Research Article pubs.acs.org/acscatalysis
Amine-Functionalized Graphene/CdS Composite for Photocatalytic Reduction of CO2 Kyeong Min Cho,† Kyoung Hwan Kim,† Kangho Park,† Chansol Kim,† Sungtak Kim,‡ Ahmed Al-Saggaf,§ Issam Gereige,§ and Hee-Tae Jung*,†
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†
Department of Chemical & Biomolecular Engineering (BK-21 plus) and KAIST Institute for Nanocentury, Korea Advanced Institute of Science & Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡ Carbon Resources Conversion Catalytic Research Center, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeongro, Yuseong-gu, Daejeon 34114, Republic of Korea § Saudi Aramco, Research and Development Center, Dhahran 31311, Saudi Arabia S Supporting Information *
ABSTRACT: This study provides a significant enhancement in CO2 photoconversion efficiency by the functionalization of a reduced graphene oxide/cadmium sulfide composite (rGO/ CdS) with amine. The amine-functionalized graphene/CdS composite (AG/CdS) was obtained in two steps. First, graphene oxide (GO) was selectively deposited via electrostatic interaction with CdS nanoparticles modified with 3-aminopropyltriethoxysilane. Subsequently, ethylenediamine (NH2C2H4NH2) was grafted by an N,N′-dicyclohexylcarbodiimide coupling reaction between the amine group of ethylenediamine and the carboxylic group of GO. As a result, a few layers of amine-functionalized graphene wrapped CdS uniformly, forming a large interfacial area. Under visible light, the photocurrent through the AG/CdS significantly increased because of enhanced charge separation in CdS. The CO2 adsorption capacity on AG/CdS was 4 times greater than that on rGO/ CdS at 1 bar. These effects resulted in a methane formation rate of 2.84 μmol/(g h) under visible light and CO2 at 1 bar, corresponding to 3.5 times that observed for rGO/CdS. Interestingly, a high methane formation rate (1.62 μmol/(g h)) was observed for AG/CdS under CO2 at low pressure (0.1 bar), corresponding to a value 20 times greater than that observed for the rGO/CdS. Thus, the enhanced performance for photocatalytic reduction of CO2 on the AG/CdS is due to the improved CO2 adsorption related to the amine groups on amine-functionalized graphene, which sustains the strong absorption of visible light and superior charge-transfer properties in comparison with those of graphene. KEYWORDS: photocatalyst, carbon dioxide, graphene, amine, CO2 activation
D
semiconductor composites have attracted interest because of the ability of graphene to improve the surface area and photocatalytic performance, caused by the transfer of electrons between graphene and the semiconductor, as well as light absorption.8−12 The formation rate of CH4 on CdS nanorods with graphene under visible light is 10 times that on bare CdS nanorods. This difference results from facile charge transfer from the nanorods to graphene.13 Similarly, the incorporation of graphene into two-dimensional (2D) mesoporous TiO2 significantly enhances the photodegradation of methylene blue because of the facile charge transfer resulting from the increase in the interfacial area between 2D TiO2 and 2D graphene.14 Despite the significant light-harvesting enhancement achieved via the improvement of the charge transfer using
eveloping a system for the highly efficient photoconversion of carbon dioxide (CO2) is a tremendous challenge for addressing global climate change. The photocatalyst absorbs photons in sunlight, which have energies greater than its band gap energy, leading to electron−hole pair generation. The generated electrons are then used to reduce CO2 into valuable fuels such as carbon monoxide (CO), methane (CH4), and methanol (CH3OH).1,2 Generally, singlecomponent inorganic semiconductors are not suitable photocatalysts for CO2 photoreduction, as they barely satisfy all critical requirements of the process, including strong light absorption, rapid charge separation, favorable CO2 adsorption, and selective CO2 reduction.3,4 A number of hybrid materials, which include heterojunction inorganic semiconductors, immobilized organic photosensitizers, functional metals, and conductive carbon materials, have been incorporated into inorganic semiconductors to improve their photoreduction performance.4−7 In particular, graphene/ © 2017 American Chemical Society
Received: June 12, 2017 Revised: August 31, 2017 Published: September 6, 2017 7064
DOI: 10.1021/acscatal.7b01908 ACS Catal. 2017, 7, 7064−7069
Research Article
ACS Catalysis
Figure 1. Morphology of AG/CdS: (a) schematic of the synthesis of the AG/CdS; (b) SEM image of the CdS nanoparticles; (c) SEM image of the AG/CdS (yellow arrows denote graphene); (d) TEM image of AG/CdS (the inset shows a high-magnification image; the scale bar is 3 nm).
promising composite that can markedly enhance the photocatalytic conversion of CO2. Figure 1a shows the overall procedure for fabricating AG/ CdS. CdS nanoparticles were synthesized by solvothermal reactions at 140 °C using cadmium acetate and excess thiourea. Graphene oxide (GO) was incorporated into the surface of CdS particles to prepare GO/CdS. First, the CdS surface was treated with 3-aminopropropyltriethoxysilane (APTES) to generate positive charges on the CdS surface.18 During mixing with a few exfoliated GO layers, positively charged CdS particles electrostatically interacted with negatively charged GO at pH 6.19 The GO/CdS was then functionalized with amine by grafting ethylenediamine (NH2C2H4NH2) via a coupling reaction with N,N′-dicyclohexylcarbodiimide (DCC) at 85 °C. In this step, the amine group of ethylenediamine reacts with the carboxylic group of GO. The resulting AG/CdS consisted of CdS nanoparticles coated with AG (Figure 1a). Figure 1b shows the synthesized CdS nanoparticles were predominantly spherical, with an average diameter of ∼140 nm (±17.3 nm). The diameter was determined by averaged measurements of approximately 100 particles in the scanning electron microscopy (SEM) images. Uniform CdS nanospheres are obtained by the aggregation of small nanocrystallites with hexagonal phase in the presence of excess thiourea, which inhibited the growth of CdS nanocrystallites. The particle size could be controlled by varying the duration of the solvothermal reaction (Figures S1 and S2 in the Supporting Information).20 Synthesized GO consisted of 2D flakes with dimensions of ∼5 μm, which predominantly exfoliated into single layers in water (Figure S3 in the Supporting Information).21 After the hybridization of CdS and GO followed by amine functionalization, careful inspection of the SEM images of AG/CdS revealed that graphene layers are uniformly distributed on the CdS nanoparticle surfaces (Figure 1c). As estimated from the TEM images, 7−10 graphene layers were observed on the CdS
graphene-based photocatalysts, CO2 photoreduction is still not efficient because of shortcomings in the design of these catalysts for CO2 activation.15 Herein, we developed an amine-functionalized graphene/ CdS composite (AG/CdS) that exhibits significant CO2 adsorption and efficient light absorption and charge-transfer properties, which enhance CO2 photoconversion performance. So far, all of the reported graphene-based photocatalysts are based on reduced graphene oxide (rGO).16,17 The use of rGO in heterogeneous photocatalyst enhances electron transfer because of its high electrical conductivity. It simultaneously loses its functional groups, enabling interaction with CO2. Therefore, a crucial research topic related to graphene-based photocatalysts is the preparation of surface-modified graphene that retains its good charge-transfer properties. The developed AG/CdS exhibits several significant advantages over those used in previously reported CO2 photoconversion systems. First, the CdS is an attractive visible-light-driven photocatalyst for CO2 conversion among the various inorganic semiconductors due to its narrow band gap of 2.4 eV. In addition, the negative conduction band edge position of CdS could provide electrons with higher reduction potential.13,16,31 Second, amine groups on graphene contribute to CO2 activation, which boosts the catalytic conversion of CO2. AG on the CdS surface enhances CO2 adsorption by 13 times at 0.1 bar and by 4 times at 1 bar. Third, AG wrapping of CdS nanoparticles provides a path for the efficient transfer of photogenerated electrons. Thus, the photocurrent through the AG/CdS under visible light is 10 times that through bare CdS. This increase is indicative of the efficient light harvesting caused by the introduction of graphene electron-transfer channels. On this composite, the conversion rate of CO2 into CH4 is 2.84 μmol/(g h) under visible light, 3.5 times that of the conversion on rGO/CdS. Interestingly, AG/ CdS enables CH4 production (1.62 μmol/(g h)) under lowpressure CO2 (0.1 bar). Therefore, we expect AG/CdS to be a 7065
DOI: 10.1021/acscatal.7b01908 ACS Catal. 2017, 7, 7064−7069
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ACS Catalysis
Figure 2. Chemical states of AG/CdS: (a) FTIR spectra of GO/CdS (black) and AG/CdS (red); (b, c) XPS C 1s spectra of GO and AG, respectively; (d) results for carbon and nitrogen content analysis of AG/CdS (the inset shows a photograph of each sample; (e) suggested mechanism for the amine functionalization of graphene oxide.
Figure 3. CO2 adsorption and light-harvesting property of CdS composite: (a) CO2 adsorption isotherms at 25 °C; (b) UV−vis diffuse reflectance spectra of AG/CdS; (c) transient photocurrent under visible light (420 nm) (Figure 3c). Photocurrent measurements were carried out using a three-electrode system. The working photoelectrode was prepared by deposition onto ITO glass and immersed in aqueous 0.5 M Na2SO4 electrolyte, and the applied voltage was 0.2 V versus a Ag/AgCl reference electrode. CdS exhibited a small photocurrent (0.18 mA/cm2) under visible light. Despite the strong visible-light absorption of CdS, the rapid recombination of electrons and holes in inorganic semiconductors lowers the light-harvesting efficiency.31 On the other hand, the rGO/CdS and AG/CdS electrodes exhibited a highly improved photocurrent (1.7 and 2.4 mA/cm2) in comparison with that of the bare CdS electrode. Such marked enhancement is caused by the efficient electron-transfer path provided by graphene. The photocurrent through the AG/CdS electrode was slightly stronger, with slow saturation response and recovery behavior in comparison with those observed for the rGO/CdS electrode. These results may be due to the trapping of electrons by amine groups at the graphene/ electrode interface.32,33 Further, electrochemical impedance spectroscopy (EIS) was conducted to characterize the charge separation property (Figure 3d). The EIS measurement was carried out under dark conditions with 0.5 M Na2SO4 electrolyte. The semicircular diameter of the Nyquist plot indicates the charge transfer resistance between catalyst and electrolyte.34 While CdS has a very large diameter due to its low conductivity, rGO/CdS improved charge transfer properties due to the heterojunction of conductive graphene. Similarly, AG/CdS shows low charge transfer resistance, indicating that amine-functionalized graphene provides an effective charge transfer pathway. Therefore, AG increases CO2 adsorption while maintaining good charge-transfer properties and light absorption, which are advantages of a graphene-based photocatalyst. To evaluate the activity of the AG/CdS for CO2 photoreduction, a photocatalytic reaction was conducted under visible light (λ >420 nm) in the presence of pure CO2 and water vapor at 40 °C. Control experiments were conducted without light, CO2, and an inorganic photocatalyst (CdS), 7067
DOI: 10.1021/acscatal.7b01908 ACS Catal. 2017, 7, 7064−7069
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ACS Catalysis
Figure 4. CO2 photoreduction on the CdS composites: (a) photoreduction of CO2 (1 bar) on AG/CdS with varying graphene contents under visible light (>420 nm); (b, c) CO2 photoreduction and CH4/CO ratio on CdS, G/CdS, and Ag/CdS (b) under CO2 at 1 bar (40 °C) and (c) under low-pressure CO2 (0.1 bar, 40 °C); (d) cyclic photoreduction of CO2 on rGO/CdS and AG/CdS; (e) graphical illustration of the proposed mechanism of CO2 photoreduction on the AG/CdS.
states, as confirmed by the SEM image and FTIR spectra (Figure S11 in the Supporting Information). Similarly, rGO/ CdS maintained its photocatalytic efficiency over 10 cycles (92.5%). Indeed, the poor photostability of CdS is one of the major issues associated with the CdS photocatalyst, related to photooxidation.33 Notably, the introduction of AG layers considerably enhanced the stability of the CdS photocatalyst. We conducted in situ FTIR spectroscopy at 313 K under 0.1 bar CO2 to verify the adsorption species on AG/CdS (Figure S12 in the Supporting Information). The peaks in the region 1300−1700 cm−1 can be assigned to chemisorbed CO2. Bands at 1626, 1551, 1486, 1423, 1396, and 1325 cm−1 were present, which have been associated with alkylammonium carbamate ion pairs. This species is obtained by interaction with two adjacent amine molecules. In addition, the peaks at 1657 and 1527 cm−1 were assigned to carbamic acid formed on an isolated amine group.35 These chemisorbed species formed in the presence of amine-functionalized graphene; therefore, the functionalized amine can activate the CO2. Figure 4e shows the possible mechanism for the photocatalytic reduction of CO2 by the AG/CdS system. CO2 is adsorbed and activated on AG via interaction with amine groups even at low CO2 concentrations. CdS absorbs visible light and generates energetic electrons and holes. A few layers of graphene then transfer the electrons before recombination. The activated CO2 molecule is reduced into CH4, which requires eight electrons under visible light and diluted CO2. Therefore, AG is a promising cocatalyst material for various inorganic photocatalyst composites for CO2 reduction. In summary, we prepared an AG/CdS composite that exhibits a high rate of photoreduction of CO2 into CH4 due to enhanced light harvesting and CO2 activation. The composite was obtained by the wrapping of GO on CdS by an electrostatic
which is inactive in the reaction (Figure S9 in the Supporting Information). Methane (CH4) and carbon monoxide (CO) were the main products obtained from CO2 reduction. Bare CdS produced trace amounts of CO (0.088 μmol/(g h)) and CH4 (0.124 μmol/(g h)) predominantly by rapid fast electron−hole recombination. In contrast, AG exhibited substantial enhancement of CO2 reduction. A drastic increase in the CH4 formation rate was observed with increasing graphene content. At high graphene loading (5 wt %), the activity decreased, which is due to shielding of light absorption in CdS and poor interfacial charge transfer of the composite.7,28 AG/CdS-2 exhibited a high CH4 formation rate (2.84 μmol/(g h); Figure 4a). In the presence of rGO, the conversion rate of CO2 on rGO/CdS similarly increased. The CH4 formation rate reached 0.804 μmol/(g h) at 2 wt % graphene (Figure S10 in the Supporting Information). Notably, the rate of photoreduction of CO2 into CH4 on the AG/CdS was enhanced by 23 and 3.5 times relative to those on CdS and rGO/CdS, respectively. In addition, the CH4/CO ratio increased to 15.8, indicating that AG promotes the multielectron reduction of CO2. These enhancements in AG/CdS in comparison with rGO/CdS are mainly due to CO2 activation, which is the ratedetermining step in CO2 reduction.15 Under CO2 at low pressure (0.1 bar), a high formation rate of CH4 on AG/CdS was observed (1.62 μmol/(g h)), whereas a low formation rate of CH4 on rGO/CdS was observed (0.087 μmol/(g h); Figure 4c). Thus, AG permits the conversion of CO2 at low pressure into CH4, which is in good agreement with the high adsorption of CO2 at low pressure. Interestingly, our AG/CdS exhibited long-term stability for CO2 photoreduction under visible light. It maintained high CO2 conversion over 10 cycles (84.7%; Figure 4d). It showed no obvious changes in macroscopic morphology and chemical 7068
DOI: 10.1021/acscatal.7b01908 ACS Catal. 2017, 7, 7064−7069
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reaction and the grafting of ethylenediamine via interaction with carboxylic groups on GO. AG played significant roles in enhancing CO2 photoreduction. The intimate contact of AG with CdS enhanced charge transfer, resulting in a marked increase in photocurrent under visible light. CO2 was chemisorbed on the amine groups, with adsorption capacities corresponding to 4 and 13 times as high as that of rGO/CdS under 1 and 0.1 bar, respectively. Indeed, AG activated CO2 by maintaining efficient electron transfer and light absorption properties. The resulting methane formation rate was 2.84 μmol/(g h), with high selectivity, corresponding to 3.5 times that observed with rGO/CdS under visible light. In addition, AG/CdS converted CO2 to methane at low pressure, with a rate of 1.62 μmol/(g h); in contrast, a low methane formation rate was obtained on rGO/CdS. Graphene enhanced the photostability of the CdS-based photocatalyst, maintaining a conversion rate of 87% over 10 cycles. We anticipate that amine-functionalized graphene may be useful in various heterogeneous catalyst systems for the high photoconversion performance of CO2.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b01908. Description of sample preparation and photoconversion tests, SEM, AFM, and TEM images, XRD, XPS, element analysis, CO2 isotherms, UV−vis diffuse reflectance, and CO2 photoreduction results (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for H.-T.J.:
[email protected]. ORCID
Sungtak Kim: 0000-0001-5818-9174 Hee-Tae Jung: 0000-0002-5727-6732 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by Saudi Aramco-KAIST CO 2 Management Center and supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (MSIP; grant no. 2015R1A2A1A05001844). In addition, this research was technically supported by the Korean Basic Science Institute (KBSI) research Grant No. E36800.
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REFERENCES
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DOI: 10.1021/acscatal.7b01908 ACS Catal. 2017, 7, 7064−7069
