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Cu2O heterojunctions: Preparation, characterization, and enhanced photocatalytic activity under visible light
Journal of Solid State Chemistry 212 (2014) 1–6
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Graphitic carbon nitride/Cu2O heterojunctions: Preparation, characterization, and enhanced photocatalytic activity under visible light Yanlong Tian, Binbin Chang, Jie Fu, Baocheng Zhou, Jiyang Liu, Fengna Xi, Xiaoping Dong n Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 October 2013 Received in revised form 1 January 2014 Accepted 9 January 2014 Available online 15 January 2014
As a metal-free semiconductor material, graphitic carbon nitride (C3N4), the high recombination rate of photogenerated charges and insufficient sunlight absorption limit its solar-based photocatalytic activity. Here, we reported the heterojunctions of C3N4–Cu2O with a p–n junction structure, which was synthesized by a hydrothermal method. The HR-TEM result revealed an intimate interface between C3N4 and Cu2O in the heterojunction, and UV–vis diffuse reflection spectra showed their extended spectral response in the visible region compared with pure C3N4. These excellent structural and spectral properties, as well as p–n junction structures, endowed the C3N4–Cu2O heterojunctions with enhanced photocatalytic activities. The possible photocatalytic mechanism that photogenerated holes as the mainly oxidant species in photocatalysis was proposed base on the trapping experiments. & 2014 Elsevier Inc. All rights reserved.
Keywords: Semiconductor Photocatalysis Heterojunctions Graphitic carbon nitride Cu2O
1. Introduction The semiconductor photocatalysis technique has been regarded as a promising approach to address the increasing global energy and environmental crises [1,2]. Among various photocatalytic materials, titania (TiO2) has been the subject of the most extensive investigation owing to its nontoxicity, chemical stability, water insolubility, low price, and favorable photochemical property [3]. Unfortunately, TiO2 can only be excited by ultraviolet (UV) light that takes up less than 5% of sunlight, which limits the application to a great extent. Not with standing extensive efforts to modify TiO2 (e.g. doping [4,5], coupling [6,7], dye-sensitization [8,9]), photocatalytic activity in the visible has remained quite low. Given this, a great deal of research is focused on finding novel semiconductor photocatalytic materials to replace TiO2-based materials. Recently, Wang et al. [10] reported to develop a novel metalfree polymeric material, graphitic carbon nitride (C3N4), which exhibits excellent performance of hydrogen or oxygen production from water splitting under visible light irradiation. Compared with TiO2, C3N4 possesses a smaller band-gap of 2.7 eV, which can make it absorb more natural sunlight. Moreover, C3N4 is extremely stable with respect to thermal, chemical, and photochemical attack
n
Corresponding author. Tel: þ 86 571 86843228; fax: þ86 571 86843653. E-mail addresses: [email protected], [email protected] (X. Dong).
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otherwise than the photocatalysts of sulfide and oxynitride semiconductor [11]. Therefore, C3N4 should be an ideal photocatalytic material to replace TiO2-based materials. Nevertheless, the photocatalytic efficiency of the pristine C3N4 is still low under natural sunlight irradiation because of its high recombination rate of photogenerated electron–hole pairs and insufficient sunlight absorption originating from onset wavelength of 460 nm [12]. Thus, many methods have been adopted to overcome above two barriers, including chemical doping with metal or nonmetal elements [13,14], coupling with graphene [15,16] and designing an appropriate textural porosity [17,18], etc. The formation of heterojunction by combination of two semiconductors with different band-gap is an effective route to promote the separation of charges and the subsequent collection of electrons and holes at the interfaces of two semiconductors, thus reducing their recombination. In the last decades, intensive efforts have been devoted to create various kinds of heterojunctions, such as ZnO–TiO2 [19], CdS–TiO2 [20], AgBr–BiPO4 [21], and SnO2–ZnO [22]. Not surprisingly, this strategy was immediately employed to improve the photocatalytic activity of C3N4 once this material was introduced into photocatalysis. Several kinds of C3N4 based heterojunctions have been developed, for instance, C3N4–TaON [23], C3N4–TiO2 [24], C3N4–ZnO [25], C3N4/SrTiO3 [26], C3N4–Bi2WO6 [27], C3N4–BiOCl [28], C3N4–BiPO4 [29], and C3N4/ ZnWO4 [30]. Very recently, we reported the fabrication of novel heterojunctions of BiOBr–C3N4 by directly depositing BiOBr nanoflakes onto the surface of C3N4 [31]. As expected, these C3N4 based
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heterojunctions exhibited enhanced photocatalytic activities compared to the single component in heterojunction, because of their well-matched overlapping band-structures and the resulting highefficient separation of photogenerated charges. Whereas, the visible light absorption ability of these C3N4 based heterojunctions has not been simultaneously extended due to the relatively large band-gaps of these coupled semiconductors. It is therefore very necessary to design new C3N4 based heterojunctions by coupling C3N4 with narrow band-gap semiconductors that doesn0 t only effectively suppress the charge recombination, but also extend photoabsorption range of C3N4. Cuprous oxide (Cu2O), a direct band-gap (2.0 eV) semiconductor, has been regarded as a promising photocatalytic material since its nontoxicity, good environmental acceptability and natural abundance, which has potential applications in solar cells and photocatalytic degradation of organic pollutants [32–34]. In this study, we report the fabrication of C3N4–Cu2O heterojunction via simply depositing Cu2O to the surface of C3N4. The visible light photocatalytic tests show that the present C3N4–Cu2O heterojunctions possess excellent photocatalytic activity for degrading methyl orange (MO) under visible light irradiation, much higher than those of individual C3N4 or Cu2O. Meanwhile, the resulting heterojunction also exhibits a high stability and durability after six successive cycles. In addition, the possible photocatalytic mechanism of this heterojunction was also discussed.
2. Experimental section 2.1. Synthesis All reagents were analytical grade and used without further treatments. C3N4 was synthesized by thermal polycondensation of melamine, which was described in our previous report [31]. C3N4–Cu2O heterojunctions were obtained by a hydrothermal method with C3N4 and the precursor of Cu2O. In a typical procedure, a mixture of C3N4 and Cu(NO3)2 were added into 50 mL of deionized water, followed by vigorous stirring overnight for ensuring adsorption of copper precursor onto C3N4 surface. After carefully adjusting the pH value to 8 using NaOH solution (1 M), a measured amount of glucose with the molar glucose/Cu ratio at 1.5:1 was added. The resulting mixture was then transferred into a Teflon-lined steel autoclave, which was heated in an oven at 100 1C for 10 h. Subsequently, the precipitate was obtained by filtration, washed with distilled water several times, dried at 60 1C for 12 h, and finally heated at 200 1C for 2 h to obtained C3N4–Cu2O heterojunctions. According to this method, different mass ratios of C3N4–Cu2O at 1:1, 3:7, and 1:9 were prepared and denoted as 0.5C3N4–0.5Cu2O, 0.3–C3N4–0.7Cu2O, 0.1C3N4– 0.9Cu2O, respectively. The pure Cu2O sample was synthesized under the same conditions in the absence of C3N4 powder.
2.3. Photocatalytic tests The photocatalytic activities of catalysts were evaluated by the degradation of MO and 2,4-dichlorophenol (2,4-DCP) in aqueous solution under visible light irradiation of a 300 W xenon lamp (HSX-F300, Beijing NBet) with the 400 nm cutoff filter. In each experiment, 100 mg of catalyst was suspended in an aqueous solution (100 mL) of MO (5 mg L 1) or 2,4-DCP (15 mg L 1) in a quartz glass reactor, which was cooled by recycled water to prevent the thermal catalytic effect. Prior to irradiation, the suspension was stirred in the dark for 1 h to ensure the establishment of adsorption–desorption equilibrium. At given irradiation time intervals, 4 mL of the suspension was collected and subsequently centrifuged to remove the catalyst particles. The concentration of solution was analyzed by measuring the maximum absorbance at 463 nm for MO, and 283 nm for 2,4-DCP using a Shimadzu UV-2450 spectrophotometer.
3. Results and discussions 3.1. Characterization of C3N4–Cu2O Fig. 1 shows XRD patterns of the as-prepared C3N4–Cu2O heterojunctions, as well as C3N4 and Cu2O. Two distinct peaks of 13.041 and 27.471 in the C3N4 sample can be indexed as the (1 0 0) and (0 0 2) diffractions for graphitic materials, respectively corresponding to the in-plane structural packing motif and interlayer stacking of aromatic segments [13,35]. After coupling C3N4 with Cu2O, these two peaks decrease in intensity gradually with the enhancement of Cu2O content, which suggests the successful sedimentation of Cu2O onto the surface of C3N4. A series of narrow and sharp diffraction peaks, assigned to a cubic Cu2O structure (JCPDS 78-2076), can be observed in the pattern of Cu2O sample, indicating the well-crystallization of Cu2O particles. Besides the Cu2O phase, some weak impurity peaks were checked, which are raised from cubic Cu (JCPDS 85-1326). The existence of Cu should be favorable for the enhancement of photocatalytic activity, because metallic Cu can accept photogenerated electrons from the conductor band of Cu2O or C3N4, subsequently improving the charge separation. All diffraction peaks of individual C3N4 and Cu2O are present in XRD patterns of the as-prepared C3N4–Cu2O heterojunctions, indicating a two-phase composition of C3N4 and Cu2O in these heterojunctions. SEM analysis was adopted to investigate the morphology of C3N4–Cu2O heterojunctions. The pure Cu2O sample shows a series of spherical and cubic morphology with a porous structure (Fig. 2a). After introducing C3N4, 0.1C3N4–0.9Cu2O heterojunction still maintains spherical and cubic morphology, which is similar
2.2. Characterization X-ray diffraction (XRD) patterns were monitored by a DX-2700 diffractometer (Dandong Haoyuan Instrument Co. Ltd., China) using Cu Kα radiation (λ ¼ 0.15418 nm). Scanning electron microscopy (SEM) images was obtained with a Hitachi S-4800 Field emission scanning electron microscope. Transmission electron microscope (TEM) images were obtained on a JEOL JEM-2100 electron microscope with an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Avatar 370 spectrophotometer using the standard KBr disk method. Diffuse reflectance spectra were recorded on a Shimadzu 2450 UV–vis spectrometer with an integrating sphere using BaSO4 as the reference.
Fig. 1. XRD patterns of C3N4, Cu2O, and C3N4–Cu2O heterojunctions.
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Fig. 2. SEM image (a) Cu2O, (b) 0.1C3N4–0.9Cu2O heterojunction, (c) 0.3C3N4–0.7Cu2O heterojunction, and (d) 0.5C3N4–0.5Cu2O heterojunction.
Fig. 3. TEM image (a) and HR-TEM image (b) of 0.1C3N4–0.9Cu2O heterojunction.
with pure Cu2O (Fig. 2b). However, when the content of C3N4 continues to increase, a typically aggregated morphology of C3N4 begins to appear in C3N4–Cu2O heterojunctions of 0.3C3N4– 0.7Cu2O and 0.5C3N4–0.5Cu2O (Fig. 2c and d). The microstructure of C3N4–Cu2O heterojunctions was investigated by TEM. The low-magnification TEM image (Fig. 3a) demonstrates that C3N4 and Cu2O have successfully coupled together. From the high-resolution TEM image of C3N4–Cu2O heterojunction (Fig. 3b), two phases of C3N4 and Cu2O are clearly observed and
closely contact to form an intimate interface. The lattice fringe of nanoparticles displaying an interplanar spacing of 0.25 nm, conforms to the (1 1 1) crystallographic plane of cubic Cu2O, which is in accordance with the XRD result. With respect to the invisibility of the lattice fringe of metallic Cu, this is because the content of Cu in the prepared heterojunctions is very little. The tight coupling of C3N4 and Cu2O is favorable for the charge transfer between these two semiconductors, and results in a high separation rate of photogenerated electron–hole pairs than a physical mixture of
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heterojunctions would absorb more visible light than C3N4, which subsequently results in a higher photocatalytic activity. 3.2. Photocatalytic study of C3N4–Cu2O
Fig. 4. FT-IR spectra of C3N4, Cu2O, and C3N4–Cu2O heterojunctions.
Fig. 5. UV–vis diffuse reflectance spectra of C3N4, Cu2O, and g-C3N4/Cu2O heterojunctions.
two separate phases, subsequently improving the photocatalytic activity. Fig. 4 presents the FT-IR spectra of as-prepared C3N4–Cu2O, together with C3N4 and Cu2O samples. Three characteristic absorption regions, 43000 cm 1, 1200–1700 cm 1 and o1000 cm 1, are observed in C3N4 sample. The former adsorption band is broad and centered at 3156 cm 1, which can be ascribed to the stretching mode of N–H bond [35,36]. Several typical absorption peaks in the second region of 1200–1700 cm 1 are attributed to the stretching vibrations of CN heterocycles [35,37]. The band at 809 cm 1 originates from an breathing mode of s-triazine ring system [35,38]. In the case of Cu2O sample, the band at about 623 cm 1 corresponds to the stretching vibration of Cu (I)–O bond [39]. These characteristic absorptions of C3N4 and Cu2O all appear in the spectra of C3N4–Cu2O heterojunctions, indicating the coexistence of these two semiconductors, and meanwhile, the stepwise enhancement of Cu (I)–O vibration in intensity and the gradual decrease of absorptions corresponding to C3N4 imply the different contents of C3N4 and Cu2O in each C3N4–Cu2O heterojunction. The optical properties of the as-prepared C3N4–Cu2O, C3N4 and Cu2O samples were examined using UV–vis diffuse reflectance spectroscopy. As shown in Fig. 5, the pristine C3N4 holds an absorption edge of 460 nm, which can be assigned to a bandgap of 2.70 eV [10]. Cu2O possesses a broad absorption in visible region from 400 to 600 nm, which is similar to those reported in literatures [40,41]. After combining the two semiconductors, the absorptions of C3N4–Cu2O heterojunctions within the visible light range remarkably increase and appear a red shift in comparison with C3N4. The absorption intensity of C3N4–Cu2O heterojunctions also increase with increasing Cu2O content. As a result,
To evaluate the photocatalytic activity of as-synthesized C3N4– Cu2O heterojunctions, we have investigated the photodegradation of MO under visible light irradiation. As shown in Fig. 6a, the absorption of MO in the visible light region significantly deceases with the increase of irradiation time, and nearly disappears after 3 h. In the meantime, no additional absorption appears that indicates the complete destruction of aromatic structures. The characteristic absorption peak at 463 nm was employed to determine the degradation degree of MO, and the C/C0 vs irradiation time was plotted in Fig. 6b. MO is very stable and almost no decomposition in the absence of catalyst, which excludes the possible of self-photolysis process. The C3N4 also shows poor activity, on which 28% of MO is decomposed after irradiation for 60 min. As for Cu2O, it can remove 75% of MO at a same irradiation time. Significantly, the C3N4–Cu2O heterojunctions exhibit higher photocatalytic activity than either individual C3N4 or Cu2O. The photocatalytic activity first increases from 0.5C3N4– 0.5Cu2O to 0.3C3N4–0.7Cu2O, and then changes relatively little with the further increasing of Cu2O content. This result implies that C3N4 content is a very crucial factor for improving the photocatalytic activity of C3N4–Cu2O heterojunctions. Suitable mass ratio could form an efficient heterojunction interface between two components, and thus suppresses the recombination of photogenerated charges effectively. On the contrary, unsuitable mass ratio would make an agglomerating state, thus reducing the contact area of heterojunction and decreasing photocatalytic activity [26–29]. The highest activity is obtained over 0.1C3N4–0.9Cu2O heterojunction, resulting 95% degradation for MO within 60 min visible light irradiation. For comparison, we also tested the degradation of MO over the commercial TiO2 of P25. As expected, P25 TiO2 exhibits weak photocatalytic activity due to because it is inactive in the visible region. In order to further proving the visible light photocatalytic activity of C3N4–Cu2O heterojunction, 2,4-DCP was also used as the another target that has no absorption in the visible light region (Fig. S1). Apparently, the colorless 2,4-DCP was effectively degraded over C3N4–Cu2O heterojunction under visible light irradiation, indicating that C3N4–Cu2O heterojunction possesses an excellent visible-light photocatalytic performance. The stability is extremely important for practical applications of photocatalyst. The cycling runs for the photodegradation of MO with 0.1C3N4–0.9Cu2O heterojunction were performed to evaluate its photocatalytic stability. Fig. 7a illustrates that the degradation rates of MO in every run. After reusing 6 cycles, the photodecomposition rate of MO still remains over 80%. In addition, XRD patterns of 0.1C3N4–0.9Cu2O heterojunction before and after reaction, as shown in Fig. 7b, distinctly reveal that the heterojunction is stable during the reaction. 3.3. Possible photocatalytic mechanism of C3N4–Cu2O On the basis of experimental and theoretical results, a possible visible light photocatalytic mechanism of C3N4–Cu2O heterojunction is proposed as illustrated in Fig. 8. The conduction band (CB) and valence band (VB) potentials of C3N4 are 1.13 V and 1.57 V, and those of Cu2O are 0.7 V and 1.3 V, respectively [27,34]. C3N4 is a typical n-type semiconductor and the position of its Fermi level is close to CB, whereas Cu2O is a p-type semiconductor and the position of its Fermi level is close to VB [12,42]. Apparently, the band-structure of C3N4 and Cu2O is straddling, which is unsuitable to construct heterojunction for effectively separating photogenerated electrons and holes. However, when Cu2O was deposited on
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Fig. 6. (a) Absorption spectra of MO with irradiation time over 0.1C3N4–0.9Cu2O heterojunction; (b) degradation rates of MO under visible light irradiation without catalyst and in the presence of C3N4, Cu2O, and C3N4–Cu2O heterojunctions.
Fig. 7. (a) Cycling runs for the photocatalytic degradation of MO over 0.1C3N4–0.9Cu2O sample under visible light irradiation; (b) XRD patterns of the 0.1g–C3N4/0.9Cu2O sample before and after the cycling photocatalytic experiments.
Fig. 8. Schematic illustration of C3N4–Cu2O heterojunction under visible light irradiation.
the surface of C3N4, the electrons would diffuse from C3N4 to Cu2O, and the holes would diffuse from Cu2O to C3N4 in their contacted interface, thus resulting in an internal electric field formed until the Fermi levels of C3N4 and Cu2O reached equilibration [43,44]. Meanwhile, the energy band positions of C3N4 and Cu2O also change along with the shift of their Fermi levels, and thus
eventually formed band-structure of C3N4 and Cu2O is overlapping. Once the heterojunction was irradiated with visible light, both C3N4 and Cu2O can be excited and produce photogenerated electron–hole pairs. Under the action of the internal electric field, photogenerated electrons moved to the positive field (n-C3N4), and holes moved to the negative field (p-Cu2O) [45]. As a consequence,
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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2014.01.011.
References
Fig. 9. Photocatalytic degradation of MO over C3N4–Cu2O heterojunction alone, and with the addition of TBA, or EDTA.
photogenerated electrons would accumulate in the n-C3N4 region, and holes accumulated in the p-Cu2O region. Moreover, it is worth pointing out that trace amounts of metallic Cu in C3N4–Cu2O heterojunction is able to act as an acceptor to collect photogenerated electrons from the CB of C3N4 and Cu2O, further promoting the photogenerated electron–hole pairs separation. All of these transferences effectively suppress the recombination of photogenerated electron–hole pairs, leading to an enhanced photocatalytic activity. To further reveal the photocatalytic mechanism of C3N4–Cu2O heterojunction, we also used the trapping experiments of hydroxyl radicals or photogenerated holes to determine the main oxidant species. Disodium ethylenediaminetetraacetate (EDTA, 2 mM) and tertbutyl alcohol (TBA, 2 mM) were used as a hole scavenger and a radical scavenger, respectively [46,47]. Fig. 9 clearly reveals that adding TBA did not bring any obvious difference for degradation behavior of MO, whereas, which was completely suppressed as EDTA was added. Thus, a possible mechanism with photogenerated holes as the main oxidants was proposed. This result also suggests that dye-sensitization is negligible compare with photocatalysis in which hydroxyls radical is the main active species [48–50]. 4. Conclusions In summary, we have successfully developed a C3N4–Cu2O heterojunction via simply depositing Cu2O to the surface of C3N4. The resulting C3N4–Cu2O heterojunctions possess broader absorption in the visible region compare with pure C3N4. Of special significance is that the photocatalytic activities of C3N4–Cu2O heterojunctions are obviously enhanced for degradation of organic pollutants under visible light irradiation. This enhancement has been proven to be attributed to the high separation and easy transfer of photogenerated electron–hole pairs at the interface of heterojunctions, which can be reasonably ascribed to the p–n junction structures of C3N4 and Cu2O. Furthermore, the stability of heterojunction is very excellent that no obvious deactivation occurs after six cycles. Acknowledgments We greatly acknowledge financial support from the National Natural Science Foundation of China (21001093), 521 talent project of ZSTU, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, Zhejiang Provincial Natural Science Foundation of China (NO. LQ13B020006), and Educational Commission of Zhejiang Province of China (Y201225426).
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Graphitic carbon nitride/Cu2O heterojunctions: Preparation, characterization, and enhanced photocatalytic activity under visible light Yanlong Tian, Binbin Chang, Jie Fu, Baocheng Zhou, Jiyang Liu, Fengna Xi, Xiaoping Dong n Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou, China
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a b s t r a c t
Article history: Received 28 October 2013 Received in revised form 1 January 2014 Accepted 9 January 2014 Available online 15 January 2014
As a metal-free semiconductor material, graphitic carbon nitride (C3N4), the high recombination rate of photogenerated charges and insufficient sunlight absorption limit its solar-based photocatalytic activity. Here, we reported the heterojunctions of C3N4–Cu2O with a p–n junction structure, which was synthesized by a hydrothermal method. The HR-TEM result revealed an intimate interface between C3N4 and Cu2O in the heterojunction, and UV–vis diffuse reflection spectra showed their extended spectral response in the visible region compared with pure C3N4. These excellent structural and spectral properties, as well as p–n junction structures, endowed the C3N4–Cu2O heterojunctions with enhanced photocatalytic activities. The possible photocatalytic mechanism that photogenerated holes as the mainly oxidant species in photocatalysis was proposed base on the trapping experiments. & 2014 Elsevier Inc. All rights reserved.
Keywords: Semiconductor Photocatalysis Heterojunctions Graphitic carbon nitride Cu2O
1. Introduction The semiconductor photocatalysis technique has been regarded as a promising approach to address the increasing global energy and environmental crises [1,2]. Among various photocatalytic materials, titania (TiO2) has been the subject of the most extensive investigation owing to its nontoxicity, chemical stability, water insolubility, low price, and favorable photochemical property [3]. Unfortunately, TiO2 can only be excited by ultraviolet (UV) light that takes up less than 5% of sunlight, which limits the application to a great extent. Not with standing extensive efforts to modify TiO2 (e.g. doping [4,5], coupling [6,7], dye-sensitization [8,9]), photocatalytic activity in the visible has remained quite low. Given this, a great deal of research is focused on finding novel semiconductor photocatalytic materials to replace TiO2-based materials. Recently, Wang et al. [10] reported to develop a novel metalfree polymeric material, graphitic carbon nitride (C3N4), which exhibits excellent performance of hydrogen or oxygen production from water splitting under visible light irradiation. Compared with TiO2, C3N4 possesses a smaller band-gap of 2.7 eV, which can make it absorb more natural sunlight. Moreover, C3N4 is extremely stable with respect to thermal, chemical, and photochemical attack
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0022-4596/$ - see front matter & 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2014.01.011
otherwise than the photocatalysts of sulfide and oxynitride semiconductor [11]. Therefore, C3N4 should be an ideal photocatalytic material to replace TiO2-based materials. Nevertheless, the photocatalytic efficiency of the pristine C3N4 is still low under natural sunlight irradiation because of its high recombination rate of photogenerated electron–hole pairs and insufficient sunlight absorption originating from onset wavelength of 460 nm [12]. Thus, many methods have been adopted to overcome above two barriers, including chemical doping with metal or nonmetal elements [13,14], coupling with graphene [15,16] and designing an appropriate textural porosity [17,18], etc. The formation of heterojunction by combination of two semiconductors with different band-gap is an effective route to promote the separation of charges and the subsequent collection of electrons and holes at the interfaces of two semiconductors, thus reducing their recombination. In the last decades, intensive efforts have been devoted to create various kinds of heterojunctions, such as ZnO–TiO2 [19], CdS–TiO2 [20], AgBr–BiPO4 [21], and SnO2–ZnO [22]. Not surprisingly, this strategy was immediately employed to improve the photocatalytic activity of C3N4 once this material was introduced into photocatalysis. Several kinds of C3N4 based heterojunctions have been developed, for instance, C3N4–TaON [23], C3N4–TiO2 [24], C3N4–ZnO [25], C3N4/SrTiO3 [26], C3N4–Bi2WO6 [27], C3N4–BiOCl [28], C3N4–BiPO4 [29], and C3N4/ ZnWO4 [30]. Very recently, we reported the fabrication of novel heterojunctions of BiOBr–C3N4 by directly depositing BiOBr nanoflakes onto the surface of C3N4 [31]. As expected, these C3N4 based
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heterojunctions exhibited enhanced photocatalytic activities compared to the single component in heterojunction, because of their well-matched overlapping band-structures and the resulting highefficient separation of photogenerated charges. Whereas, the visible light absorption ability of these C3N4 based heterojunctions has not been simultaneously extended due to the relatively large band-gaps of these coupled semiconductors. It is therefore very necessary to design new C3N4 based heterojunctions by coupling C3N4 with narrow band-gap semiconductors that doesn0 t only effectively suppress the charge recombination, but also extend photoabsorption range of C3N4. Cuprous oxide (Cu2O), a direct band-gap (2.0 eV) semiconductor, has been regarded as a promising photocatalytic material since its nontoxicity, good environmental acceptability and natural abundance, which has potential applications in solar cells and photocatalytic degradation of organic pollutants [32–34]. In this study, we report the fabrication of C3N4–Cu2O heterojunction via simply depositing Cu2O to the surface of C3N4. The visible light photocatalytic tests show that the present C3N4–Cu2O heterojunctions possess excellent photocatalytic activity for degrading methyl orange (MO) under visible light irradiation, much higher than those of individual C3N4 or Cu2O. Meanwhile, the resulting heterojunction also exhibits a high stability and durability after six successive cycles. In addition, the possible photocatalytic mechanism of this heterojunction was also discussed.
2. Experimental section 2.1. Synthesis All reagents were analytical grade and used without further treatments. C3N4 was synthesized by thermal polycondensation of melamine, which was described in our previous report [31]. C3N4–Cu2O heterojunctions were obtained by a hydrothermal method with C3N4 and the precursor of Cu2O. In a typical procedure, a mixture of C3N4 and Cu(NO3)2 were added into 50 mL of deionized water, followed by vigorous stirring overnight for ensuring adsorption of copper precursor onto C3N4 surface. After carefully adjusting the pH value to 8 using NaOH solution (1 M), a measured amount of glucose with the molar glucose/Cu ratio at 1.5:1 was added. The resulting mixture was then transferred into a Teflon-lined steel autoclave, which was heated in an oven at 100 1C for 10 h. Subsequently, the precipitate was obtained by filtration, washed with distilled water several times, dried at 60 1C for 12 h, and finally heated at 200 1C for 2 h to obtained C3N4–Cu2O heterojunctions. According to this method, different mass ratios of C3N4–Cu2O at 1:1, 3:7, and 1:9 were prepared and denoted as 0.5C3N4–0.5Cu2O, 0.3–C3N4–0.7Cu2O, 0.1C3N4– 0.9Cu2O, respectively. The pure Cu2O sample was synthesized under the same conditions in the absence of C3N4 powder.
2.3. Photocatalytic tests The photocatalytic activities of catalysts were evaluated by the degradation of MO and 2,4-dichlorophenol (2,4-DCP) in aqueous solution under visible light irradiation of a 300 W xenon lamp (HSX-F300, Beijing NBet) with the 400 nm cutoff filter. In each experiment, 100 mg of catalyst was suspended in an aqueous solution (100 mL) of MO (5 mg L 1) or 2,4-DCP (15 mg L 1) in a quartz glass reactor, which was cooled by recycled water to prevent the thermal catalytic effect. Prior to irradiation, the suspension was stirred in the dark for 1 h to ensure the establishment of adsorption–desorption equilibrium. At given irradiation time intervals, 4 mL of the suspension was collected and subsequently centrifuged to remove the catalyst particles. The concentration of solution was analyzed by measuring the maximum absorbance at 463 nm for MO, and 283 nm for 2,4-DCP using a Shimadzu UV-2450 spectrophotometer.
3. Results and discussions 3.1. Characterization of C3N4–Cu2O Fig. 1 shows XRD patterns of the as-prepared C3N4–Cu2O heterojunctions, as well as C3N4 and Cu2O. Two distinct peaks of 13.041 and 27.471 in the C3N4 sample can be indexed as the (1 0 0) and (0 0 2) diffractions for graphitic materials, respectively corresponding to the in-plane structural packing motif and interlayer stacking of aromatic segments [13,35]. After coupling C3N4 with Cu2O, these two peaks decrease in intensity gradually with the enhancement of Cu2O content, which suggests the successful sedimentation of Cu2O onto the surface of C3N4. A series of narrow and sharp diffraction peaks, assigned to a cubic Cu2O structure (JCPDS 78-2076), can be observed in the pattern of Cu2O sample, indicating the well-crystallization of Cu2O particles. Besides the Cu2O phase, some weak impurity peaks were checked, which are raised from cubic Cu (JCPDS 85-1326). The existence of Cu should be favorable for the enhancement of photocatalytic activity, because metallic Cu can accept photogenerated electrons from the conductor band of Cu2O or C3N4, subsequently improving the charge separation. All diffraction peaks of individual C3N4 and Cu2O are present in XRD patterns of the as-prepared C3N4–Cu2O heterojunctions, indicating a two-phase composition of C3N4 and Cu2O in these heterojunctions. SEM analysis was adopted to investigate the morphology of C3N4–Cu2O heterojunctions. The pure Cu2O sample shows a series of spherical and cubic morphology with a porous structure (Fig. 2a). After introducing C3N4, 0.1C3N4–0.9Cu2O heterojunction still maintains spherical and cubic morphology, which is similar
2.2. Characterization X-ray diffraction (XRD) patterns were monitored by a DX-2700 diffractometer (Dandong Haoyuan Instrument Co. Ltd., China) using Cu Kα radiation (λ ¼ 0.15418 nm). Scanning electron microscopy (SEM) images was obtained with a Hitachi S-4800 Field emission scanning electron microscope. Transmission electron microscope (TEM) images were obtained on a JEOL JEM-2100 electron microscope with an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Avatar 370 spectrophotometer using the standard KBr disk method. Diffuse reflectance spectra were recorded on a Shimadzu 2450 UV–vis spectrometer with an integrating sphere using BaSO4 as the reference.
Fig. 1. XRD patterns of C3N4, Cu2O, and C3N4–Cu2O heterojunctions.
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Fig. 2. SEM image (a) Cu2O, (b) 0.1C3N4–0.9Cu2O heterojunction, (c) 0.3C3N4–0.7Cu2O heterojunction, and (d) 0.5C3N4–0.5Cu2O heterojunction.
Fig. 3. TEM image (a) and HR-TEM image (b) of 0.1C3N4–0.9Cu2O heterojunction.
with pure Cu2O (Fig. 2b). However, when the content of C3N4 continues to increase, a typically aggregated morphology of C3N4 begins to appear in C3N4–Cu2O heterojunctions of 0.3C3N4– 0.7Cu2O and 0.5C3N4–0.5Cu2O (Fig. 2c and d). The microstructure of C3N4–Cu2O heterojunctions was investigated by TEM. The low-magnification TEM image (Fig. 3a) demonstrates that C3N4 and Cu2O have successfully coupled together. From the high-resolution TEM image of C3N4–Cu2O heterojunction (Fig. 3b), two phases of C3N4 and Cu2O are clearly observed and
closely contact to form an intimate interface. The lattice fringe of nanoparticles displaying an interplanar spacing of 0.25 nm, conforms to the (1 1 1) crystallographic plane of cubic Cu2O, which is in accordance with the XRD result. With respect to the invisibility of the lattice fringe of metallic Cu, this is because the content of Cu in the prepared heterojunctions is very little. The tight coupling of C3N4 and Cu2O is favorable for the charge transfer between these two semiconductors, and results in a high separation rate of photogenerated electron–hole pairs than a physical mixture of
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heterojunctions would absorb more visible light than C3N4, which subsequently results in a higher photocatalytic activity. 3.2. Photocatalytic study of C3N4–Cu2O
Fig. 4. FT-IR spectra of C3N4, Cu2O, and C3N4–Cu2O heterojunctions.
Fig. 5. UV–vis diffuse reflectance spectra of C3N4, Cu2O, and g-C3N4/Cu2O heterojunctions.
two separate phases, subsequently improving the photocatalytic activity. Fig. 4 presents the FT-IR spectra of as-prepared C3N4–Cu2O, together with C3N4 and Cu2O samples. Three characteristic absorption regions, 43000 cm 1, 1200–1700 cm 1 and o1000 cm 1, are observed in C3N4 sample. The former adsorption band is broad and centered at 3156 cm 1, which can be ascribed to the stretching mode of N–H bond [35,36]. Several typical absorption peaks in the second region of 1200–1700 cm 1 are attributed to the stretching vibrations of CN heterocycles [35,37]. The band at 809 cm 1 originates from an breathing mode of s-triazine ring system [35,38]. In the case of Cu2O sample, the band at about 623 cm 1 corresponds to the stretching vibration of Cu (I)–O bond [39]. These characteristic absorptions of C3N4 and Cu2O all appear in the spectra of C3N4–Cu2O heterojunctions, indicating the coexistence of these two semiconductors, and meanwhile, the stepwise enhancement of Cu (I)–O vibration in intensity and the gradual decrease of absorptions corresponding to C3N4 imply the different contents of C3N4 and Cu2O in each C3N4–Cu2O heterojunction. The optical properties of the as-prepared C3N4–Cu2O, C3N4 and Cu2O samples were examined using UV–vis diffuse reflectance spectroscopy. As shown in Fig. 5, the pristine C3N4 holds an absorption edge of 460 nm, which can be assigned to a bandgap of 2.70 eV [10]. Cu2O possesses a broad absorption in visible region from 400 to 600 nm, which is similar to those reported in literatures [40,41]. After combining the two semiconductors, the absorptions of C3N4–Cu2O heterojunctions within the visible light range remarkably increase and appear a red shift in comparison with C3N4. The absorption intensity of C3N4–Cu2O heterojunctions also increase with increasing Cu2O content. As a result,
To evaluate the photocatalytic activity of as-synthesized C3N4– Cu2O heterojunctions, we have investigated the photodegradation of MO under visible light irradiation. As shown in Fig. 6a, the absorption of MO in the visible light region significantly deceases with the increase of irradiation time, and nearly disappears after 3 h. In the meantime, no additional absorption appears that indicates the complete destruction of aromatic structures. The characteristic absorption peak at 463 nm was employed to determine the degradation degree of MO, and the C/C0 vs irradiation time was plotted in Fig. 6b. MO is very stable and almost no decomposition in the absence of catalyst, which excludes the possible of self-photolysis process. The C3N4 also shows poor activity, on which 28% of MO is decomposed after irradiation for 60 min. As for Cu2O, it can remove 75% of MO at a same irradiation time. Significantly, the C3N4–Cu2O heterojunctions exhibit higher photocatalytic activity than either individual C3N4 or Cu2O. The photocatalytic activity first increases from 0.5C3N4– 0.5Cu2O to 0.3C3N4–0.7Cu2O, and then changes relatively little with the further increasing of Cu2O content. This result implies that C3N4 content is a very crucial factor for improving the photocatalytic activity of C3N4–Cu2O heterojunctions. Suitable mass ratio could form an efficient heterojunction interface between two components, and thus suppresses the recombination of photogenerated charges effectively. On the contrary, unsuitable mass ratio would make an agglomerating state, thus reducing the contact area of heterojunction and decreasing photocatalytic activity [26–29]. The highest activity is obtained over 0.1C3N4–0.9Cu2O heterojunction, resulting 95% degradation for MO within 60 min visible light irradiation. For comparison, we also tested the degradation of MO over the commercial TiO2 of P25. As expected, P25 TiO2 exhibits weak photocatalytic activity due to because it is inactive in the visible region. In order to further proving the visible light photocatalytic activity of C3N4–Cu2O heterojunction, 2,4-DCP was also used as the another target that has no absorption in the visible light region (Fig. S1). Apparently, the colorless 2,4-DCP was effectively degraded over C3N4–Cu2O heterojunction under visible light irradiation, indicating that C3N4–Cu2O heterojunction possesses an excellent visible-light photocatalytic performance. The stability is extremely important for practical applications of photocatalyst. The cycling runs for the photodegradation of MO with 0.1C3N4–0.9Cu2O heterojunction were performed to evaluate its photocatalytic stability. Fig. 7a illustrates that the degradation rates of MO in every run. After reusing 6 cycles, the photodecomposition rate of MO still remains over 80%. In addition, XRD patterns of 0.1C3N4–0.9Cu2O heterojunction before and after reaction, as shown in Fig. 7b, distinctly reveal that the heterojunction is stable during the reaction. 3.3. Possible photocatalytic mechanism of C3N4–Cu2O On the basis of experimental and theoretical results, a possible visible light photocatalytic mechanism of C3N4–Cu2O heterojunction is proposed as illustrated in Fig. 8. The conduction band (CB) and valence band (VB) potentials of C3N4 are 1.13 V and 1.57 V, and those of Cu2O are 0.7 V and 1.3 V, respectively [27,34]. C3N4 is a typical n-type semiconductor and the position of its Fermi level is close to CB, whereas Cu2O is a p-type semiconductor and the position of its Fermi level is close to VB [12,42]. Apparently, the band-structure of C3N4 and Cu2O is straddling, which is unsuitable to construct heterojunction for effectively separating photogenerated electrons and holes. However, when Cu2O was deposited on
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Fig. 6. (a) Absorption spectra of MO with irradiation time over 0.1C3N4–0.9Cu2O heterojunction; (b) degradation rates of MO under visible light irradiation without catalyst and in the presence of C3N4, Cu2O, and C3N4–Cu2O heterojunctions.
Fig. 7. (a) Cycling runs for the photocatalytic degradation of MO over 0.1C3N4–0.9Cu2O sample under visible light irradiation; (b) XRD patterns of the 0.1g–C3N4/0.9Cu2O sample before and after the cycling photocatalytic experiments.
Fig. 8. Schematic illustration of C3N4–Cu2O heterojunction under visible light irradiation.
the surface of C3N4, the electrons would diffuse from C3N4 to Cu2O, and the holes would diffuse from Cu2O to C3N4 in their contacted interface, thus resulting in an internal electric field formed until the Fermi levels of C3N4 and Cu2O reached equilibration [43,44]. Meanwhile, the energy band positions of C3N4 and Cu2O also change along with the shift of their Fermi levels, and thus
eventually formed band-structure of C3N4 and Cu2O is overlapping. Once the heterojunction was irradiated with visible light, both C3N4 and Cu2O can be excited and produce photogenerated electron–hole pairs. Under the action of the internal electric field, photogenerated electrons moved to the positive field (n-C3N4), and holes moved to the negative field (p-Cu2O) [45]. As a consequence,
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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2014.01.011.
References
Fig. 9. Photocatalytic degradation of MO over C3N4–Cu2O heterojunction alone, and with the addition of TBA, or EDTA.
photogenerated electrons would accumulate in the n-C3N4 region, and holes accumulated in the p-Cu2O region. Moreover, it is worth pointing out that trace amounts of metallic Cu in C3N4–Cu2O heterojunction is able to act as an acceptor to collect photogenerated electrons from the CB of C3N4 and Cu2O, further promoting the photogenerated electron–hole pairs separation. All of these transferences effectively suppress the recombination of photogenerated electron–hole pairs, leading to an enhanced photocatalytic activity. To further reveal the photocatalytic mechanism of C3N4–Cu2O heterojunction, we also used the trapping experiments of hydroxyl radicals or photogenerated holes to determine the main oxidant species. Disodium ethylenediaminetetraacetate (EDTA, 2 mM) and tertbutyl alcohol (TBA, 2 mM) were used as a hole scavenger and a radical scavenger, respectively [46,47]. Fig. 9 clearly reveals that adding TBA did not bring any obvious difference for degradation behavior of MO, whereas, which was completely suppressed as EDTA was added. Thus, a possible mechanism with photogenerated holes as the main oxidants was proposed. This result also suggests that dye-sensitization is negligible compare with photocatalysis in which hydroxyls radical is the main active species [48–50]. 4. Conclusions In summary, we have successfully developed a C3N4–Cu2O heterojunction via simply depositing Cu2O to the surface of C3N4. The resulting C3N4–Cu2O heterojunctions possess broader absorption in the visible region compare with pure C3N4. Of special significance is that the photocatalytic activities of C3N4–Cu2O heterojunctions are obviously enhanced for degradation of organic pollutants under visible light irradiation. This enhancement has been proven to be attributed to the high separation and easy transfer of photogenerated electron–hole pairs at the interface of heterojunctions, which can be reasonably ascribed to the p–n junction structures of C3N4 and Cu2O. Furthermore, the stability of heterojunction is very excellent that no obvious deactivation occurs after six cycles. Acknowledgments We greatly acknowledge financial support from the National Natural Science Foundation of China (21001093), 521 talent project of ZSTU, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, Zhejiang Provincial Natural Science Foundation of China (NO. LQ13B020006), and Educational Commission of Zhejiang Province of China (Y201225426).
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