Photocatalytic Reduction of Carbon Dioxide over Quinacridone

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General Research

Photocatalytic Reduction of Carbon Dioxide over Quinacridone Nanoparticles Supported on Reduced Graphene Oxide Peng Yang, Shien Guo, Xiaoxiao Yu, Fengtao Zhang, Bo Yu, Hongye Zhang, Yanfei Zhao, and Zhimin Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00242 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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Industrial & Engineering Chemistry Research

Photocatalytic

Reduction

of

Quinacridone

Nanoparticles

Carbon Supported

Dioxide on

over

Reduced

Graphene Oxide Peng Yang †, ‡, Shien Guo †, ‡, Xiaoxiao Yu †, ‡, Fengtao Zhang †, ‡, Bo Yu †, Hongye Zhang †, Yanfei Zhao †and Zhimin Liu *, †, ‡. † Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ‡ University of Chinese Academy of Sciences, Beijing 100049, China. *E-mail: [email protected]

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ABSTRACT. Photoreduction of carbon dioxide to chemicals or fuels is very interesting from the viewpoint of green chemistry. Herein, we report the photoreduction of CO2 catalyzed by a metalfree photocatalyst: reduced graphene oxide (rGO) supported quinacridone (QA) particles (QA/rGO), which were prepared via aggregation of QA on the surface of GO through forming Hbonding with GO. The resultant QA/rGO composites exhibited improved activity for CO2 photocatalytic reduction using TEOA as a sacrifice reagent under ultraviolet light irradiation; especially, the composite with rGO content of 2 wt% (i.e., QA/rGO-2) produced CO and CH4 with rates of 450 and 275 μmol g-1 h-1, respectively. It was indicated that the QA particles served as photosensitizer and photocatalyst, and the rGO nanosheets promoted the transfer of the photogenerated carriers and their separation, thus improving the catalytic activity of QA for catalysing CO2 reduction.

KEYWORDS: Quinacridone, Photoredution, Carbon dioxide, Self-assembly, rGO.

1. Introduction Photocatalytic reduction of carbon dioxide to chemicals and fuels such as carbon oxide

1-4,

formic acid5, 6, methanol7, 8, methane9, 10 is a promising way for CO2 conversion, which has been paid much attention in recent years. In CO2 photoreduction, photocatalysts play the crucial role, and an efficient photocatalyst should meet the requirements that can absorb light and adsorb/activate CO2 simultaneously. To date, various photocatalysts including inorganic semiconductors (e.g., metal oxides, sulfides)11-14, conjugated polymers15-18 and hybrids19 have been reported for photoreduction of CO2. However, most photocatalysts have the disadvantages such as low efficiency and poor stability, thus limiting their applications. The hybrid catalysts can


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combine the properties of each component to generate enhanced activity, and show promising potential in CO2 photoreduction. For instance, as a 2D carbon support reduced graphene oxide (rGO) has been applied in the photocatalysts such as ZnO/rGO20, CdS/rGO21, CuO/rGO22, CoTHPP/G23, rGO/pCN24, which can promote transfer and separation of the photogenerated carriers because of its unique structure. Recently, non-covalent self-assembly organic molecules that possess unique optical and electronic properties have been utilized as photocatalysts. For example, self-assembled perylene3,4,9,10-tetracarboxylic diimide25 and carboxy-substituent perylene tetracarboxylic diimide26 supramolecular systems have been reported to be very effective for photodegrading pollutants. One-dimensional self-assembled porphyrin nanowires27 and perylene monoimide chromophore amphiphiles28-30 showed enhanced photocatalytic activity in hydrogen generation. Since there are large amount of organic molecules that possess optical and electronic properties, the organic molecule-based photocatalysts may have promising applications in photocatalysis. Quinacridone (QA) is an organic molecule with -NH2 and -C=O groups, which has strong ability to form H-bond with H-acceptor and H-donator molecules. As illustrated in Scheme 1, the γ-phase QA has a crisscross lattice, in which each QA molecule can hydrogen-bond with four neighbouring molecules, forming thermodynamically stable H-bond aggregated solid semiconductor with the hole and electron mobilities of 0.2 and 0.01 cm2/Vs, respectively.31, 32 Therefore, the QA semiconductor has applied in organic light-emitting diode,33, 34 organic solar cell,35, 36 and organic field-effect transistor.31, 37, 38 It may have potential in photocatalysis, but has not been reported in a literature survey. The presence of -NH2 and -C=O in QA molecules provides their strong ability to form Hbond with H-acceptor and H-donator molecules. Graphene oxides (GO) have plenty of O-


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containing groups on their surface, which may form H-bond with QA molecules. Moreover, the NH2 and O-containing groups are favorable to the adsorption of CO2. 39, 40 With these in mind, we intend to prepare the composite of QA and rGO for CO2 photoreduction. We supported QA nanoparticles onto the surface of rGO nanosheets via H-bonding, resulting in a series of QA/rGO composites with different rGO contents, as illustrated in Scheme 1. The resultant QA/rGO samples were examined using TEM and SEM, FTIR, Raman spectroscopy, and their photoelectronic properties were also detected. Furthermore, they were applied in photoreduction of CO2. Interestingly, the QA/rGO composite displayed high performance for CO2 photoreduction using TEOA as a sacrifice reagent under ultraviolet light irradiation, and QA/rGO with 2wt% rGO afforded carbon oxide and methane in production rates of 450 and 275 μmol g-1 h-1, respectively. The reaction mechanism was discussed as well. This work presents a metal-free photocatalyst composed of organic semiconductor QA and rGO for CO2 photoreduction.

Scheme 1. Chemical structure of QA and schematic illustration of synthesis of QA/rGO composite.

2. Experimental Section


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2.1 Materials. CO2 (99.999%) was supplied by Beijing Analytical Instrument Factory. QA (99%), graphite powder (300 meshes), dimethly sulfoxide (99%), H2SO4 (98%), KMnO4 (99%) and H2O2 (30%) were commercially available. All chemicals were used without further treatment, and deionized water was used throughout the course of the experiments. 2.2 Preparation of QA/rGO composites. GO was first prepared via oxidation of graphite powders (3g, >99.8%), according to the modified Hummers methods,41 and an aqueous dispersion of GO with concentration around 1.0 mg/mL was obtained. The aqueous dispersions of GO with desired concentrations were prepared via dilution. The procedures to prepare QA/rGO composite are schematically shown in Scheme 1. Typically, a desired amount of QA solution (e.g., 0.2 mM in DMSO) was drop-added into 200 mL of GO aqueous dispersion and violently stirred for 1h, which was labelled as QA/GO. Subsequently, the QA/GO dispersion was irradiated under a Xe lamp (300 W) with stirring for 2 h, for reducing GO to rGO. The resultant precipitates were filtered with membrane filter (0.25μm) and washed by deionized water, which was denoted as QA/rGO. To remove the residual water, QA/rGO was freeze-dried at -50 °C for 48 h. To get QA nanoparticles, the recrystallization was performed as follows. First, 1g of commercial QA (labelled as CQA) was dissolved in 200mL of DMSO in a round-bottom flask at 80°C. The solution was then naturally cooled to room temperature, resulting in the formation of red precipitate, which was labelled as RQA after being filtered, washed with deionized water, and dried in vacuum. For comparison, TiO2/rGO with 2 wt% rGO and TiO2 particle size around 20 nm was prepared based on the reported procedures.42 2.3 Characterization


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X-ray diffraction (XRD) analysis was performed on a Rigaku D/MAX-2500 diffraction using Ni-ﬁltered Cu Kα radiation (λ = 1.5418 Å), in the range of 3-50° (2θ) with a step width of 0.02°. A JASCO UV-550 spectrophotometer was used to collect UV−Vis spectra of samples. The morphology of the samples was observed on a scanning electron microscope (S-4800, Hitachi, Japan) and on a transmission electron microscope (JEM2011, JEOL, Japan). The CO2 adsorption and desorption isotherms were measured at 273K by Autosorb-1-C Quantachrome analyzer. FT-IR analyses of the samples were performed using a Bruker Optic TENSOR-37. All samples were mixed with KBr and determined in the range of 4000-400 cm-1 wavenumbers. The UV–Vis diffused reflectance spectra (UVDRS) were collected on UV-2600 using BaSO4 as a reference. PL emission spectra were obtained on F-4500 fluorescence spectrophotometer with 290 nm excitation (Hitachi, Japan). Time-resolved transient PL decay curves were obtained using a FLS980 fluorescence lifetime spectrophotometer with 405 nm excitation (Edinburgh Instruments, UK). 2.4 Photocatalytic reduction of CO2. In a typical experiment to photocatalyze CO2 reduction, 10 mg of photocatalyst (e.g, QA/rGO, RQA, or CQA) and an aqueous solution (10 mL) containing 1 mL of TEOA as an electron donor were loaded into a 250 mL photoreactor, and sonicated for 10 min to obtain a suspension. Before illumination, the photoreactor was bubbled with CO2 for 1h to reach saturated dissolution of CO2 and adsorption/desorption equilibrium. The photoreduction of CO2 was performed at 100 kPa and 25°C with a circulating water. Then the reactor was illuminated for desired time with a 300 W Xe lamp (Aulight CEL-HX, Beijing) positioned 5cm above the reactor without any wavelength filters. During the


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reaction process, certain amount of the gaseous mixture was periodically sampled from the reactor at a desired time interval, and analyzed by gas chromatography equipped with a thermal conductivity detector (4890, Agilent Technology) with Argon as the carrier gas. The liquid reaction solution was analyzed by liquid chromatography after reaction. 2.5 Photoelectrochemical characterization A CorrTest electrochemical station was used to detect the electrochemical properties of the QA/rGO composites in a conventional three-electrode cell using a Pt plate as the counter electrode and a saturated Ag/AgCl as the reference electrode. The working electrode was prepared by spreading aqueous slurry of photocatalyst over FTO glass substrate with a coated area at 1cm-2, where the slurry was fabricated by dispersing photocatalyst (25 mg) in 2 ml of ethanol solution of Nafion (1 wt%). An aqueous solution of Na2SO4 (0.2 M) was purged with nitrogen for 1h, which was used as the supporting electrolyte. The working electrode was irradiated from its back.

3. Results and Discussion 3.1 Preparation and characterization of photocatalysts A series of QA/rGO samples with different rGO contents were prepared, labelled as QA/rGO-x (x=1, 2, 5, 20wt%, referring to the content of rGO in the QA/rGO composites). The morphology of the resultant RQA and QA/rGO together with CQA was observed by SEM and TEM. As shown in Figure 1a and 1b, RQA displayed rod-like particles, and high-resolution TEM image showed obvious crystalline structure with an interplanar distance of 1.36 nm, ascribing to (100) lattice


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plane. The SEM observation indicates that the surface of QA/rGO became rough in comparison with that of rGO (Figures 1c and S1a,b), and TEM image showed that QA particles were distributed on the rGO surface (Figure 1d and S1e).

Figure 1. a) TEM of RQA; b) HRTEM of RQA; c) SEM of QA/rGO-2; d) TEM of QA/rGO-2.

To explore their structures and the interaction between the QA particles and rGO, the resultant samples were examined by different techniques. As shown in Figure 2a, CQA, RQA and QA/rGO showed similar XRD patterns with intense diffraction peaks, indicating that QA particles in these samples were in the form of γ-phase.43, 44 In each pattern, a peak appeared at 2θ = 6.5°, suggesting the lamellar structure of QA particles, identical to the TEM observation. From this peak, a lamellar distance of 1.36 nm could be derived via Bragg formula. In contrast, the RQA particles had better crystallinity than the CQA particles, and the particles in QA/rGO-2 had similar structure to RQA.


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No typical diffraction peak of GO (002) at 11.4° or rGO (002) at 24.5° was observed in the pattern of QA/rGO-2, probably due to the small amount of rGO.

Figure 2. a) XRD patterns and b) FT-IR spectra of GO, CQA, RQA and QA/rGO-2; c) Raman spectra of GO and QA/rGO-2; d) CO2 uptake of CQA, RQA and QA/rGO-2. Figure 2b shows FTIR spectra of GO, CQA, RQA and QA/rGO-2 samples. The FTIR spectrum of GO reveals four main absorption peaks at 1223, 1402, 1620 and 1724 cm-1, ascribing to epoxy C–O–C, tertiary C–OH, aromatic C=C and carboxyl C=O, respectively, which indicates that the GO with hydroxyl, epoxy and carboxyl groups was obtained. The presence of these O-containing groups in r-GO may provide interaction sites to form H-bonding with QA molecules. As reported,


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the formation of H-bond between amino H atom and carbonyl O atom (i.e., N–H⋅⋅⋅O=C) would lead the N–H and C=O stretching vibrations to lower wavenumbers.25 From the FTIR spectra of QA/rGO-2, RQA and CQA, it was observed that the stretching bands assigning to free N-H (3500 cm-1 and) and C=O (1700 cm-1) in RQA or CQA shifted to 3273 and 1600 cm-1 in QA/rQA, respectively, indicating the formation of N–H⋅⋅⋅O=C bond between QA molecules and rGO. In the Raman spectra of GO and QA/rGO-2 (Figure 2c), GO exhibited the G-band and D-band at 1597 and 1352 cm-1, respectively, with the D/G intensity ratio (ID/IG) of 0.96. QA/rGO-2 had the similar Raman spectrum with G-band shifting to 1592 cm-1 and D-band to 1319 cm-1, accompanied with an increase in the D/G intensity ratio to ID/IG = 1.21. Raman analysis suggests that the decoration of QA particles on the surface of rGO modified the electronic structure of rGO, probably due to the formation of H-bond between the QA particles and rGO. Figure 2d showed the CO2 sorption isotherms of CQA, RQA and QA/rGO-2. Compared to CQA, RQA showed enhanced CO2 adsorption capacity, indicating the regular structure is favourable to the CO2 capture, probably due to the exposure of more interaction sites to attract CO2. Interestingly, QA/rGO-2 showed significantly increased CO2 uptake, up to 6.47 wt% at 1 bar and 273K, in spite of only small amount of rGO (2 wt%) in this sample. The higher CO2 adsorption capacity of QA/rGO-2 suggests that it had more active sites to attract and activate CO2 molecules, which may facilitate CO2 conversion over QA/rGO-2. 3.2 Photoelectrochemical and photochemical properties To investigate the generation and separation efficiency of the photogenerated electrons of the samples upon light illumination, transient photocurrent-time experiments were carried out, and a photocurrent-time plot was obtained by repeating the light on–off cycles (Figure 3a). It was


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indicated that all samples were sensitive to light irradiation with rapid generation or disappearance of photocurrents as the light source was turned on or off (Figures 3a and S2). Compared to CQA, RQA showed larger photocurrent, suggesting that the regular structure of RQA is favourable to the transfer of the photogenerated carriers. The QA/rGO samples with the rGO contents of 1~5wt% showed enhanced currents compared to RQA, indicating that rGO in the samples accelerated the transfer of photocarrieres (Figure S2a). However, the presence of large amount of rGO in QA/rGO20 decreased its photocurrent. As known, rGO can absorb light, but cannot generate electron-hole pairs. Upon illuminating QA/rGO-20, more light was absorbed by rGO, and only part of light was absorbed by QA particles to generate electron-hole pairs, thus leading to the declined photocurrent. Consequently, an appropriate content of rGO is required to obtain a catalyst with high photocurrent, and 2wt% of rGO was a suitable content in these samples. For the QA/rGO samples, rGO serves as electron acceptor that administers efficient transfer of photoinduced electrons between rGO and QA, thus retarding recombination of the electron-hole pairs. The improved conductivity and suppressed recombination of the charge carries increase amounts of the photogenerated charges, which in turn causes more electrons available to transfer to back contact and contribute to current intensity. Figure 3b shows the EIS Nyquist plots of the resultant samples, which reflect the solid interface delamination resistance and the surface charge transfer resistance. QA/rGO-2 showed the smallest radius of the EIS Nyquist plot, indicating the best performance on separation of the photogenerated charges.


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Figure 3. a) Transient photocurrent, b) Nyquist plots, c) PL spectra, d) time-resolved PL decay.

The photoluminescence (PL) spectra were collected at excitation wavelength of 250 nm, and QA/rGO-2 showed the weakest PL signal among the tested samples (Figure 3c). This indicates that the recombination of photogenerated carries in this sample was significantly reduced. It is reasonable that the photogenerated electrons are promptly transferred to the rGO support, resulting in separation in physical space and suppressing recombination of electrons with holes, which may facilitate to improve its photocatalytic activity. This means that the combination of QA with rGO significantly increased the separation efficiency of photogenerated electrons and holes, which may improve the photocatalytic activity of QA/rGO-2.


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To give insight on lifetime of charge carrier dynamics and validate charge transfer process, timeresolved photoluminescence (TRPL) studies were conducted, and the average lifetime (τave) of each sample was estimated by employing the following equation: 𝑛

𝑛 2 𝜏𝑎𝑣𝑒 = ∑𝑖 = 0𝐴𝑖𝜏𝑖 ∑𝑖 = 0𝐴𝑖𝜏𝑖

The kinetic parameters of the QA/rGO samples are shown in the Table as an inset in Figure 3d. Particularly, QA/rGO-2 possessed the lowest decay lifetime of 4.33 ns, which was very close to Instrument Response Function (IRF) and much lower than that of RQA (16.14 ns) or that of CQA (18.32 ns). This indicates that the interfacial electron injection from QA particles to rGO was very fast, probably due to the inmate contact of QA particles with rGO via the hydrogen bond at the nanoscale interface. The shortened decay life-time indicates a non-radiative pathway and prolonged lifetime of the photogenerated carriers, probably favorable to improving the activity of the catalyst.7 In addition, RQA particles exhibited shorter decay lifetime than CQA, suggesting that regular structure also could improve the photocarriers separation and transport ability. The electronic energy level structures of the resultant samples were investigated by UV–vis diffusive reflectance spectra (DRS) combined with Mott–Schottky (MS) measurements. As illustrated in Figure 4a, the introduction of rGO into QA/rGO-2 resulted in an enhanced absorption compared to that of RQA. The two absorption bands at 565 and 532 nm in the spectrum of RQA correspond to vibronic A0–n peaks (n refers to the number of vibrational quanta coupled to the electronic transition). The introduction of rGO into QA/rGO resulted in the blue shifts of the corresponding bands, consistent with the reported results,46 which further demonstrates the interaction between the QA particles and the rGO support. According to the Kubelka-Munk method,26, 45 the band gap energy was calculated to be 3.47 and 3.33 eV for RQA and QA/rGO-2,


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respectively (Figure 4b). The narrowed band gap of QA in QA/rGO-2 was caused by the interaction between the QA particles and the rGO support. Additionally, the flat band potential (Efb) of the RQA and QA/rGO-2 photocatalyst were investigated by Mott-Schottky plots. As illustrated in Figure 4c, RQA and QA/rGO-2 exhibited features of typical n-type semiconductor due to the apparent positive slopes of the MS curves. The flat-band potentials for RQA and QA/rGO-2 were identified as approximately -0.73 and -0.61 V vs. Ag/AgCl after extrapolation of the curves, respectively. As reported, the conduction bands (CBs) of n-type semiconductors are generally 0.1-0.2 eV deeper than their flat-band potentials.24 In this study, the difference between CB and flat potential was set to be 0.15 eV. Therefore, the CB positions (ECB) of RQA and QA/rGO-2 were calculated to be -0.68 and -0.56 V vs. RHE at pH 7 after conversion using equation: ENHE = EAg/AgCl + 0.20 V,47 which were more negative than the reduction potentials of CO2/CO (0.52 V) and CO2/CH4 (-0.24 V). The valence band (VB) edge potential (EVB) was determined through EVB = Eg + ECB, in tandem with the band gap value from UV-Vis results, which was estimated to be +2.89 V vs. RHE for RQA and +2.65 V vs. RHE for QA/rGO-2. Based on these results, the scheme of band structures of RQA and QA/rGO-2 is summarized in Figure 4d, in which the CB/VB of RQA and QA/rGO-2 were evaluated to be −0.56/+2.89 V and −0.68/+2.65 V, respectively.


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Figure 4. a) UV–vis diffusive reflectance spectra, b) plots of transformed Kubelka-Munk function vs. photon energy, c) Mott–Schottky plots, d) band structures of RQA and QA/rGO-2.

3.3 Photocatalytic Activity and Stability for catalyzing CO2 reduction The photocatalytic activity of the as-prepared samples was evaluated by CO2 photoreduction. Blank experiments were first carried out without either CO2 or photocatalyst or light, and the reaction did not occur in these three cases. Subsequently, the CQA, RQA and QA/rGO samples were examined for catalyzing photoreduction of CO2 using TEOA as a sacrifice reagent under UV light illumination. It was demonstrated that all these samples were effective for catalysing CO2 reduction, and CO and CH4 were obtained accompanied with generation of H2. To further confirm


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the source of the carbon in CO and CH4, isotope-labelling experiment was performed using 13CO2 instead of CO2. As a result, 13CO and 13CH4 were obtained confirmed by GC-MS spectrum with peaks at m/z = 29 and 17 (Figure S3), indicating that CO and CH4 were indeed originated from CO2. Compared to CQA, RQA showed better performance, probably because of its more regular structure and higher surface area, while the presence of small amount of rGO in the QA/rGO samples remarkably improved the activity for catalysing CO2 reduction. As shown in Figure 5a, QA/rGO-2 showed the best performance, affording CO and CH4 production rates of 450 and 275 μmol g-1 h-1, respectively, within the initial 10 h under Xe light illumination, which was much better than that of CQA (CO: 83 μmol g-1 h-1, CH4: 36 μmol g-1 h-1) and RQA (CO: 153 μmol g-1 h-1, CH4: 69 μmol g-1 h-1), respectively. The enhanced performance of QA/rGO can be explained as follows. As compared to that of RQA (24 m2 g-1), the larger specific surface area of QA/rGO-2 (120 m2/g) offers more active sites to attract CO2, which may favor to CO2 conversion. Moreover, rGO can accept photogenerated electrons from the QA particles and effectively decreases their recombination probability with photogenerated holes, forming more reactive species. However, for the sample with large rGO amount (e.g., QA/rGO-20), rGO can absorb more light, and only part of light involves to excite the QA particles to generate electron-hole pairs, thus leading to decrease in the activity of the catalyst. For example, QA/rGO-20 offered CO and CH4 production rates of 154 and 83 μmol g-1 h-1, respectively, much lower than those of QA/rGO-2. The activity of QA/rGO-2 was comparable to that of the reported Pt/C-In2O3 metal photocatalyst (CO: 633 μmol g-1 h-1; CH4: 139.5 μmol g-1 h-1) under similar conditions,48 much better than that of the asprepared TiO2/rGO (CO: 58μmol g-1 h-1; CH4: 21 μmol g-1 h-1).


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The reusability of QA/rGO-2 was examined, and the results are shown in Figure 5b. It is clear that the catalyst can be reused without obvious activity loss. The CO and CH4 products within 10 h reached 47.5 μmol and 25μmol, respectively. The slight decline in C1 (include CO and CH4) production rate could be ascribed to consumption of TEOA in the reaction. It was indicated that QA/rGO-2 could be reused for several times without obvious loss in activity, suggesting the good stability, probably originating from its stable structure.

Figure 5. a) Contrast experiment; b) Stability and recyclability of QA/rGO-2, 10% of TEOA was re-added after the fourth cycle. Based on the above results, possible reaction mechanism of CO2 photoreduction catalyzed by QA/rGO-2 is proposed, as depicted in Figure 6. Upon ultraviolet light irradiation, electrons are excited from VB to CB of the QA particles, which subsequently transfer to the sites that capture CO2, where CO2 is reduced to CO and CH4; meanwhile the holes leaving at VB of the QA particle are easily caught by TEOA, leading to effective spatial charge separation. Notably, the photogenerated electrons are apt to transfer to the rGO nanosheet due to its lower Femi energy level, which is favorable to the reduction of the CO2 adsorbed on the surface of rGO nanosheets.49,


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In general, CO is considered as an intermediate of CH4 during CO2 photoreduction. In this work,

the photoreduction of CO2 occurs at the sites that can attract CO2 on surfaces of QA particles and on the rGO nanosheets. Since the interaction sites such as -NH2, -C=O have different capability to stabilize CO, CO is released to free CO or is further reduced to CH4. Therefore, the selectivity towards CO and CH4 is difficult to control.

Figure 6 Schematic diagram of the photoreduction of CO2 over the QA/rGO composite..

4. Conclusions In summary, QA/rGO nanocomposites were prepared for CO2 photoreduction, in which QA nanoparticles employed as photosensitizer and photocatalyst, and rGO as an electron acceptor to promote charge separation and as a support to provide plentiful active sites for adsorbing CO2. QA/rGO-2 showed the best performance, producing CO and CH4 with rates of 450 and 275 μmol g-1 h-1, respectively, together with good stability. This work presents a metal-free photocatalyst composed of organic semiconductor and rGO, with strong ability to adsorb CO2 and excellent optical property, which may have promising applications in CO2 photoreduction. ASSOCIATED CONTENT


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Supporting Information. Details of the characterization and measurements and Figures S1−S8. The following files are available free of charge. AUTHOR INFORMATION Funding Sources This work was financially supported by the National Natural Science Foundation of China (Grant No. 21773266),

Beijing Municipal Science & Technology Commission (Grant No.

Z181100004218004), and Chinese Academy of Sciences (Grant No. QYZDY-SSW-SLH013). Notes The authors declare no competing financial interest. REFERENCES (1) Lin, J.; Pan, Z.; Wang, X., Photochemical Reduction of CO2 by Graphitic Carbon Nitride Polymers. ACS Sustainable Chem. Eng. 2014, 2, (3), 353-358. (2) Claudio, C.; Ryo, K.; Lingjing, C.; Kazuhiko, M.; Tai-Chu, L.; Osamu, I.; Marc, R., A Carbon Nitride/Fe Quaterpyridine Catalytic System for Photostimulated CO2-to-CO Conversion with Visible Light. J. Am. Chem. Soc. 2018, 140, (24), 7437-7440. (3) Yu, X.; Yang, Z.; Qiu, B.; Guo, S.; Yang, P.; Yu, B.; Zhang, H.; Zhao, Y.; Yang, X.; Han, B.; Liu, Z., Eosin Y-Functionalized Conjugated Organic Polymers for Visible-Light-Driven CO2 Reduction with H2O to CO with High Efficiency. Angew. Chem. Int. Ed. 2019, 58, (2), 632636. (4) Huang, P.; Huang, J.; Pantovich, S. A.; Carl, A. D.; Fenton, T. G.; Caputo, C. A.; Grimm, R. L.; Frenkel, A. I.; Li, G., Selective CO2 Reduction Catalyzed by Single Cobalt Sites on Carbon Nitride under Visible-Light Irradiation. J. Am. Chem. Soc. 2018, 140, (47), 16042-16047. (5) Sadeghi, N.; Sharifnia, S.; Do, T., Enhanced CO2 Photoreduction by a Graphene–Porphyrin Metal–Organic Framework under Visible Light Irradiation. J. Mater. Chem. A 2018, 6, (37), 18031-18035. (6) Nakada, A.; Nakashima, T.; Sekizawa, K.; Maeda, K.; Ishitani, O., Visible-Light-Driven CO2 Reduction on A Hybrid Photocatalyst Consisting of A Ru(II) Binuclear Complex and A AgLoaded TaON in Aqueous Solutions. Chem. Sci. 2016, 7, (7), 4364-4371. (7) Xu, D.; Cheng, B.; Wang, W.; Jiang, C.; Yu, J., Ag2CrO4/g-C3N4/Graphene Oxide Ternary Nanocomposite Z-scheme Photocatalyst with Enhanced CO2 Reduction Activity. Appl. Catal., B 2018, 231, 368-380.


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Photocatalytic Reduction of Carbon Dioxide over Quinacridone

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