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Article

Sustainable Recovery of CO2 by Using Visible-Light-Responsive Crystal Cuprous Oxide/Reduced Graphene Oxide

by
Shou-Heng Liu
1,*,
Jun-Sheng Lu
1 and
Yi-Chiun Chen
1,2
1
Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan
2
Department of Resources Engineering, National Cheng Kung University, Tainan 701, Taiwan
*
Author to whom correspondence should be addressed.
Sustainability 2018, 10(11), 4145; https://doi.org/10.3390/su10114145
Submission received: 5 September 2018 / Revised: 24 October 2018 / Accepted: 31 October 2018 / Published: 11 November 2018
(This article belongs to the Section Energy Sustainability)
Browse Figures
Versions Notes

Abstract

:
A simple solution-chemistry method has been investigated to prepare crystal cuprous oxide (Cu2O) incorporated with reduced graphene oxide (designated as Cu2O-rGO-x, where x represents the contents of rGO = 1%, 5% and 10%) in this work. These Cu2O-rGO-x composites combine the prospective advantages of rhombic dodecahedra Cu2O together with rGO nanosheets which have been studied as visible-light-sensitive catalysts for the photocatalytic production of methanol from CO2. Among the Cu2O-rGO-x photocatalysts, the methanol yield photocatalyzed by Cu2O-rGO-5% can be observed to be 355.26 μmol g−1cat, which is ca. 36 times higher than that of pristine Cu2O nanocrystal in the 20th hour under visible light irradiation. The improved activity may be attributed to the enhanced absorption ability of visible light, the superior separation of electron–hole pairs, well-dispersed Cu2O nanocrystals and the increased photostability of Cu2O, which are evidenced by employing UV-vis diffuse reflection spectroscopy, photoluminescence, scanning electron microscopy/transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. This work demonstrates an easy and cost-effective route to prepare non-noble photocatalysts for efficient CO2 recovery in artificial photosynthesis.

1. Introduction

In the past decades, global warming, which is causing serious global climate change problems and ocean acidification, has been becoming more and more severe with the increase of greenhouse gases (e.g., carbon dioxide) [1]. A global agreement to reduce greenhouse gases has been reached on the COP21 in Paris on December 2015. In order to reduce CO2 emissions, technologies developed for carbon capture, storage and utilization (CCSU) have been widely implemented [2,3]. Among these possible technologies, the conversion of CO2 into useful chemicals or fuels is considered as one of the most promising approaches to deal with energy and environmental issues at the same time [4,5,6]. However, extra energy is required for the transformation of CO2 because it is a stable molecule (ΔG = −400 kJ mol−1) value [7]. Therefore, CO2 conversion via photocatalytic reactions, which are involved in the utilization of solar energy, is an eco-friendly attractive route [8,9,10,11,12]. Previous studies [13,14,15,16] revealed that photocatalysts consisting of metal oxides are shown to be active in the UV-light region. However, it is difficult to practically use the aforementioned photocatalysts due to the limited fraction of the UV spectrum in natural sunlight [17,18,19]. Therefore, developments in visible-light-driven photocatalysts for artificial photosynthesis are crucial. To address this issue, many works have focused on enhancing the visible-light-active photocatalysts by using methods such as doping the semiconductors with non-metal/metal ions [20,21,22,23], engineering semiconductor structures [24], exposing a facet of the semiconductors [21,25,26,27], and semiconductor coupling [28,29]. As a result, these prepared photocatalysts, by using the above-mentioned routes, possess better charge carrier transfer, hence leading to a notable improvement in the photocatalytic performance under visible light.
Cuprous oxides (Cu2O), which are p-type semiconductors, have been widely investigated for visible-light-responsive photocatalysis in artificial photosynthesis owing to their low cost, small band gap (ca. 2.0–2.2 eV) and suitable energy level of conduction and valence bands [15,30,31,32]. It is extensively reported that the Cu2O nanocrystals can highly enhance the photocatalytic oxidation of organic pollutants under visible-light irradiation [33,34,35,36,37]. However, the studies on the photocatalytic reduction of CO2 via crystal Cu2O are limited. In the earlier report [38], the different facets of Cu2O were investigated to perform the photocatalytic reduction of CO2, indicating that the low-index facets of Cu2O possessed higher activity than Cu2O with higher index facets. In addition, Cu2O with a stone-like shape exhibited superior activity to that with nanobelt arrays in terms of CO2 photoreduction [39]. Nevertheless, the crystal Cu2O applied in the photocatalytic reactions suffers from the separation of photogenerated electron–hole pairs [31] and long-term stability [40]. Recently, the enhanced durability and activity [41,42,43,44,45,46] can be achieved by the Cu2O-based photocatalysts. Among them, the incorporation of graphene with the 2D sp2-hybridized carbons has received much attention due to its high electron mobility [47], theoretical surface area [48], and chemical stability [49]. For instance, a microwave-assisted chemical method was proposed to prepare Cu2O/reduced graphene oxide (rGO) nanocomposites which have improved activity and stability toward the photocatalytic reduction of CO2 [50]. Spherical Cu2O nanoparticles were incorporated with rGO by using in-situ reduction method, and their photocatalytic activities toward CO2 reduction were better than that of pristine Cu2O nanoparticles [51]. Although the synthesis of Cu2O–graphene composites with visible-light-active CO2 reduction was proposed previously, the crystal structure of Cu2O decorated with rGO in the photoreduction of CO2 was reported for the first time.
In the present research, the nanocrystal Cu2O-possessing facets decorated with various amounts of rGO nanosheets (named as Cu2O-rGO-x, x = amounts of rGO) were synthesized by using a simple solution-chemical method. The resultant Cu2O-rGO-x photocatalysts were thoroughly characterized via a variety of spectroscopic analyses. Also, the tests regarding the photoreduction of CO2 under visible light show that rhombic dodecahedral Cu2O decorated with 5 wt.% of rGO exhibited superior photocatalytic activity to the other Cu2O-rGO-x and pristine Cu2O. The reaction mechanism in the enhancement of CO2 photoreduction by Cu2O-rGO-5% composites is studied and discussed later.

2. Materials and Methods

2.1. Preparation of Crystal Cu2O Decorated with rGO

Graphene oxide (GO) was fabricated by using the modified Hummer’s method based on the earlier work [52]. For the synthesis of crystal Cu2O decorated with rGO, fixed amounts of 0.1 M CuCl2 and sodium dodecyl sulfate (SDS, Sigma, St. Louis, MO, USA, 99%) were added into various weight ratios of GO solution (1–10%) under stirring in a water bath at 34 °C. After 20 min, 1.8 mL of 1 M NaOH solution was added into the mixed solution, and then it was stirred for 20 s. Twenty-four milliliters of 0.1 M NH2OH·HCl (Alfa Aesar, Haverhill, MA, USA, 99%) solution was slowly poured into the mixture. The mixture was stirred for 20 s and kept in the water bath for an hour. Finally, the product was washed and then separated by a centrifuge several times. After that, these precipitates were dried in vacuum at 30 °C for 8 h. The resultant products were symbolized as Cu2O-rGO-x (x represents the amounts of rGO = 1%, 5% and 10%).

2.2. Characterizations

X-ray diffraction (XRD) measurements were carried out by a PANalytical X’Pert PRO diffractometer (Cu Kα, λ = 1.541 Å) at a scan of 3° min−1 from 5 to 80°. The morphologies of samples were identified by scanning electron microscopy (SEM) and transmission electron microscopy on a field-emission scanning electron microscope (JSM-7000F, JEOL, Akishima, Tokyo, Japan) and transmission electron microscope (JEM-1400, JEOL, Akishima, Tokyo, Japan) with respective accelerating voltages of 20 and 200 kV. The rate of the electron–hole recombination of photocatalysts can be determined by photo-luminescence (HR800, HORIBA, Kyoto, Japan) from 450 to 650 nm. X-ray photoelectron spectra (XPS) were recorded via a Kratos AXIS Ultra DLD spectrometer system with an excitation source of Al Kα (1486 eV). The peak of adventitious carbon (284.6 eV) was referenced to calibrate the binding energies of the photoelectrons. The electrons could be excited from the valance band to the conduction band of the sample by excitation light with 325 nm. The spectra of diffuse reflectance ultraviolet/visible (UV/Vis) light were taken on a spectrophotometer (Cary 100, Varian, Palo Alto, CA, USA).

2.3. Photoreduction of CO2 Experiments

The photocatalytic activity measurements were carried out in a batch reactor (200 mL) equipped with a water-circulation cooling system. About 25 mg of samples and 50 mL of deionized water (DI) water were added into the aforementioned reactor, followed by dispersing homogeneously in water by sonication. Then, pure CO2 gas (99.995%) was introduced into the reactor for 45 min to keep the solution saturated and ensure adsorption–desorption equilibrium in the dark. After the reactor was sealed, the photocatalytic reaction was performed by using a top-irradiated 300W Xe lamp (Model 66142, Newport, Irvine, CA, USA) with a cut-off filter to remove UV illumination. During the reaction process, the gas (1 mL) and liquid (1 μL) products were withdrawn periodically and examined by gas chromatography (GC, GC-430, Bruker, Billerica, MA, USA) with a thermal conductivity detector (TCD, equipped with molecular sieve 5A column) and a barrier discharge ionization detector (BID, GC-2010 Plus equipped with DB-WAXETR column, Shimadzu, Kyoto, Japan), respectively.

3. Results and Discussion

The X-ray diffraction patterns of GO, crystal Cu2O and Cu2O-rGO-x composites (x represents the weight percentage of rGO to Cu2O) are shown in Figure 1. The results show that all the diffraction features of crystal Cu2O-rGO-x composites are located at 2θ of 29.6°, 36.4°, 42.3°, 61.4°, 73.6° and 77.5° which can be assigned to (110), (111), (200), (220), (311), and (222), respectively. This indicates the presence of a crystal phase of Cu2O (JCPDS #78-2076) in the Cu2O-rGO-x composites. No characteristic peak of GO at 2θ = 10.6° can be observed for all samples, suggesting that the GO may be reduced to rGO. No characteristic peaks of rGO can be found for all Cu2O-rGO-x samples in the X-ray diffraction patterns, which may be due to the lower contents of rGO and higher intensity of crystal Cu2O. As reported previously [53], the crystal structure of Cu2O is a pure rhombic dodecahedron when the intensity ratio of the (220) facet to the (200) facet (I220/I200) is 0.79. The values of I220/I200 of Cu2O and Cu2O-rGO-x (x = 1%, 5% and 10%) are calculated to be 0.76, 0.72, 0.58, and 0.56, respectively. It can be found that the values of I220/I200 are decreased when the contents of rGO in the Cu2O-rGO-x composites are increased. The I220/I200 of Cu2O-rGO-10% is decreased to 0.56 while the weight ratio of rGO is increased to 10%. There are two possible reasons which could explain why the crystal structure of rhombic dodecahedral Cu2O cannot be completely formed. First, the pH of the solution was changed from neutral to acidic by adding GO, which may hinder the formation of a crystal structure of Cu2O. Second, GO would react with the reductant to form rGO during the synthesis process. This would lead to insufficient amounts of the reductant for the formation of a crystal structure of rhombic dodecahedral Cu2O [53].
The high-resolution scanning electron microscope (SEM) was used to investigate the morphology of Cu2O-rGO-x composites. The SEM clearly presents the morphology of rGO with numerous wrinkles on the surface, as shown in Figure 2. In addition, the rhombic dodecahedral structure of Cu2O nanocrystals can be observed. The SEM images of Cu2O-rGO-1% demonstrates that the morphologies of Cu2O nanocrystals remain as rhombic dodecahedra and the rGO sheet was successfully incorporated onto Cu2O nanocrystals. However, the aggregation of Cu2O nanocrystals in the Cu2O-rGO-1% composites can be observed since the amounts of rGO in the composites is limited. Upon incorporating higher amounts of rGO into the composites, the Cu2O crystals in the Cu2O-rGO-5% and Cu2O-rGO-10% are found to have a better dispersion on rGO sheets. Nevertheless, the destruction of rhombic dodecahedral Cu2O nanocrystals can be observed for Cu2O-rGO-x (x = 5% and 10%). In particular, the crystallization structure of Cu2O-rGO-10% composites could hardly be seen, as shown in Figure 2. This is in good accordance with the aforementioned XRD results.
The transmission electron microscopy (TEM) image of rGO shows a thin sheet with wrinkles, similar to paper, as displayed in Figure 2. The deeper-colored region can be observed in the TEM image of rGO, which may be caused by the stack of rGO sheets. The wrinkles on the rGO sheets could result in a higher surface area of rGO, which may enhance the capacity of CO2 adsorption. Therefore, it is helpful that rGO was decorated onto the surface of Cu2O nanocrystals. The morphological and structural features of Cu2O and Cu2O-rGO-x composites (x = 1%, 5% and 10%) were also studied by TEM and their corresponding images are shown in Figure 2. It can be seen that the morphological features of rhombic dodecahedral Cu2O are similar to a honey-comb shape with a particle size of 400–500 nm. As can be seen from the inset of Figure 2, the corresponding selected-area electron diffraction (SAED) pattern of rhombic dodecahedra can be observed along the [100] direction. The TEM images also show that Cu2O nanocrystals are decorated with thin rGO sheets with some wrinkles. This result confirms that rGO is successfully decorated on rhombic dodecahedral Cu2O.
The elemental compositions and the chemical surface states of GO, rGO, crystal Cu2O and Cu2O-rGO-x composites were examined by XPS. In Figure 3a, the survey scans of the samples suggest the existence of carbon, oxygen and copper. The high-resolution C1s XPS spectra of GO (see Figure 3b) indicate three deconvoluted peaks at 284.6, 286.7, and 289.1 eV, which are ascribed to C–C/C=C, C–O, and C=O/COOH bonds, respectively. The intensities of C–O bindings found for rGO and Cu2O-rGO-x samples are decreased, indicating that GO can be effectively reduced to rGO in the Cu2O-rGO-x. As can be seen from Figure 3c, the Cu2p XPS spectra of Cu2O-rGO-x composites display two major peaks at ca. 932.6 and 952.4 eV corresponding to Cu(I) 2p3/2 and Cu(I) 2p1/2, respectively. Moreover, the binding energies of 934.8 and 954.3 eV are attributed to the formation of Cu(II), indicating the presence of Cu(II) on the surface of Cu2O and Cu2O-rGO-x photocatalysts. However, the intensities of aforementioned peaks (i.e., Cu(II)) are obviously reduced for Cu2O-rGO-5% and Cu2O-rGO-10% composites, implying the prevention of surface oxidation of Cu2O. It can be attributed to the enhancement of chemical stability caused by improving the electronic structure of Cu2O nanocrystals in the presence of carbon vacancies and dangling bonds in the rGO [54]. Figure 3d shows high-resolution of O1s XPS spectra of Cu2O and Cu2O-rGO-x. The features at 531.0 and 531.8 eV can be attributed to oxygen vacancy and adsorbed oxygen, respectively [44]. It can be seen that the intensities of two components are increased upon the increased decoration of rGO. As reported earlier [55], the binding of oxygen vacancies in the nanocrystalline Cu2O with bicarbonate ions may be enhanced, which could promote the photoreduction of CO2 discussed below.
The optical absorption of Cu2O and Cu2O-rGO-x composites was investigated by diffuse reflectance ultraviolet/visible (UV/Vis) spectra. The UV/Vis spectra of Cu2O and Cu2O-rGO-x show a wide absorption profile from 300 to 600 nm, as displayed in Figure 4a. In addition, the absorption intensities of Cu2O-rGO-x composites are relatively enhanced in the absorption wavelength >600 nm regions if compared with those of Cu2O. The increased absorption intensity of Cu2O-rGO-x composites depends on the amounts of rGO in the composites; i.e., the intensity is increased with increasing amounts of rGO. Therefore, the rGO could cause a red shift to promote the absorption ability in the visible region of Cu2O. Figure 4b shows that the band gap of Cu2O is reduced as Cu2O combined with rGO. Their corresponding threshold wavelength can be obtained and is shown in Table 1. The band gap of the pure Cu2O nanocrystal is assessed to be 2.18 and those of Cu2O-rGO-x composites are decreased to be 2.16–2.06 with increasing contents of rGO in the Cu2O-rGO-x composites. A red-shift absorbance and the narrower bad gap of composites means that it can easily absorb the visible light. Therefore, the incorporation of rGO onto the Cu2O is good for the absorption of visible light during photocatalytic reduction.
The rates of the electron–hole recombination can be determined by photo-luminescence (PL) with the exited wavelength at 325 nm. The peak position is related to the band gap value of photocatalysts and the intensity of the peak represents the rates of the electron–hole recombination. A higher intensity of peaks indicates a greater rate of electron–hole recombination. On the other hand, a lower intensity of peaks means an inferior rate of electron–hole recombination. As can be seen in Figure 5, the Cu2O-rGO-10% composites have the lowest intensity of PL peaks, which suggests that they have the lowest recombination rates. Accordingly, the rate of electron–hole recombination can be reduced by decorating rGO on Cu2O, and the photo-exited electrons can be effectively transferred from Cu2O to rGO to separate electrons and holes [56,57,58,59].
The photoconversion of CO2 into methanol photocatalyzed by Cu2O and Cu2O-rGO-x composites was examined under visible light for 20 h. In this study, the possible products, such as HCHO and HCOOH, which were confirmed by injecting the standard solution of respective compounds, were not detected. Only methanol was observed as the main product. In order to check the formation of methanol during photocatalytic reaction, we performed three blank experiments. It can be seen that no methanol can be obtained without the assistance of photocatalysts, light irradiation and the existence of CO2 (see Table 2). Moreover, Figure 6 shows that the contents of rGO have a great effect on the photocatalytic activity of Cu2O nanocrystals. The methanol yields varied with the contents of rGO follow the order Cu2O-rGO-5% > Cu2O-rGO-1% > Cu2O-rGO-10% > Cu2O. The yields of methanol are 9.76, 47.91, 335.26 and 12.41 μmol g−1cat, corresponding to Cu2O, Cu2O-rGO-1%, Cu2O-rGO-5% and Cu2O-rGO-10%, respectively after 20 h of visible-light irradiation. Among these prepared photocatalysts, the Cu2O-rGO-5% composites possess the best activity for the photoreduction of CO2. It is concluded that the suitable amounts of rGO which are incorporated onto Cu2O would have a positive impact on the photocatalytic performance of CO2 reduction. According to the UV-Vis spectrum, the adsorption intensity of Cu2O in the visible regions is promoted and the energy gap of Cu2O is reduced by combining with rGO, indicating that Cu2O-rGO-x can easily absorb visible light to generate electron–hole pairs which can be utilized to reduce CO2. The PL patterns indicate that the peak intensities are declined with increasing ratios of rGO to Cu2O, suggesting that the rGO can effectively prevent the electron–hole pairs from recombination. The SEM/TEM images show that the aggregation of Cu2O nanocrystals can be suppressed by decorating with rGO, and thus the activity sites are also increased. XPS spectra show that rGO nanosheets served as the protective layers to prevent the photoreduction and oxidation of Cu2O by photogenerated electrons and water, respectively, which enhance the stability of Cu2O during the photocatalytic reaction. Consequently, the methanol yield of Cu2O-rGO-5% composites (355.26 μmol g−1cat) is ca. 36 times higher than that of pure Cu2O nanocrystal (9.76 μmol g−1cat). Additionally, the apparent quantum efficiency (AQE) of Cu2O-rGO-5% under visible-light illumination was estimated to be 0.724%. However, the photocatalytic activity is observed to be decreased when the ratio (rGO/Cu2O) is further increased to 10%. This result may be due to the fact that excessive rGO would deteriorate the crystal facet of Cu2O as well as obstructing light from reaching the surface of Cu2O nanocrystals which generated fewer electron–hole pairs, thereby decreasing the reduction of CO2 [52].
Combining the results mentioned above, a possible mechanism of photocatalytic CO2 reduction by Cu2O-rGO-x composites is illustrated in Figure 7. The Cu2O-rGO-x composites possess band gaps of ca. 2.1 eV with more negative reduction potential than that of methanol formation (E = −0.38 V vs. NHE at pH = 7) and slightly more positive oxidation potential than that of oxygen evolution (E = 0.81 V vs. NHE at pH = 7) [24]. The photogenerated electrons are transferred to highly conductive rGO to react with CO2 or O2 which have two lone pairs of electrons for the formation of π-electrons bindings on the rGO surface. As such, the recombination of electron–hole pairs would be hindered significantly, which is important for the electron-dominated reduction reaction [50]. More importantly, the removal of electrons from the conduction bands of Cu2O via rGO can prevent the photoreduction of Cu2O and thus enhance the photostability [60]. In addition, the protection of rGO from the oxidation of Cu2O by water may maintain the copper in the Cu(I) state [61]. In Figure S1, the XRD pattern confirms that the crystal phase of Cu2O is intact after 20 h of photocatalytic reaction. It is noteworthy that the dominant product of methanol may be caused by the exposed (110) facets of Cu2O on which hydrogen may perform the addition to the oxygen atom in the bicarbonate rather than a carbon atom. This finding is similar to the pervious study [62]. In terms of photogenerated holes, the evolution of oxygen gas (ca. 4.9 μmol g−1cat after 20 h of visible-light irradiation) implies that the occurrence of water splitting would serve as the hole scavenger, which hinders the recombination of electron–hole pairs. However, the lower amounts of evoluted oxygen after 20 h of photocatalytic reaction may be ascribed to the possibility of carbon corrosion and oxygen reduction.

4. Conclusions

In summary, crystal Cu2O photocatalysts incorporated with various amounts of rGO (1–10%) are prepared via a simple solution-chemistry route. Among the prepared Cu2O-rGO-x composites, we found that the Cu2O-rGO-5% (5 wt.% of rGO) photocatalysts possess the superior photocatalytic performance toward CO2 reduction; i.e., after visible light illumination for 20 h, the methanol yield of Cu2O-rGO-5% composites can reach 355.26 μmol g−1cat, which is about 36 times that of pure Cu2O photocatalysts. This remarkable enhancement in the CO2 photoreduction activity may be due to the increased intensity of visible-light absorption in the Cu2O (identified by UV-VIS), reduced recombination of electron–hole pairs (proven by PL), enhanced dispersion of crystal Cu2O (examined by SEM/TEM) and stabilization of Cu2O (evidenced by XPS). Accordingly, the visible-light-driven photocatalytic reduction of CO2 by Cu2O-rGO-x composites could provide a possible opportunity for CO2 conversion by using sunlight energy.

Supplementary Materials

The following are available online at https://www.mdpi.com/2071-1050/10/11/4145/s1, Figure S1: XRD patterns of the fresh and used photocatalysts after 20 h of photocatalytic reaction.

Author Contributions

Designing, writing, review & editing the paper, S.-H.L.; doing the experiments and analyzing the data, J.-S.L. All authors reviewed and approved the manuscript.

Acknowledgments

We are gratefully appreciated the support of this study by the Ministry of Science and Technology of Taiwan (grant number: MOST 105-2221-E-006-015-MY5).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Powdered XRD patterns of graphene oxide (GO), Cu2O and various Cu2O-rGO-x photocatalysts.
Figure 1. Powdered XRD patterns of graphene oxide (GO), Cu2O and various Cu2O-rGO-x photocatalysts.
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Figure 2. SEM and TEM images of rGO, Cu2O and Cu2O-rGO-x (x = 1%, 5% and 10%).
Figure 2. SEM and TEM images of rGO, Cu2O and Cu2O-rGO-x (x = 1%, 5% and 10%).
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Figure 3. XPS spectra of various photocatalysts: (a) survey, (b) C 1s (c) Cu 2p and (d) O 1s regions.
Figure 3. XPS spectra of various photocatalysts: (a) survey, (b) C 1s (c) Cu 2p and (d) O 1s regions.
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Figure 4. The photo-luminescence (PL) pattern of Cu2O and Cu2O-rGO-x (x = 1%, 5% and 10%).
Figure 4. The photo-luminescence (PL) pattern of Cu2O and Cu2O-rGO-x (x = 1%, 5% and 10%).
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Figure 5. (a) UV-VIS spectra and (b) their corresponding Kubelka–Munk plots of Cu2O and various Cu2O-rGO-x (x = 1%, 5% and 10%) photocatalysts.
Figure 5. (a) UV-VIS spectra and (b) their corresponding Kubelka–Munk plots of Cu2O and various Cu2O-rGO-x (x = 1%, 5% and 10%) photocatalysts.
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Figure 6. Photocatalytic methanol formation by Cu2O and Cu2O-rGO-x (x = 1%, 5% and 10%) under visible light irradiation.
Figure 6. Photocatalytic methanol formation by Cu2O and Cu2O-rGO-x (x = 1%, 5% and 10%) under visible light irradiation.
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Figure 7. A possible mechanism of CO2 photoreduction catalyzed by Cu2O-rGO-x under visible light irradiation.
Figure 7. A possible mechanism of CO2 photoreduction catalyzed by Cu2O-rGO-x under visible light irradiation.
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Table
Table 1. Energy gaps and threshold wavelength of Cu2O and Cu2O-rGO-x (x = 1%, 5% and 10%).
Table 1. Energy gaps and threshold wavelength of Cu2O and Cu2O-rGO-x (x = 1%, 5% and 10%).
SampleEnergy Gap (eV)Threshold Wavelength (nm)
Cu2O2.18570.11
Cu2O-rGO-1%2.16573.01
Cu2O-rGO-5%2.16574.87
Cu2O-rGO-10%2.06603.11
Table
Table 2. Conditions for the photocatalytic reduction of CO2.
Table 2. Conditions for the photocatalytic reduction of CO2.
SampleReaction PrecursorVisible Light Illumination
(λ > 400 nm)		Methanol Yield
(μmol g−1-cat)
NoneCO2ONone
Cu2OCO2O9.76
Cu2O-rGO-1%CO2O47.91
Cu2O-rGO-5%CO2O355.26
Cu2O-rGO-5%CO2XNone
Cu2O-rGO-5%N2ONone
Cu2O-rGO-10%CO2O12.41
All experiments were carried out at 27 °C with 0.5 mg-cat/mL-solvent.

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MDPI and ACS Style

Liu, S.-H.; Lu, J.-S.; Chen, Y.-C. Sustainable Recovery of CO2 by Using Visible-Light-Responsive Crystal Cuprous Oxide/Reduced Graphene Oxide. Sustainability 2018, 10, 4145. https://doi.org/10.3390/su10114145

AMA Style

Liu S-H, Lu J-S, Chen Y-C. Sustainable Recovery of CO2 by Using Visible-Light-Responsive Crystal Cuprous Oxide/Reduced Graphene Oxide. Sustainability. 2018; 10(11):4145. https://doi.org/10.3390/su10114145

Chicago/Turabian Style

Liu, Shou-Heng, Jun-Sheng Lu, and Yi-Chiun Chen. 2018. "Sustainable Recovery of CO2 by Using Visible-Light-Responsive Crystal Cuprous Oxide/Reduced Graphene Oxide" Sustainability 10, no. 11: 4145. https://doi.org/10.3390/su10114145

APA Style

Liu, S. -H., Lu, J. -S., & Chen, Y. -C. (2018). Sustainable Recovery of CO2 by Using Visible-Light-Responsive Crystal Cuprous Oxide/Reduced Graphene Oxide. Sustainability, 10(11), 4145. https://doi.org/10.3390/su10114145

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