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Photocatalytic reduction of CO2 to CO over copper decorated g-C3N4 nanosheets with enhanced yield and selectivity
Applied Surface Science 427 (2018) 1165–1173
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Full Length Article
Photocatalytic reduction of CO2 to CO over copper decorated g-C3 N4 nanosheets with enhanced yield and selectivity Guodong Shi, Lin Yang, Zhuowen Liu, Xiao Chen, Jianqing Zhou, Ying Yu ∗ Institute of Nanoscience and Nanotechnology, College of Physical Science and Technology, Central China Normal University, Wuhan 430079, China
a r t i c l e
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Article history: Received 10 July 2017 Received in revised form 18 August 2017 Accepted 23 August 2017 Available online 26 August 2017 Keywords: Cu decoration g-C3 N4 Photocatalysis CO2 reduction
a b s t r a c t Photocatalytic reduction of CO2 to fuel has attracted considerable attention due to the consumption of fossil fuels and serious environmental problems. Although there are many photocatalysts reported for CO2 reduction, the improvement of activity and selectivity is still in great need of. In this work, a series of Cu nanoparticle decorated g-C3 N4 nanosheets with different Cu loadings were fabricated by a facile secondary calcination and subsequent microwave hydrothermal method. The designed catalysts shown good photocatalytic activity and selectivity for CO2 reduction to CO. The optimal sample exhibited a 3-fold augmentation of the CO yield in comparison with pristine g-C3 N4 under visible light. It is revealed that with the loading of Cu nanoparticles, the resulting photocatalyst possessed an improved charge carrier transfer and separation efficiency as well as increased surface reactive sites, resulting in a significant enhancement of CO yield. It is anticipated that the designed Cu/C3 N4 photocatalyst may provide new insights for two dimensional layer materials and non-noble particles applied to CO2 reduction. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the development of society, the continuous consumption of fossil energy and environmental pollution is two major challenges for human beings. Since the discovery of the photocatalytic reduction of carbon dioxide (CO2 ) into organic fuel of semiconductors by Inoue in 1979, artificial photocatalytic reduction of CO2 to fuel has been proven to be a fantastic method to simultaneously solve the above two problems [1–5]. As one of the widely studied photocatalysts, titanium dioxide (TiO2 ) had been intensely used because of its environmentally friendly, low cost, superior stability. It is a pity that the ineffective visible light utilization caused by its wide bandgap and high recombination rate of light-induced electron-hole pairs restricts its further application [6–9]. So a lot of work has been done to prepare all kinds of other photocatalysts such as metal oxide, metal sulfides, organic compounds and graphitic compounds. However, those semiconductor photocatalysts also suffer from the many problems of low stability, photocorrosion and high cost [10–15]. As such, it is necessary to explore some new catalysts with splendid performance for photocatalytic reduction of CO2 .
∗ Corresponding author. E-mail address: [email protected] (Y. Yu). http://dx.doi.org/10.1016/j.apsusc.2017.08.148 0169-4332/© 2017 Elsevier B.V. All rights reserved.
For the past few years, two-dimensional layered materials, such as graphene, molybdenum disulfide and graphitic carbon nitride (g-C3 N4 ) exhibited excellent performance in photocatalytic and electrocatalytic reaction [16–20]. In particular, g-C3 N4 has attracted a great deal of consideration owing to its good photocatalytic performance for pollutant degradation, hydrogen production and CO2 reduction [21–23]. For example, Zhu et al. reported the C3 N4 -agar composite shows enhanced performance in photocatalytic degradation of phenol and MB under visible light [24]; Wang’s group demonstrated that carbon nitride as a commonly available photocatalyst is able to generate H2 from water [25]. But nonetheless, the practical application of g-C3 N4 in photocatalysis also faces challenges due to its inherent disadvantages, including low specific surface area, a small amount of surface active sites and high recombination rate of photo-generated carriers [26–29]. So, in order to overcome those deficiency, many strategies have been taken to design and modify g-C3 N4 , including fabrication of C3 N4 quantum dot and single or few layer nanosheet [30,31], coupling with other semiconductors to form heterojunction structure [32,33], non-metal doping [34–36], metal doping [37], surface functionalization [38] and metal deposition [39,40]. For example, Yu’s group combined g-C3 N4 with Ag2 WO4 to form direct Z-scheme photocatalyst, which displayed a better performance toward the degradation of methyl orange [41]. Zou et al. fabricated borondoped g-C3 N4 with enhanced efficiency for rhodamine degradation
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Scheme 1. Schematic illustration for the synthetic process of Cu/C3 N4 .
[42]. Zhang’s group reported g-C3 N4 with tunable band structure for efficient visible light driven hydrogen evolution [43]. Moreover, the enhanced activity of photocatalytic CO2 reduction, especially CH4 and CH3 OH formation over functionalized g-C3 N4 ultrathin nanosheets was demonstrated by Yu’s group [44]. Among the above strategies for the activity improvement of semiconductors, non-precious metal deposition is considered to an efficient and facile method to significantly improve the photocatalytic activity since the deposited metals can act as “charge-carrier traps” and suppress the recombination process of electrons-holes. As a typical non-precious metal, Cu has been widely investigated because of its low cost and good catalytic effect. For instance, Yu et al. reported Cu-decorated microsized nanoporous TiO2 photocatalyst was fabricated for CO2 reduction, resulting in a 21-fold augmentation of the CH4 yield in comparsion with commercial TiO2 [45]. Zhao et al. used metallic Cu decorated Co3 O4 nanotubes as photoelectrocatalyst, leading to an outstanding photoelectrochemical response toward CO2 reduction [46]. Graphene oxide decorated with Cu nanoparticles was demonstrated by Chen et al. [16] with 60 times higher CO2 reduction rate than pristine GO. Moreover, Cu nanoparticles interspersed MoS2 with enhanced efficiency for CO2 electrochemical reduction was recently reported by our group [18]. It is believed that the deposition of Cu can effectively suppress the unwelcome photoinduced electro-hole recombination, increase active sites and enhance electrical conductivity, all of which may improve catalytic activity. Provoked by the aforesaid study, in this work, few layer g-C3 N4 nanosheets modified by Cu nanoparticles were successfully synthesized for efficient CO2 photocatalytic reduction. Although Cu/C3 N4 composites have been reported earlier [39,47,48], to the best of our knowledge, so far there has been no publication about the combination of Cu nanoparticles and ultrathin g-C3 N4 as catalyst for photocatalytic CO2 reduction. The prepared Cu/C3 N4 catalytic composite was optimized by varying the amount of loading Cu nanoparticles. The result demonstrates the yield of CO2 reduction of g-C3 N4 was significantly enhanced by the deposition of Cu nanoparticles. Moreover, the obtained Cu/C3 N4 catalytic exhibited excellent selectivity for carbon monoxide (CO) formation. Therefore, we believe that this kind of Cu/C3 N4 catalytic may shed light on the design and preparation of novel two-dimensional layer materials for efficient CO2 photochemical reduction.
2. Experiment section 2.1. Materials synthesis All of the chemicals used in this study were purchased from Shanghai Guoyao Chemicals Ltd. Co. The chemicals were analytical reagents and used without further purification. Deionized water was used for the preparation of aqueous solutions. In this study, the synthetic process consisted of two steps as shown in Scheme 1. In the first step, g-C3 N4 nanosheet were formed through secondary calcination method. In brief, 10 g dicyandiamide was added to a crucible and then calcined in a muffle furnace at 550 ◦ C for 4 h with a ramp rate of 2.5 ◦ C/min. After heat treatment, yellow powder was collected and named as bulk g-C3 N4 . Then, bulk g-C3 N4 was ground into fine powder and 0.1 g fine powder was subsequently placed into a ceramic boat, which was secondary calcining at 510 ◦ C for 2 h with a ramp rate of 5 ◦ C/min. Finally, g-C3 N4 with few layers was obtained. In the second step, Cu nanoparticle modified ultrathin g-C3 N4 nanosheets were synthesized via a simple microwave hydrothermal method. Detailed information is that 20 mg g-C3 N4 powder with few layers was suspended into 20 mL ethylene glycol and then had been ultrasonicated for 1 h. Then, different amount of copper (II) nitrate trihydrate (Cu(NO3 )2 ·3H2 O) was added to the suspension, and the solution was stirred. The weight ratios of Cu(NO3 )2 . 3H2 O to g-C3 N4 were 3%, 6%, 9% and the corresponding fabricated three Cu modified g-C3 N4 samples were denoted as Cu/C3 N4 -3, Cu/C3 N4 -6, Cu/C3 N4 -9, respectively. After ultrasonication for 60 min, the homogeneous mixture was placed into a microwave oven and had been kept for the duration of 180 s at 350 W power. Black precipitates obtained by post microwave treatment were cooled down to room temperature, and then were separated by centrifugation and washed with ethanol for several times to remove the ethylene glycol residue. The final product was dried overnight in a vacuum oven at 50 ◦ C. 2.2. Materials characterization The morphology and structure of the prepared Cu/C3N4 samples were analyzed by a field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) at 10 kV acceleration voltage and a highresolution transmission electron microscopy (HRTEM, Titan G2
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Fig. 1. XRD pattern comparison for g-C3 N4 , Cu/C3 N4 -3, Cu/C3 N4 -6 and Cu/C3 N4 -9 (a) and the corresponding enlarged parts in the range of 30–50◦ (b).
60-300 Probe Cs Corrector) at 200 kV acceleration voltage. The phase composition of the prepared Cu/C3N4 samples were characterized by X-ray diffractometer (XRD, X’PertPRO MRD, PANalytical, Netherlands, Cu K␣ radiation). The surface chemical component analysis of the g-C3N4 and Cu/C3N4 samples was confirmed by Xray photoelectron spectroscopy (XPS) (VG Multiab-2000) equipped with a monochromatic Al K␣ source. Brunauer-Emmett-Teller (BET) specific surface area of the prepared samples was measured with Micromeritics ASAP2020 nitrogen adsorption apparatus (USA). UV–vis diffuse reflectance spectra (UV–vis DRS) were chronicled with PerkinElmer Lambda 35 spectrophotometer in the range of 400–800 nm using BaSO4 as a reference. Fourier transform infrared spectra (FTIR) were measured on an IR Affinity-1 FTIR spectrometer (Shimadzu, Japan). 2.3. Electrochemical measurements Electrochemical impedance spectroscopy (EIS) were measured with an electrochemical working station (CHI660E) within the range of 10 mHz–100 kHz in a standard three electrode system. The prepared samples were used as working electrodes, a platinum plate and standard Ag/AgCl electrode (3 M KCl) used as the counter and reference respectively. A 0.02 M Na2 SO4 aqueous solution was used as the electrolyte. The photocurrent of the samples was performed under a chopped irradiation from a 350 W xenon lamp (Lap Pu, XQ). The working electrode was fabricated as follow. 20 mg of prepared samples was dispersed in 10 mL mixed solution with ethanol and nafion by ultrasonication and then the slurry was spread onto the surface of a cleaning ITO glass, followed by drying in vacuum oven. 2.4. Photocatalytic performance tests The photocatalytic CO2 reduction experiments were carried out in a homemade sealed cell (250 mL) [10]. The amount of prepared sample was 20 mg in each test and electrolyte solution was 0.1 M KHCO3 with the volume of 170 mL. The whole photocatalytic CO2 reduction test was performed without additional power. Then, high purity CO2 (99.999%) gas was flowed and bubbled through the reactor for 30 min until the CO2 concentration reached saturation and the dissolved oxygen was removed completely. A 350 W xenon lamp was used as the light source and all of the experiments were performed at room temperature and ambient pressure. The gas products from CO2 photocatalytic reduction were analyzed by using gas chromatograph instruments (GC-2014, SHIMADZU) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD) for organic and inorganic compound detection respectively. A packed column (PROPAK-Q, 2 m × 3 mm)
and a different packed column (C36880-14, Restek, 2 m × 1 mm) were used for GC-2014 with FID and TCD, respectively. Nitrogen (N2 ) was used as carrier gas. The gas products were collected by a 1 m syringe and then injected into the gas chromatograph with an interval of 60 min. Control experiments were performed under identical conditions but in the absence of CO2 (under an N2 atmosphere) or light irradiation.
3. Results and discussion 3.1. Characterization of the catalysts The XRD patterns of g-C3 N4 and Cu/C3 N4 composites are shown in Fig. 1. It can be clearly seen that in both g-C3 N4 and Cu/C3 N4 composites, the distinct peak positioned at 27.4◦ can be assigned to plane (002) of hexagonal g-C3 N4 (JCPDS 87-1526), corresponding to the typical interlayer stacking of aromatic systems [42]. It explicitly indicates that the g-C3 N4 were successfully fabricated and the molecular structure of graphitic carbon nitride was basically preserved with the deposition of Cu nanoparticles [49]. Moreover, with the introduction of Cu, the diffraction intensity of (002) plane became weaker. The g-C3 N4 surface covered by Cu nanoparticle probably caused the decrease of XRD intensity. However, according to the previous report, this also may be due to the crystallinity decrease of Cu/C3 N4 composites, leading to more edge defects generated [50]. It is reasonable to speculate that those edge defects will be beneficial to CO2 photocatalytic reduction. In contrast to the gC3 N4 , all the diffraction peaks of the hybrids were well indexed to g-C3 N4 phase, and no significant characteristic diffraction peaks for Cu nanoparticles was shown up until the content of Cu reached to 9%. The possible reason was that the Cu nanoparticles were well dispersed on the surface of g-C3 N4 . Fig. 1b displays the enlarged XRD patterns in the 35–50◦ range. A distinct broad peak around 43.7◦ was detected for Cu/C3 N4 -9, which was ascribed to Cu species. It is well known that the small Cu particles are easy to be oxidized and it is inevitable that a small quantity of Cu2 O was present on the surface. However, the XRD and later XPS and HRTEM results reveal that the metal Cu was dominant on the surface of the prepared catalyst. So, this result confirmed that desired Cu/C3 N4 materials were successfully fabricated. Moreover, in order to confirm the real content of Cu in the composites, EDS analysis was performed for all Cu/C3 N4 samples. As shown in Fig. S1 in Supporting information, the weight percentages of element Cu in Cu/C3 N4 -3, Cu/C3 N4 -6, Cu/C3 N4 -9 samples were 2.37%, 4.23% and 4.88%, respectively, which was close to that in the raw precursors. The XPS spectra were employed to investigate the composition and element chemical status of the pristine g-C3 N4 and Cu/C3 N4 6 samples. Fig. 2a displays the survey spectrum comparison of
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Fig. 2. Comparison of XPS survey spectra (a), high-resolution XPS spectra of C 1s (b) and N 1s (c) for g-C3 N4 and Cu/C3 N4 -6 samples; and high-resolution XPS spectra of Cu 2p for Cu/C3 N4 -6 sample (d).
the g-C3 N4 and Cu/C3 N4 -6 samples. The two XPS survey spectra demonstrate that C and N were the main elements for the g-C3 N4 and Cu/C3 N4 -6 samples, and the strong peak located at 285 eV and 399 eV belonged to C 1s and N 1s respectively. In addition, the characteristic peaks centered at 931.7 eV and 951.5 eV were obvious, corresponding to Cu 2p3/2 and Cu 2p1/2 respectively. Fig. 3b–d
shows the high-resolution XPS spectrum of the C 1s, N 1s and Cu 2p, respectively. As shown in Fig. 2b, the high-resolution spectra of C 1s for both of the g-C3 N4 and Cu/C3 N4 -6 samples were able to be fitted into two peaks at 284.8 and 288.1 eV, attributed to sp2 C C bonding from surface adventitious carbon and N C N2 bonding in the molecular structure of g-C3 N4 respectively. The high-resolution
Fig. 3. SEM images of g-C3 N4 nanosheets (a), Cu/C3 N4 -3 (b), Cu/C3 N4 -6 (c) and Cu/C3 N4 -9 (d).
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Fig. 4. FT-IR spectra of g-C3 N4 and different Cu/C3 N4 composite samples.
XPS spectra of N 1s for both of the g-C3 N4 and Cu/C3 N4 -6 samples were fitted into four peaks located at binding energies of 398.7, 400.2, 401.3 and 404.1 eV. The two former peaks were assigned to sp2 -hybridized nitrogen atoms (C N C) and bridging nitrogen atoms (C3 N) respectively. The peak located at 401.3 eV belonged to the N H bonding, and the peak centered at 404.1 eV owned to the charging effects caused by -excitations. Additionally, two symmetrical peaks centered at 931.7 and 951.5 eV were observed in Fig. 2d, corresponding to Cu 2p3/2 and Cu 2p1/2 respectively. No evident statellite peak was found, indicating the absence of Cu2+ in the Cu/C3 N4 -6 sample. The above peak locations were in conformity of previous reports [34,38]. It is noteworthy that all the peaks in the high-resolution spectra for C and N elements showed no shift, indicating that the main molecular structure of g-C3 N4 was not influenced by Cu deposition. SEM images are shown in Fig. 3 to display the morphological information of the samples with different amount of Cu. It can be seen that the prepared g-C3 N4 nanosheets showed a thin cloud-
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like morphology, while the ultrathin nanosheets were unfurled and porous structure was presented (Fig. 3a). The thickness of individual nanosheet was speculated to be just a few nanometers. It is believed that such enormous and ultrathin nanosheets could provide a large number of active sites for CO2 reduction owing to large specific surface area. The morphology of Cu/C3 N4 with different Cu content is presented in Fig. 3b–d. It can be obviously seen that with the augmented amount of Cu, the Cu/C3 N4 composite remained the layer structure, but the surface became matte in comparison with that of the g-C3 N4 . A number of nanoparticles attached to the surface of the C3 N4 nanosheet were looming. It may be probably due to the reason that in the process of microwave hydrothermal, some of Cu nanoparticles were deposited on the surface of g-C3 N4 nanosheet while the other Cu nanoparticles were wrapped by g-C3 N4 nanosheets. It worth highlighting that the intimate contact between g-C3 N4 and Cu nanoparticles can result in a rapid transfer of electrons, which will be beneficial to CO2 photocatalytic reduction. In order to confirm the large specific surface area of the prepared samples, the surface area was measured by Brunauer-Emmett-Teller method. Nitrogen adsorption and desorption isotherms for all prepared samples are displayed in Fig. S2 in Supporting information. It can be confirm that the BET specific surface area for g-C3 N4 and Cu/C3 N4 -3, Cu/C3 N4 -6 and Cu/C3 N4 -9 was 34.04 m2 g−1 , 103.2 m2 g−1 , 58.39 m2 g−1 and 75.24 m2 g−1 , respectively. Therefore, it is reasonable to believe that the larger specific surface area endow the samples with more reaction activity sites, which plays an important role in the photocatalytic reduction of CO2 . FT-IR spectra were a useful tool to investigate the surface properties of the g-C3 N4 and Cu/C3 N4 samples. As shown in Fig. 4, for both the pristine g-C3 N4 and Cu/C3 N4 samples, several strong bands in the region of 1200–1700 cm−1 and a sharp peak at 810 cm−1 were observed, ascribed to the different vibration modes of C N bond within the tri-s-triazine rings and the characteristic breathing mode of tri-s-triazine heterocycles respectively [51–53]. Additional, the broad absorption band at around 3200 cm−1 was originated from various vibration modes of N H and O H bond, which is related to uncondensed amino group [54]. All above results indicate that the local structure of the prepared Cu/C3 N4 com-
Fig. 5. TEM images of g-C3 N4 in low (a) and high (b) magnification; and TEM images for Cu/C3 N4 -6 (c) (the insets are the detailed image of black frame).
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Fig. 6. UV–vis diffuse reflectance spectra for pristine g-C3 N4 and different Cu/C3 N4 composite samples.
posites was also composed of triazine units and the fundamental structure was not affected by the introduction of Cu, which is in accordance with reported results [39,49]. Note that with the augmented amount of Cu, all the absorption intensity of characteristic peak decreased gradually, indicating the change of the crystallinity degree, which corresponds to the XRD analysis. Moreover, to further understanding the structural feature of the composite with the Cu nanoparticles on the surface, TEM images were measured. Owing to the remarkable similarity of the fabrication method and components for the Cu/C3 N4 samples, for simplicity, only the images of g-C3 N4 and Cu/C3 N4 -6 are shown as the proof in Fig. 5. Fig. 5a and b displays the low and high magnification image of pristine g-C3 N4 respectively. Substantially, the ultrathin layer structure was clearly observed in the TEM images because of the extraordinary penetrability through the gC3 N4 , which is in accordance with the SEM images. Those ultrathin nanosheets may not only possess many reaction activity sites but also equip with large specific surface area, which will be beneficial to CO2 photocatalytic reduction. The more detailed information on the structure of the Cu/C3 N4 sample is presented in Fig. 5c. It can be clearly seen that pearl-like Cu nanoparticles interspersed on the surface of the g-C3 N4 nanosheets. Moreover, the distinctive lattice fringes with d spacing of 0.207 nm corresponded to the plane (111) of Cu phase was clearly seen. Therefore, the distinct lattice fringes of Cu phase also offered an objective and direct evidence for the presence of pure metal Cu. In addition, from the insets TEM images, it can be inferred that the Cu nanoparticles were deposited on the
surface of g-C3 N4 to achieve an intimate contact status, which is also in agreement with the SEM results. In order to probe the optical absorption properties of the gC3 N4 and Cu/C3 N4 samples, UV–vis diffuse reflectance spectra was measured. The colors of the prepared samples were found to gradually change from gray to dark yellow with the increase of Cu loading amount (inset photos). As presented in Fig. 6, pristine gC3 N4 demonstrated excellent visible light absorption in the range of less than 450 nm, corresponding to a band gap of 2.7 eV. It is consistent with the previous literature results [27]. However, in comparison with pristine g-C3 N4 , Cu/C3 N4 samples showed an increased absorption in visible light region, which implied that the deposition of Cu nanoparticles facilitated light harvesting for the composites to some extent. Moreover, the absorption edges of Cu/C3 N4 samples displayed a significant red shift with the increase of Cu content. These results indicated that the Cu/C3 N4 samples possessed an enhanced visible light absorption ability, resulting in more electron-hole carrier generation. It can be predicted that Cu/C3 N4 samples may possess a better photocatalytic performance than pristine g-C3 N4 owing to more photogenerated electrons involved in CO2 reduction. Fig. 7a and b shows the photocurrent density and electrochemical impedance spectra respectively for pristine g-C3 N4 and Cu/C3 N4 samples. It can be clearly seen that pristine g-C3 N4 had a very weak photocurrent density with several on-off cycles of visible light irradiation. However, the photocurrent density of Cu/C3 N4 samples, by contrast, exhibited a significant improvement. It is found that the photocurrent density for Cu/C3 N4 -6 reached about −3.1 mA cm−2 , the largest, while the pristine g-C3 N4 only had the photocurrent density of only about −0.8 mA cm−2 . The enhanced photocurrent of Cu/C3 N4 samples was due to the reason that with the load of Cu nanoparticles, a more efficient electron-hole pair transfer and separation was achieved on the Cu/C3 N4 photocatalysts. It speculates that Cu nanoparticles can act as an electron traps and help suppress undesired photogenerated electro-hole recombination. Similar conclusion was also reported by Yu group about Cu nanoparticle modified TiO2 for CO2 reduction [45]. The electrochemical impedance spectra (EIS) shown in Fig. 7b is used to analyze the interfacial charge transfer process for different electrodes in electrolyte. For the analog equivalent circuit (inset of Fig. 7b), it is well known that the diameter of semicircle is indicative of the interfacial charge transfer resistance, Rct , between electrode and electrolyte interface [55], Rsr the electrolytic solution resistance and CPE the constant phase element between the electrode and electrolyte solution. The size of semicircular arc means the value of Rct , which was estimated as ∼9.7, ∼8.6, ∼7.4, ∼8.0 k for g-C3 N4 , Cu/C3 N4 -3, Cu/C3 N4 -6 and Cu/C3 N4 -9, respectively. Obviously, Cu/C3 N4 -6 had the smallest semicircle radius comparted with the other samples, which is in accordance with
Fig. 7. Photocurrent density (a) and electrochemical impedance spectra (EIS) (b) for pristine g-C3 N4 and different Cu/C3 N4 composite samples.
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Fig. 8. Time-dependent product evolution over all samples (a) and the corresponding histogram of yield (b).
the photocurrent density result. It means a fast and efficient charge transfer and separation. Previous studies have authenticated that efficient charge transfer at semiconductor/electrolyte interface can enhance the catalytic efficiency. Since the smallest charge transfer resistance of Cu/C3 N4 -6 as well as the largest photocurrent density, this hybrid showed a preferable efficiency of charge transfer, which will have outstanding contribution for CO2 photocatalytic reduction.
3.2. Photocatalytic activity measurement To obtain insight into the activity of the samples, CO2 photocatalytic reduction measurements were performed over pristine g-C3 N4 and Cu/C3 N4 samples. As shown in Fig. 8a, the photocatalytic performance comparison of all the prepared samples toward CO2 reduction is presented. It is worth noting that carbon monoxide (CO) was found to be the major product of CO2 reduction. When pristine g-C3 N4 was used as photocatalyst, the yield of CO increased gradually with the increase of reaction time and reached 15.8 mol/g after 5 h visible light irradiation. On the other hand, the Cu nanoparticle modified g-C3 N4 exhibited good activity toward the photoreduction of CO2 to CO. The CO yield of 5 h visible light irradiation for Cu/C3 N4 -3, Cu/C3 N4 -6, Cu/C3 N4 -9 was about 24.31, 49.43 and 39.75 mol/g, respectively, which was much higher than pristine g-C3 N4 . It is worth noting that Cu/C3 N4 -6, the optimal sample, resulted in a 3-fold increase of CO formation yield in comparison with pristine g-C3 N4 , demonstrating the excellent photocatalytic property after Cu nanoparticle deposition. The histogram of the product yield in Fig. 8b displays a further comparison for pristine g-C3 N4 and Cu/C3 N4 samples. It can be clearly seen that the yield of CO for all samples increased gradually during 5 h irradi-
ation and among them Cu/C3 N4 -6 possessed the best photocatalytic property. Additionally, some control experiments under identical conditions was also carried out. As shown in Fig. 9a, when there is no light irradiation or in the absence of CO2 (under saturated N2 atmosphere), no CO product was detected. Therefore, it is proved that the carbon source for the evolved products is from CO2 but not from any other carbon source [10]. The activity of bulk-C3 N4 prepared by first calcination method was also measured for comparison under the same conditions. It can be seen that the bulk-C3 N4 had negligible activity toward CO2 photoreduction in comparison with the g-C3 N4 and Cu/C3 N4 -6 samples. 3.3. Mechanism discussions Some logical speculations may account for the good activity toward the photoreduction of CO2 to CO. (1) It is well known that g-C3 N4 often suffers from a rapid electron-hole recombination process, leading to a negligible photocatalytic activity [56,57]. Moreover, the relatively few active sites also restrict its CO2 photoreduction performance. Hence, it is reasonable that pristine g-C3 N4 exhibited a negligible CO2 reduction capacity. (2) With the Cu nanoparticle loading, the visible light adsorption ability of the samples exhibited a significant enhancement, resulting in the increase of the photogenerated electron amount. In addition, the Cu nanoparticle deposition was beneficial to the separation of photogenerated electron-holes. It has been reported that in the presence of Cu, which can act as the trapping center for the photogenerated electrons, the chances of electrons-holes recombination process can be suppressed effectively [45]. That means, in theory, more trapping electrons can easily migrate to the surface of the photocatalyst to participate in CO2 reduction reaction. Moreover, in addition
Fig. 9. Control experiments of different photocatalysts under 5 h visible light irradiation (a) and time dependent photocatalytic O2 production for Cu/C3 N4 -6 (b).
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Scheme 2. Schematic diagram of proposed process of CO2 photoreduction on Cu/C3 N4 samples.
to inhibiting electrons-holes recombination, Cu nanoparticles also serve as the active sites for the photocatalytic reduction of CO2 [45,46]. Therefore, compared with pristine g-C3 N4 , Cu nanoparticle modified g-C3 N4 possessed more catalytic sites to adsorb and activate CO2 . So, it is not surprised that the Cu/C3 N4 samples presented a better performance for CO2 reduction. (3) In our system, there existed an optimal copper loading amount, which could bring superior active performance for Cu/C3 N4 samples as photocatalysts for CO2 reduction and the Cu/C3 N4 -6 sample with 6 wt.% Cu loading amount was confirmed as the optimized sample. On one hand, when the Cu loading amount was lower than 6 wt.%, relatively low density of Cu in the composite to function electrons trapping as well as catalytic sites. On the other hand, when the Cu loading amount was higher than the optimum value, the presence of excess Cu is likely to become new electrons-hole recombination center, leading to the decrease of electron utilization and thereafter the limitation of the further improvement of CO2 reduction activity [58–60]. That is why the yield of CO2 reduction reaction increased initially and then decreased as the Cu loading amount of on the surface of g-C3 N4 increased gradually. It is believed that photogenerated holes can also play a pivotal role in CO2 reduction because of the catalytic oxidation of water to O2 by the photogenerated holes, which can promote the separation of photogenerated charge carriers and result in the enhancement in activity for CO2 photoreduction [61]. Therefore, the amount detection of O2 evolved in the photocatalytic CO2 reduction was also performed for Cu/C3 N4 -6. As shown in Fig. 9b, the evolved O2 amount increased with the increase of photocatalytic reaction time, indicating that photogenerated holes indeed took part in the reaction with H2 O for O2 generation. According to the above discussion, a schematic diagram of the proposed process of CO2 reduction over Cu/C3 N4 sample is displayed in Scheme 2. In brief, photogenerated electrons excited by g-C3 N4 migrated to the surface of Cu nanoparticles, which sever as trapping center as well as catalytic sites, for the photocatalytic CO2 reduction to CO. Moreover, photogenerated holes migrated to the surface of the photocatalyst for water oxidation to O2 . 4. Conclusions Cu nanoparticle modified g-C3 N4 hybrid composites have been successfully synthesized via a facile secondary calcination and subsequent microwave hydrothermal method for efficient photocatalytic reduction of CO2 into CO and the optimal amount of deposited Cu has been confirmed. Furthermore, the Cu/C3 N4 sam-
ples possessed not only improved visible light absorption ability but also enhanced efficiency of photogenerated electrons-holes separation and increased catalytic activity sites, resulted in a significantly enhancement of CO2 reduction activity in comparison with the pristine g-C3 N4 . In addition, O2 has been collected in the reaction system, which indicates photogenerated hole can oxidize water, and no sacrificial agent is needed here. It is anticipated that the Cu/C3 N4 composite photocatalyst may provide new insights for two dimensional layered materials and non-noble particles applied in CO2 conversion to useful chemicals. Acknowledgements This work was financially supported by National Natural Science Foundation of China (Nos. 21377044 and 21573085) and the Key Project of Natural Science Foundation of Hubei Province (No. 2015CFA037). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.08. 148. References [1] J. Yu, J. Low, W. Xiao, P. Zhou, M. Jaroniec, J. Am. Chem. Soc. 136 (2014) 8839–8842. [2] C. Li, J. Ciston, M. Kanan, Nature 508 (2014) 504–507. [3] B. Rosen, A. Khojin, M. Thorson, W. Zhu, D. Whipple, P. Kenis, R. Masel, Science 334 (2011) 643–644. [4] J. Su, L. Vayssieres, ACS Energy Lett. 1 (2016) 121–135. [5] T. Inoue, A. Fujishima, S. Konishi, K. Honda, Nature 227 (1979) 637–638. [6] X. Chen, S. Mao, Chem. Rev. 107 (2007) 2891–2959. [7] T. Gordon, M. Cargnello, T. Paik, F. Mangolini, R. Weber, P. Fornasiero, C. Murray, J. Am. Chem. Soc. 134 (2012) 6751–6761. [8] Y. Li, D. Xu, J. Oh II, W. Shen, X. Li, Y. Yu, ACS Catal. 2 (2012) 391–398. [9] H. Wang, Y. Li, X. Ba, L. Huang, Y. Yu, Appl. Surf. Sci. 345 (2015) 49–56. [10] L. Yu, G. Li, X. Zhang, X. Ba, G. Shi, Y. Li, P. Wong, J.C. Yu, Y. Yu, ACS Catal. 6 (2016) 6444–6454. [11] J. Zhang, Y. Wang, J. Jin, J. Zhang, Z. Lin, F. Huang, J. Yu, ACS Appl. Mater. Interfaces 5 (2013) 10317–10324. [12] K. Choi, D. Kim, B. Rungtaweevoranit, C. Trickett, J. Barmanbek, A. Alshammari, P. Yang, O. Yaghi, J. Am. Chem. Soc. 139 (2017) 356–362. [13] L. Tan, W. Ong, S. Chai, A. Mohamed, Chem. Eng. J. 308 (2017) 248–255. [14] Y. Li, W. Zhang, X. Shen, P. Peng, L. Xiong, Y. Yu, Chin. J. Catal. 36 (2015) 2229–2236. [15] J. Yang, R. Liu, S. Huang, Y. Shao, Y. Huang, Y. Yu, Catal. Today 224 (2014) 104–113. [16] I. Shown, H. Hsu, Y. Chang, C. Lin, P. Roy, A. Ganguly, C. Wang, J. Chang, C. Wu, L. Chen, Nano Lett. 14 (2014) 6097–6103.
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Photocatalytic reduction of CO2 to CO over copper decorated g-C3 N4 nanosheets with enhanced yield and selectivity Guodong Shi, Lin Yang, Zhuowen Liu, Xiao Chen, Jianqing Zhou, Ying Yu ∗ Institute of Nanoscience and Nanotechnology, College of Physical Science and Technology, Central China Normal University, Wuhan 430079, China
a r t i c l e
i n f o
Article history: Received 10 July 2017 Received in revised form 18 August 2017 Accepted 23 August 2017 Available online 26 August 2017 Keywords: Cu decoration g-C3 N4 Photocatalysis CO2 reduction
a b s t r a c t Photocatalytic reduction of CO2 to fuel has attracted considerable attention due to the consumption of fossil fuels and serious environmental problems. Although there are many photocatalysts reported for CO2 reduction, the improvement of activity and selectivity is still in great need of. In this work, a series of Cu nanoparticle decorated g-C3 N4 nanosheets with different Cu loadings were fabricated by a facile secondary calcination and subsequent microwave hydrothermal method. The designed catalysts shown good photocatalytic activity and selectivity for CO2 reduction to CO. The optimal sample exhibited a 3-fold augmentation of the CO yield in comparison with pristine g-C3 N4 under visible light. It is revealed that with the loading of Cu nanoparticles, the resulting photocatalyst possessed an improved charge carrier transfer and separation efficiency as well as increased surface reactive sites, resulting in a significant enhancement of CO yield. It is anticipated that the designed Cu/C3 N4 photocatalyst may provide new insights for two dimensional layer materials and non-noble particles applied to CO2 reduction. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the development of society, the continuous consumption of fossil energy and environmental pollution is two major challenges for human beings. Since the discovery of the photocatalytic reduction of carbon dioxide (CO2 ) into organic fuel of semiconductors by Inoue in 1979, artificial photocatalytic reduction of CO2 to fuel has been proven to be a fantastic method to simultaneously solve the above two problems [1–5]. As one of the widely studied photocatalysts, titanium dioxide (TiO2 ) had been intensely used because of its environmentally friendly, low cost, superior stability. It is a pity that the ineffective visible light utilization caused by its wide bandgap and high recombination rate of light-induced electron-hole pairs restricts its further application [6–9]. So a lot of work has been done to prepare all kinds of other photocatalysts such as metal oxide, metal sulfides, organic compounds and graphitic compounds. However, those semiconductor photocatalysts also suffer from the many problems of low stability, photocorrosion and high cost [10–15]. As such, it is necessary to explore some new catalysts with splendid performance for photocatalytic reduction of CO2 .
∗ Corresponding author. E-mail address: [email protected] (Y. Yu). http://dx.doi.org/10.1016/j.apsusc.2017.08.148 0169-4332/© 2017 Elsevier B.V. All rights reserved.
For the past few years, two-dimensional layered materials, such as graphene, molybdenum disulfide and graphitic carbon nitride (g-C3 N4 ) exhibited excellent performance in photocatalytic and electrocatalytic reaction [16–20]. In particular, g-C3 N4 has attracted a great deal of consideration owing to its good photocatalytic performance for pollutant degradation, hydrogen production and CO2 reduction [21–23]. For example, Zhu et al. reported the C3 N4 -agar composite shows enhanced performance in photocatalytic degradation of phenol and MB under visible light [24]; Wang’s group demonstrated that carbon nitride as a commonly available photocatalyst is able to generate H2 from water [25]. But nonetheless, the practical application of g-C3 N4 in photocatalysis also faces challenges due to its inherent disadvantages, including low specific surface area, a small amount of surface active sites and high recombination rate of photo-generated carriers [26–29]. So, in order to overcome those deficiency, many strategies have been taken to design and modify g-C3 N4 , including fabrication of C3 N4 quantum dot and single or few layer nanosheet [30,31], coupling with other semiconductors to form heterojunction structure [32,33], non-metal doping [34–36], metal doping [37], surface functionalization [38] and metal deposition [39,40]. For example, Yu’s group combined g-C3 N4 with Ag2 WO4 to form direct Z-scheme photocatalyst, which displayed a better performance toward the degradation of methyl orange [41]. Zou et al. fabricated borondoped g-C3 N4 with enhanced efficiency for rhodamine degradation
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Scheme 1. Schematic illustration for the synthetic process of Cu/C3 N4 .
[42]. Zhang’s group reported g-C3 N4 with tunable band structure for efficient visible light driven hydrogen evolution [43]. Moreover, the enhanced activity of photocatalytic CO2 reduction, especially CH4 and CH3 OH formation over functionalized g-C3 N4 ultrathin nanosheets was demonstrated by Yu’s group [44]. Among the above strategies for the activity improvement of semiconductors, non-precious metal deposition is considered to an efficient and facile method to significantly improve the photocatalytic activity since the deposited metals can act as “charge-carrier traps” and suppress the recombination process of electrons-holes. As a typical non-precious metal, Cu has been widely investigated because of its low cost and good catalytic effect. For instance, Yu et al. reported Cu-decorated microsized nanoporous TiO2 photocatalyst was fabricated for CO2 reduction, resulting in a 21-fold augmentation of the CH4 yield in comparsion with commercial TiO2 [45]. Zhao et al. used metallic Cu decorated Co3 O4 nanotubes as photoelectrocatalyst, leading to an outstanding photoelectrochemical response toward CO2 reduction [46]. Graphene oxide decorated with Cu nanoparticles was demonstrated by Chen et al. [16] with 60 times higher CO2 reduction rate than pristine GO. Moreover, Cu nanoparticles interspersed MoS2 with enhanced efficiency for CO2 electrochemical reduction was recently reported by our group [18]. It is believed that the deposition of Cu can effectively suppress the unwelcome photoinduced electro-hole recombination, increase active sites and enhance electrical conductivity, all of which may improve catalytic activity. Provoked by the aforesaid study, in this work, few layer g-C3 N4 nanosheets modified by Cu nanoparticles were successfully synthesized for efficient CO2 photocatalytic reduction. Although Cu/C3 N4 composites have been reported earlier [39,47,48], to the best of our knowledge, so far there has been no publication about the combination of Cu nanoparticles and ultrathin g-C3 N4 as catalyst for photocatalytic CO2 reduction. The prepared Cu/C3 N4 catalytic composite was optimized by varying the amount of loading Cu nanoparticles. The result demonstrates the yield of CO2 reduction of g-C3 N4 was significantly enhanced by the deposition of Cu nanoparticles. Moreover, the obtained Cu/C3 N4 catalytic exhibited excellent selectivity for carbon monoxide (CO) formation. Therefore, we believe that this kind of Cu/C3 N4 catalytic may shed light on the design and preparation of novel two-dimensional layer materials for efficient CO2 photochemical reduction.
2. Experiment section 2.1. Materials synthesis All of the chemicals used in this study were purchased from Shanghai Guoyao Chemicals Ltd. Co. The chemicals were analytical reagents and used without further purification. Deionized water was used for the preparation of aqueous solutions. In this study, the synthetic process consisted of two steps as shown in Scheme 1. In the first step, g-C3 N4 nanosheet were formed through secondary calcination method. In brief, 10 g dicyandiamide was added to a crucible and then calcined in a muffle furnace at 550 ◦ C for 4 h with a ramp rate of 2.5 ◦ C/min. After heat treatment, yellow powder was collected and named as bulk g-C3 N4 . Then, bulk g-C3 N4 was ground into fine powder and 0.1 g fine powder was subsequently placed into a ceramic boat, which was secondary calcining at 510 ◦ C for 2 h with a ramp rate of 5 ◦ C/min. Finally, g-C3 N4 with few layers was obtained. In the second step, Cu nanoparticle modified ultrathin g-C3 N4 nanosheets were synthesized via a simple microwave hydrothermal method. Detailed information is that 20 mg g-C3 N4 powder with few layers was suspended into 20 mL ethylene glycol and then had been ultrasonicated for 1 h. Then, different amount of copper (II) nitrate trihydrate (Cu(NO3 )2 ·3H2 O) was added to the suspension, and the solution was stirred. The weight ratios of Cu(NO3 )2 . 3H2 O to g-C3 N4 were 3%, 6%, 9% and the corresponding fabricated three Cu modified g-C3 N4 samples were denoted as Cu/C3 N4 -3, Cu/C3 N4 -6, Cu/C3 N4 -9, respectively. After ultrasonication for 60 min, the homogeneous mixture was placed into a microwave oven and had been kept for the duration of 180 s at 350 W power. Black precipitates obtained by post microwave treatment were cooled down to room temperature, and then were separated by centrifugation and washed with ethanol for several times to remove the ethylene glycol residue. The final product was dried overnight in a vacuum oven at 50 ◦ C. 2.2. Materials characterization The morphology and structure of the prepared Cu/C3N4 samples were analyzed by a field emission scanning electron microscopy (FESEM, JEOL JSM-6700F) at 10 kV acceleration voltage and a highresolution transmission electron microscopy (HRTEM, Titan G2
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Fig. 1. XRD pattern comparison for g-C3 N4 , Cu/C3 N4 -3, Cu/C3 N4 -6 and Cu/C3 N4 -9 (a) and the corresponding enlarged parts in the range of 30–50◦ (b).
60-300 Probe Cs Corrector) at 200 kV acceleration voltage. The phase composition of the prepared Cu/C3N4 samples were characterized by X-ray diffractometer (XRD, X’PertPRO MRD, PANalytical, Netherlands, Cu K␣ radiation). The surface chemical component analysis of the g-C3N4 and Cu/C3N4 samples was confirmed by Xray photoelectron spectroscopy (XPS) (VG Multiab-2000) equipped with a monochromatic Al K␣ source. Brunauer-Emmett-Teller (BET) specific surface area of the prepared samples was measured with Micromeritics ASAP2020 nitrogen adsorption apparatus (USA). UV–vis diffuse reflectance spectra (UV–vis DRS) were chronicled with PerkinElmer Lambda 35 spectrophotometer in the range of 400–800 nm using BaSO4 as a reference. Fourier transform infrared spectra (FTIR) were measured on an IR Affinity-1 FTIR spectrometer (Shimadzu, Japan). 2.3. Electrochemical measurements Electrochemical impedance spectroscopy (EIS) were measured with an electrochemical working station (CHI660E) within the range of 10 mHz–100 kHz in a standard three electrode system. The prepared samples were used as working electrodes, a platinum plate and standard Ag/AgCl electrode (3 M KCl) used as the counter and reference respectively. A 0.02 M Na2 SO4 aqueous solution was used as the electrolyte. The photocurrent of the samples was performed under a chopped irradiation from a 350 W xenon lamp (Lap Pu, XQ). The working electrode was fabricated as follow. 20 mg of prepared samples was dispersed in 10 mL mixed solution with ethanol and nafion by ultrasonication and then the slurry was spread onto the surface of a cleaning ITO glass, followed by drying in vacuum oven. 2.4. Photocatalytic performance tests The photocatalytic CO2 reduction experiments were carried out in a homemade sealed cell (250 mL) [10]. The amount of prepared sample was 20 mg in each test and electrolyte solution was 0.1 M KHCO3 with the volume of 170 mL. The whole photocatalytic CO2 reduction test was performed without additional power. Then, high purity CO2 (99.999%) gas was flowed and bubbled through the reactor for 30 min until the CO2 concentration reached saturation and the dissolved oxygen was removed completely. A 350 W xenon lamp was used as the light source and all of the experiments were performed at room temperature and ambient pressure. The gas products from CO2 photocatalytic reduction were analyzed by using gas chromatograph instruments (GC-2014, SHIMADZU) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD) for organic and inorganic compound detection respectively. A packed column (PROPAK-Q, 2 m × 3 mm)
and a different packed column (C36880-14, Restek, 2 m × 1 mm) were used for GC-2014 with FID and TCD, respectively. Nitrogen (N2 ) was used as carrier gas. The gas products were collected by a 1 m syringe and then injected into the gas chromatograph with an interval of 60 min. Control experiments were performed under identical conditions but in the absence of CO2 (under an N2 atmosphere) or light irradiation.
3. Results and discussion 3.1. Characterization of the catalysts The XRD patterns of g-C3 N4 and Cu/C3 N4 composites are shown in Fig. 1. It can be clearly seen that in both g-C3 N4 and Cu/C3 N4 composites, the distinct peak positioned at 27.4◦ can be assigned to plane (002) of hexagonal g-C3 N4 (JCPDS 87-1526), corresponding to the typical interlayer stacking of aromatic systems [42]. It explicitly indicates that the g-C3 N4 were successfully fabricated and the molecular structure of graphitic carbon nitride was basically preserved with the deposition of Cu nanoparticles [49]. Moreover, with the introduction of Cu, the diffraction intensity of (002) plane became weaker. The g-C3 N4 surface covered by Cu nanoparticle probably caused the decrease of XRD intensity. However, according to the previous report, this also may be due to the crystallinity decrease of Cu/C3 N4 composites, leading to more edge defects generated [50]. It is reasonable to speculate that those edge defects will be beneficial to CO2 photocatalytic reduction. In contrast to the gC3 N4 , all the diffraction peaks of the hybrids were well indexed to g-C3 N4 phase, and no significant characteristic diffraction peaks for Cu nanoparticles was shown up until the content of Cu reached to 9%. The possible reason was that the Cu nanoparticles were well dispersed on the surface of g-C3 N4 . Fig. 1b displays the enlarged XRD patterns in the 35–50◦ range. A distinct broad peak around 43.7◦ was detected for Cu/C3 N4 -9, which was ascribed to Cu species. It is well known that the small Cu particles are easy to be oxidized and it is inevitable that a small quantity of Cu2 O was present on the surface. However, the XRD and later XPS and HRTEM results reveal that the metal Cu was dominant on the surface of the prepared catalyst. So, this result confirmed that desired Cu/C3 N4 materials were successfully fabricated. Moreover, in order to confirm the real content of Cu in the composites, EDS analysis was performed for all Cu/C3 N4 samples. As shown in Fig. S1 in Supporting information, the weight percentages of element Cu in Cu/C3 N4 -3, Cu/C3 N4 -6, Cu/C3 N4 -9 samples were 2.37%, 4.23% and 4.88%, respectively, which was close to that in the raw precursors. The XPS spectra were employed to investigate the composition and element chemical status of the pristine g-C3 N4 and Cu/C3 N4 6 samples. Fig. 2a displays the survey spectrum comparison of
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Fig. 2. Comparison of XPS survey spectra (a), high-resolution XPS spectra of C 1s (b) and N 1s (c) for g-C3 N4 and Cu/C3 N4 -6 samples; and high-resolution XPS spectra of Cu 2p for Cu/C3 N4 -6 sample (d).
the g-C3 N4 and Cu/C3 N4 -6 samples. The two XPS survey spectra demonstrate that C and N were the main elements for the g-C3 N4 and Cu/C3 N4 -6 samples, and the strong peak located at 285 eV and 399 eV belonged to C 1s and N 1s respectively. In addition, the characteristic peaks centered at 931.7 eV and 951.5 eV were obvious, corresponding to Cu 2p3/2 and Cu 2p1/2 respectively. Fig. 3b–d
shows the high-resolution XPS spectrum of the C 1s, N 1s and Cu 2p, respectively. As shown in Fig. 2b, the high-resolution spectra of C 1s for both of the g-C3 N4 and Cu/C3 N4 -6 samples were able to be fitted into two peaks at 284.8 and 288.1 eV, attributed to sp2 C C bonding from surface adventitious carbon and N C N2 bonding in the molecular structure of g-C3 N4 respectively. The high-resolution
Fig. 3. SEM images of g-C3 N4 nanosheets (a), Cu/C3 N4 -3 (b), Cu/C3 N4 -6 (c) and Cu/C3 N4 -9 (d).
G. Shi et al. / Applied Surface Science 427 (2018) 1165–1173
Fig. 4. FT-IR spectra of g-C3 N4 and different Cu/C3 N4 composite samples.
XPS spectra of N 1s for both of the g-C3 N4 and Cu/C3 N4 -6 samples were fitted into four peaks located at binding energies of 398.7, 400.2, 401.3 and 404.1 eV. The two former peaks were assigned to sp2 -hybridized nitrogen atoms (C N C) and bridging nitrogen atoms (C3 N) respectively. The peak located at 401.3 eV belonged to the N H bonding, and the peak centered at 404.1 eV owned to the charging effects caused by -excitations. Additionally, two symmetrical peaks centered at 931.7 and 951.5 eV were observed in Fig. 2d, corresponding to Cu 2p3/2 and Cu 2p1/2 respectively. No evident statellite peak was found, indicating the absence of Cu2+ in the Cu/C3 N4 -6 sample. The above peak locations were in conformity of previous reports [34,38]. It is noteworthy that all the peaks in the high-resolution spectra for C and N elements showed no shift, indicating that the main molecular structure of g-C3 N4 was not influenced by Cu deposition. SEM images are shown in Fig. 3 to display the morphological information of the samples with different amount of Cu. It can be seen that the prepared g-C3 N4 nanosheets showed a thin cloud-
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like morphology, while the ultrathin nanosheets were unfurled and porous structure was presented (Fig. 3a). The thickness of individual nanosheet was speculated to be just a few nanometers. It is believed that such enormous and ultrathin nanosheets could provide a large number of active sites for CO2 reduction owing to large specific surface area. The morphology of Cu/C3 N4 with different Cu content is presented in Fig. 3b–d. It can be obviously seen that with the augmented amount of Cu, the Cu/C3 N4 composite remained the layer structure, but the surface became matte in comparison with that of the g-C3 N4 . A number of nanoparticles attached to the surface of the C3 N4 nanosheet were looming. It may be probably due to the reason that in the process of microwave hydrothermal, some of Cu nanoparticles were deposited on the surface of g-C3 N4 nanosheet while the other Cu nanoparticles were wrapped by g-C3 N4 nanosheets. It worth highlighting that the intimate contact between g-C3 N4 and Cu nanoparticles can result in a rapid transfer of electrons, which will be beneficial to CO2 photocatalytic reduction. In order to confirm the large specific surface area of the prepared samples, the surface area was measured by Brunauer-Emmett-Teller method. Nitrogen adsorption and desorption isotherms for all prepared samples are displayed in Fig. S2 in Supporting information. It can be confirm that the BET specific surface area for g-C3 N4 and Cu/C3 N4 -3, Cu/C3 N4 -6 and Cu/C3 N4 -9 was 34.04 m2 g−1 , 103.2 m2 g−1 , 58.39 m2 g−1 and 75.24 m2 g−1 , respectively. Therefore, it is reasonable to believe that the larger specific surface area endow the samples with more reaction activity sites, which plays an important role in the photocatalytic reduction of CO2 . FT-IR spectra were a useful tool to investigate the surface properties of the g-C3 N4 and Cu/C3 N4 samples. As shown in Fig. 4, for both the pristine g-C3 N4 and Cu/C3 N4 samples, several strong bands in the region of 1200–1700 cm−1 and a sharp peak at 810 cm−1 were observed, ascribed to the different vibration modes of C N bond within the tri-s-triazine rings and the characteristic breathing mode of tri-s-triazine heterocycles respectively [51–53]. Additional, the broad absorption band at around 3200 cm−1 was originated from various vibration modes of N H and O H bond, which is related to uncondensed amino group [54]. All above results indicate that the local structure of the prepared Cu/C3 N4 com-
Fig. 5. TEM images of g-C3 N4 in low (a) and high (b) magnification; and TEM images for Cu/C3 N4 -6 (c) (the insets are the detailed image of black frame).
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Fig. 6. UV–vis diffuse reflectance spectra for pristine g-C3 N4 and different Cu/C3 N4 composite samples.
posites was also composed of triazine units and the fundamental structure was not affected by the introduction of Cu, which is in accordance with reported results [39,49]. Note that with the augmented amount of Cu, all the absorption intensity of characteristic peak decreased gradually, indicating the change of the crystallinity degree, which corresponds to the XRD analysis. Moreover, to further understanding the structural feature of the composite with the Cu nanoparticles on the surface, TEM images were measured. Owing to the remarkable similarity of the fabrication method and components for the Cu/C3 N4 samples, for simplicity, only the images of g-C3 N4 and Cu/C3 N4 -6 are shown as the proof in Fig. 5. Fig. 5a and b displays the low and high magnification image of pristine g-C3 N4 respectively. Substantially, the ultrathin layer structure was clearly observed in the TEM images because of the extraordinary penetrability through the gC3 N4 , which is in accordance with the SEM images. Those ultrathin nanosheets may not only possess many reaction activity sites but also equip with large specific surface area, which will be beneficial to CO2 photocatalytic reduction. The more detailed information on the structure of the Cu/C3 N4 sample is presented in Fig. 5c. It can be clearly seen that pearl-like Cu nanoparticles interspersed on the surface of the g-C3 N4 nanosheets. Moreover, the distinctive lattice fringes with d spacing of 0.207 nm corresponded to the plane (111) of Cu phase was clearly seen. Therefore, the distinct lattice fringes of Cu phase also offered an objective and direct evidence for the presence of pure metal Cu. In addition, from the insets TEM images, it can be inferred that the Cu nanoparticles were deposited on the
surface of g-C3 N4 to achieve an intimate contact status, which is also in agreement with the SEM results. In order to probe the optical absorption properties of the gC3 N4 and Cu/C3 N4 samples, UV–vis diffuse reflectance spectra was measured. The colors of the prepared samples were found to gradually change from gray to dark yellow with the increase of Cu loading amount (inset photos). As presented in Fig. 6, pristine gC3 N4 demonstrated excellent visible light absorption in the range of less than 450 nm, corresponding to a band gap of 2.7 eV. It is consistent with the previous literature results [27]. However, in comparison with pristine g-C3 N4 , Cu/C3 N4 samples showed an increased absorption in visible light region, which implied that the deposition of Cu nanoparticles facilitated light harvesting for the composites to some extent. Moreover, the absorption edges of Cu/C3 N4 samples displayed a significant red shift with the increase of Cu content. These results indicated that the Cu/C3 N4 samples possessed an enhanced visible light absorption ability, resulting in more electron-hole carrier generation. It can be predicted that Cu/C3 N4 samples may possess a better photocatalytic performance than pristine g-C3 N4 owing to more photogenerated electrons involved in CO2 reduction. Fig. 7a and b shows the photocurrent density and electrochemical impedance spectra respectively for pristine g-C3 N4 and Cu/C3 N4 samples. It can be clearly seen that pristine g-C3 N4 had a very weak photocurrent density with several on-off cycles of visible light irradiation. However, the photocurrent density of Cu/C3 N4 samples, by contrast, exhibited a significant improvement. It is found that the photocurrent density for Cu/C3 N4 -6 reached about −3.1 mA cm−2 , the largest, while the pristine g-C3 N4 only had the photocurrent density of only about −0.8 mA cm−2 . The enhanced photocurrent of Cu/C3 N4 samples was due to the reason that with the load of Cu nanoparticles, a more efficient electron-hole pair transfer and separation was achieved on the Cu/C3 N4 photocatalysts. It speculates that Cu nanoparticles can act as an electron traps and help suppress undesired photogenerated electro-hole recombination. Similar conclusion was also reported by Yu group about Cu nanoparticle modified TiO2 for CO2 reduction [45]. The electrochemical impedance spectra (EIS) shown in Fig. 7b is used to analyze the interfacial charge transfer process for different electrodes in electrolyte. For the analog equivalent circuit (inset of Fig. 7b), it is well known that the diameter of semicircle is indicative of the interfacial charge transfer resistance, Rct , between electrode and electrolyte interface [55], Rsr the electrolytic solution resistance and CPE the constant phase element between the electrode and electrolyte solution. The size of semicircular arc means the value of Rct , which was estimated as ∼9.7, ∼8.6, ∼7.4, ∼8.0 k for g-C3 N4 , Cu/C3 N4 -3, Cu/C3 N4 -6 and Cu/C3 N4 -9, respectively. Obviously, Cu/C3 N4 -6 had the smallest semicircle radius comparted with the other samples, which is in accordance with
Fig. 7. Photocurrent density (a) and electrochemical impedance spectra (EIS) (b) for pristine g-C3 N4 and different Cu/C3 N4 composite samples.
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Fig. 8. Time-dependent product evolution over all samples (a) and the corresponding histogram of yield (b).
the photocurrent density result. It means a fast and efficient charge transfer and separation. Previous studies have authenticated that efficient charge transfer at semiconductor/electrolyte interface can enhance the catalytic efficiency. Since the smallest charge transfer resistance of Cu/C3 N4 -6 as well as the largest photocurrent density, this hybrid showed a preferable efficiency of charge transfer, which will have outstanding contribution for CO2 photocatalytic reduction.
3.2. Photocatalytic activity measurement To obtain insight into the activity of the samples, CO2 photocatalytic reduction measurements were performed over pristine g-C3 N4 and Cu/C3 N4 samples. As shown in Fig. 8a, the photocatalytic performance comparison of all the prepared samples toward CO2 reduction is presented. It is worth noting that carbon monoxide (CO) was found to be the major product of CO2 reduction. When pristine g-C3 N4 was used as photocatalyst, the yield of CO increased gradually with the increase of reaction time and reached 15.8 mol/g after 5 h visible light irradiation. On the other hand, the Cu nanoparticle modified g-C3 N4 exhibited good activity toward the photoreduction of CO2 to CO. The CO yield of 5 h visible light irradiation for Cu/C3 N4 -3, Cu/C3 N4 -6, Cu/C3 N4 -9 was about 24.31, 49.43 and 39.75 mol/g, respectively, which was much higher than pristine g-C3 N4 . It is worth noting that Cu/C3 N4 -6, the optimal sample, resulted in a 3-fold increase of CO formation yield in comparison with pristine g-C3 N4 , demonstrating the excellent photocatalytic property after Cu nanoparticle deposition. The histogram of the product yield in Fig. 8b displays a further comparison for pristine g-C3 N4 and Cu/C3 N4 samples. It can be clearly seen that the yield of CO for all samples increased gradually during 5 h irradi-
ation and among them Cu/C3 N4 -6 possessed the best photocatalytic property. Additionally, some control experiments under identical conditions was also carried out. As shown in Fig. 9a, when there is no light irradiation or in the absence of CO2 (under saturated N2 atmosphere), no CO product was detected. Therefore, it is proved that the carbon source for the evolved products is from CO2 but not from any other carbon source [10]. The activity of bulk-C3 N4 prepared by first calcination method was also measured for comparison under the same conditions. It can be seen that the bulk-C3 N4 had negligible activity toward CO2 photoreduction in comparison with the g-C3 N4 and Cu/C3 N4 -6 samples. 3.3. Mechanism discussions Some logical speculations may account for the good activity toward the photoreduction of CO2 to CO. (1) It is well known that g-C3 N4 often suffers from a rapid electron-hole recombination process, leading to a negligible photocatalytic activity [56,57]. Moreover, the relatively few active sites also restrict its CO2 photoreduction performance. Hence, it is reasonable that pristine g-C3 N4 exhibited a negligible CO2 reduction capacity. (2) With the Cu nanoparticle loading, the visible light adsorption ability of the samples exhibited a significant enhancement, resulting in the increase of the photogenerated electron amount. In addition, the Cu nanoparticle deposition was beneficial to the separation of photogenerated electron-holes. It has been reported that in the presence of Cu, which can act as the trapping center for the photogenerated electrons, the chances of electrons-holes recombination process can be suppressed effectively [45]. That means, in theory, more trapping electrons can easily migrate to the surface of the photocatalyst to participate in CO2 reduction reaction. Moreover, in addition
Fig. 9. Control experiments of different photocatalysts under 5 h visible light irradiation (a) and time dependent photocatalytic O2 production for Cu/C3 N4 -6 (b).
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Scheme 2. Schematic diagram of proposed process of CO2 photoreduction on Cu/C3 N4 samples.
to inhibiting electrons-holes recombination, Cu nanoparticles also serve as the active sites for the photocatalytic reduction of CO2 [45,46]. Therefore, compared with pristine g-C3 N4 , Cu nanoparticle modified g-C3 N4 possessed more catalytic sites to adsorb and activate CO2 . So, it is not surprised that the Cu/C3 N4 samples presented a better performance for CO2 reduction. (3) In our system, there existed an optimal copper loading amount, which could bring superior active performance for Cu/C3 N4 samples as photocatalysts for CO2 reduction and the Cu/C3 N4 -6 sample with 6 wt.% Cu loading amount was confirmed as the optimized sample. On one hand, when the Cu loading amount was lower than 6 wt.%, relatively low density of Cu in the composite to function electrons trapping as well as catalytic sites. On the other hand, when the Cu loading amount was higher than the optimum value, the presence of excess Cu is likely to become new electrons-hole recombination center, leading to the decrease of electron utilization and thereafter the limitation of the further improvement of CO2 reduction activity [58–60]. That is why the yield of CO2 reduction reaction increased initially and then decreased as the Cu loading amount of on the surface of g-C3 N4 increased gradually. It is believed that photogenerated holes can also play a pivotal role in CO2 reduction because of the catalytic oxidation of water to O2 by the photogenerated holes, which can promote the separation of photogenerated charge carriers and result in the enhancement in activity for CO2 photoreduction [61]. Therefore, the amount detection of O2 evolved in the photocatalytic CO2 reduction was also performed for Cu/C3 N4 -6. As shown in Fig. 9b, the evolved O2 amount increased with the increase of photocatalytic reaction time, indicating that photogenerated holes indeed took part in the reaction with H2 O for O2 generation. According to the above discussion, a schematic diagram of the proposed process of CO2 reduction over Cu/C3 N4 sample is displayed in Scheme 2. In brief, photogenerated electrons excited by g-C3 N4 migrated to the surface of Cu nanoparticles, which sever as trapping center as well as catalytic sites, for the photocatalytic CO2 reduction to CO. Moreover, photogenerated holes migrated to the surface of the photocatalyst for water oxidation to O2 . 4. Conclusions Cu nanoparticle modified g-C3 N4 hybrid composites have been successfully synthesized via a facile secondary calcination and subsequent microwave hydrothermal method for efficient photocatalytic reduction of CO2 into CO and the optimal amount of deposited Cu has been confirmed. Furthermore, the Cu/C3 N4 sam-
ples possessed not only improved visible light absorption ability but also enhanced efficiency of photogenerated electrons-holes separation and increased catalytic activity sites, resulted in a significantly enhancement of CO2 reduction activity in comparison with the pristine g-C3 N4 . In addition, O2 has been collected in the reaction system, which indicates photogenerated hole can oxidize water, and no sacrificial agent is needed here. It is anticipated that the Cu/C3 N4 composite photocatalyst may provide new insights for two dimensional layered materials and non-noble particles applied in CO2 conversion to useful chemicals. Acknowledgements This work was financially supported by National Natural Science Foundation of China (Nos. 21377044 and 21573085) and the Key Project of Natural Science Foundation of Hubei Province (No. 2015CFA037). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.08. 148. References [1] J. Yu, J. Low, W. Xiao, P. Zhou, M. Jaroniec, J. Am. Chem. Soc. 136 (2014) 8839–8842. [2] C. Li, J. Ciston, M. Kanan, Nature 508 (2014) 504–507. [3] B. Rosen, A. Khojin, M. Thorson, W. Zhu, D. Whipple, P. Kenis, R. Masel, Science 334 (2011) 643–644. [4] J. Su, L. Vayssieres, ACS Energy Lett. 1 (2016) 121–135. [5] T. Inoue, A. Fujishima, S. Konishi, K. Honda, Nature 227 (1979) 637–638. [6] X. Chen, S. Mao, Chem. Rev. 107 (2007) 2891–2959. [7] T. Gordon, M. Cargnello, T. Paik, F. Mangolini, R. Weber, P. Fornasiero, C. Murray, J. Am. Chem. Soc. 134 (2012) 6751–6761. [8] Y. Li, D. Xu, J. Oh II, W. Shen, X. Li, Y. Yu, ACS Catal. 2 (2012) 391–398. [9] H. Wang, Y. Li, X. Ba, L. Huang, Y. Yu, Appl. Surf. Sci. 345 (2015) 49–56. [10] L. Yu, G. Li, X. Zhang, X. Ba, G. Shi, Y. Li, P. Wong, J.C. Yu, Y. Yu, ACS Catal. 6 (2016) 6444–6454. [11] J. Zhang, Y. Wang, J. Jin, J. Zhang, Z. Lin, F. Huang, J. Yu, ACS Appl. Mater. Interfaces 5 (2013) 10317–10324. [12] K. Choi, D. Kim, B. Rungtaweevoranit, C. Trickett, J. Barmanbek, A. Alshammari, P. Yang, O. Yaghi, J. Am. Chem. Soc. 139 (2017) 356–362. [13] L. Tan, W. Ong, S. Chai, A. Mohamed, Chem. Eng. J. 308 (2017) 248–255. [14] Y. Li, W. Zhang, X. Shen, P. Peng, L. Xiong, Y. Yu, Chin. J. Catal. 36 (2015) 2229–2236. [15] J. Yang, R. Liu, S. Huang, Y. Shao, Y. Huang, Y. Yu, Catal. Today 224 (2014) 104–113. [16] I. Shown, H. Hsu, Y. Chang, C. Lin, P. Roy, A. Ganguly, C. Wang, J. Chang, C. Wu, L. Chen, Nano Lett. 14 (2014) 6097–6103.
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