation during the photocatalytic process. The injected electrons in the rGO sheets were
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subsequently captured by the adsorbed H+, producing the active CHO, CH2, and CH3 radicals during the photocatalysis. Finally, they combined with H+ on the electron-rich rGO,
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producing CH4.
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Conclusions
In this study, rGO/NiWO4@Au catalysts with electron donors of rGO and Au grafted to
an NiWO4 main catalyst were synthesized. We intended to promote the reduction of CO2 by efficiently inducing oxidation and reduction reactions at two bands through the continuous supply of electrons to both the VB and CB of NiWO4. The synthesized rGO/NiWO4@Au
three-component catalyst exhibited synergy between the constituents, in contrast to NiWO4 and bimetallic catalysts, yielding better catalytic activity in the CO2 reduction reaction. The rGO/NiWO4@Au three-component catalyst had several advantages, exhibiting improved photocatalytic and photoelectric performance. (1) The Au nanoparticles of rGO/NiWO4@Au acted as electron donors, promoting the electron transfer and improving the quantum efficiency. (2) The excellent conductivity and electron-transfer capability of rGO contributed
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to the sustained maintenance of the catalytic reaction cycle, as the rGO supplied electrons to the VB of NiWO4. (3) The irregular interfaces of the rGO/NiWO4@Au three-component catalyst not only provided many reactive sites but also absorbed and scattered light,
increasing the light utilization. These three advantages contributed to the enhancement of the
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catalytic activity through the effective separation of the charges formed on the catalyst
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Acknowledgements
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surface.
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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2018R1A2B6004746), and the authors
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appreciate these supports.
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Rodrigues, J. Braz. Chem. Soc. 30 (2019) 371.
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Fig. 1. (A) XRD patterns and (B) Raman spectra of the synthesized particles.
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Fig. 2. TEM images of synthesized particles.
Fig. 3. (a) HRTEM image, (b) specific lattice planes, (c) SAED pattern, and (d) HAADF
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element-mapping images of rGO/NiWO4@Au particles.
Fig. 4. C1s, O1s, Ni2p, W4f, and Au4f X-ray photoelectron spectra of the synthesized
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particles.
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Fig. 5. Catalytic performances of the synthesized catalysts for CO2 photoreduction.
Fig. 6. (A) Mass spectra of gas species formed on rGO/NiWO4@Au during 10 h of CO2 photoreduction; (B) IR spectra of the surface of rGO/NiWO4@Au before and after CO2
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adsorption and after CO2 reduction.
Fig. 7. (A) DIR-UV–vis spectra of the synthesized particles; (B) XPS curve for
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determining the position of the VB in the NiWO4.
Fig. 8. (A) PL curves and (B) photocurrent density cycles of the synthesized particle
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films.
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Fig. 9. (A) IMPS and (B) IMVS results for NiWO4 and rGO/NiWO4@Au.
Fig. 10. (A) Recombination rate of electrons and holes and (B) electron-transfer rate for
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NiWO4 and rGO/NiWO4@Au.
Scheme 1. Synthesis sequences for the NiWO4, NiWO4@Au, rGO/NiWO4, and
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rGO/NiWO4@Au particles.
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Scheme 2. CO2 photoreduction mechanism on the rGO/NiWO4@Au catalyst.