0The Journal of
Physical Chemistry
pubs.acs.org/JPCL
Switching Oxygen Reduction Pathway by Exfoliating Graphitic Carbon Nitride for Enhanced Photocatalytic Phenol Degradation
Hui Zhang, Liang-Hong Guo,* Lixia Zhao, Bin Wan, and Yu Yang
State Key Laboratory of Environmental Chemistry and Eco-toxicology, Research Centre for Eco-environmental Sciences, Chinese Academy of Sciences, 18 Shuangqing Road, P.O. Box 2871, Beijing 100085, China
^ Supporting Information
ABSTRACT: The selectivity of molecular oxygen activation on the exfoliated graphitic carbon nitride (g-C3N4) and its influence on the photocatalytic phenol degradation process were demonstrated. Compared with bulk g-C3N4, the exfoliated nanosheet yielded a 3-fold enhancement in photocatalytic phenol degradation. ROS trapping experiments demonstrated that although the direct hole oxidation was mainly responsible for phenol photodegradation on both g-C3N4 catalysts, molecular oxygen activation processes on their surface greatly influenced the whole phenol degradation efficiency. Reactive oxygen species and Raman spectroscopy measurements revealed that oxygen was preferentially reduced to O2— by one-electron transfer on bulk g-C3N4, while on g-C3N4 nanosheet the production of H2O2 via a
two-electron transfer process was favored due to the rapid formation of surface-stabilized 1,4-endoperoxide. The latter process not only promotes the separation of photogenerated electron—hole pairs but also greatly facilitates reactive oxygen species formation and subsequently enhances phenol degradation.
With the escalating concerns on environmental pollution, sunlight-driven photocatalytic reactions over semiconducting materials for contaminant degradation have attracted considerable attention.1-3 Upon light excitation, photogenerated electrons and holes can either recombine or migrate to the semiconductor surface, where they react with the adsorbed molecular oxygen and hydroxyls to generate reactive oxygen species (ROS, typically including -OH, -O2—, and H2O2) and eventually mineralize contaminants.4-6 In particular, as a main electron scavenger in an ambient atmosphere, molecular oxygen reduction by photogenerated electrons on the semiconductor conduction band, which generally proceeds via one-or two-electron transfer, plays a key role or even is the rate-determining step in the overall reaction because this process not only suppresses electron—hole recombination but also yields ROS.
The pathway of molecular oxygen reduction on the photocatalyst surface is critically dependent on the electronic structure and surface atomic structure of the photocatalyst due to their dominating effect on the redox potentials of photogenerated charges and the adsorption site of reactant molecules, respectively. Previous studies reported that the loading of Au nanoparticles or Cu(ll) clusters on TiO2 surface facilitated multielectron reduction of O2 to H2O2 due to the change of surface electronic structure, which efficiently promoted the degradation of aromatics.7'8 Recently, Zhao's group reported that the different surface structure between anatase TiO2 {001} and {101} facets dramatically influenced the pathways of O2 reduction in the photocatalytic reactions.9 A facet-dependent O2 reduction pathway (one- or two-electron reduction) was also observed on the {001} and {010} surface of BiOCl due to the difference in surface atom exposure.10 These
results clearly suggest that the structural properties can greatly determine the molecular oxygen reduction process. Nevertheless, knowledge on the surface-dependent O2 reduction over photoexcited semiconductor surface is still limited. An in-depth understanding of the above issues is essential to modulating reaction mechanisms and intermediate products and devising practical photocatalytic degradation applications.
Recently, layer-structured semiconductors (e.g., metal-free graphitic carbon nitride, g-C3N4) have emerged as attractive visible-light photocatalysts for hydrogen evolution, organic pollutant degradation, and organic synthesis.11-13 Layer-structured g-C3N4 makes it easy to tailor the energy band structure by controlling the layer number. Experimental results demonstrated that ultrathin g-C3N4 nanosheets promoted the photocatalytic ability with respect to bulk counterparts owing to their larger surface area and enhanced separation of electron—hole pairs.1^15 Despite numerous studies on g-C3N4 photocatalysis, the first step of molecular oxygen reduction on this promising photocatalyst has still not been clarified. Inspired by the previous elaborate work, herein we demonstrated the structure-dependent molecular oxygen reduction on bulk and exfoliated g-C3N4 during photocatalysis. The molecular oxygen reduction processes and possible mechanisms for the observed selectivity for the reduction pathway were investigated. Our work highlights the important role of structure-dependent molecular oxygen reduction during photocatalytic reactions.
Received: January 22, 2015 Accepted: February 27, 2015
ACS Publications © XXXX American Chemical Society
Letter
Figure 1. (a) AFM image of g-C3N nanosheets, (b) UV-vis absorption spectra, (c) Mott-Schottky plots, and (d) schematic illustration of the energy band structures of bulk g-C3N and g-C3N nanosheets.
Figure 2. Phenol degradation curves under visible light over (a) bulk g-C3N and (b) g-C3N nanosheets in aerated and N2 and O2 bubbling suspensions. Phenol degradation curves in (c) bulk g-C3N and (d) g-C3N nanosheet suspensions with the addition of different scavengers (TEOA for hole, tert-butyl alcohol for 'OH, catalase for H2O2).
Bulk g-C3N4 was prepared by direct pyrolysis of cyanamide. Polymeric bulk g-C3N4 is composed of C—N layers, which stack in an ABA type fashion and are formed by weak van der Waals forces. After sonication in water, the bulk g-C3N4 was gradually exfoliated into ultrathin g-C3N4 nanosheets via the interaction of water molecules.17 The AFM image (Figure 1a) exhibits a
thin layer structure of g-C3N4 nanosheets with a uniform thickness of ~1.8 nm, equal to five to six CN atomic monolayers.18 The structural features of g-C3N4 were verified by two signals in XRD pattern (Figure S1 in the SI) at 20 of 27.6 and 13.0°, which originate from the (002) interlayer diffraction of C—N layers and inplanar repeated tri-s-triazine
(a) 0.020
< 0.005
— Bulkg-СД —•— g-c,n nanosheet л I ^
— 8.0x10" «
g б.ОхЮ4-с
I 4.0xl0j-o
2.0xl04 0.0
—Bulk g-C3N4
—•— g-c,n4 nanosheet e
30 60 90 120 150 180 Irradiation time (min)
30 60 90 120 150 180 Irradiation time (min)
Figure 3. (a) Ю2 , and (b) H2O2 generation curves over bulk g-C3N and g-C3N nanosheets under visible light irradiation.
units, respectively. The sharply reduced peaks after exfoliation indicate the few-layer structure of the g-C3N4 nanosheets. Indeed, because of the absence of any oxidation treatment, the crystal structure of bulk g-C3N4 was well-retained in the exfoliated nanosheets.
The electronic structures of the two types of g-C3N4 photocatalysts were investigated by a combined analysis of the optical absorption spectroscopy (Figure 1b) and Mott-Schottky plots (Figure 1c). Compared with bulk g-C3N4, the absorption edge of g-C3N4 nanosheets displays a slight blue shift, accompanied by the bandgap broadening from 2.4 to 2.6 eV. The Mott-Schottky plots reveal the typical n-type characteristics of g-C3N4 due to the positive slope of the linear plots.19 The flat-band potentials derived from Mott-Schottky plots are about -1.30 and -1.40 V (vs NHE) for the bulk and exfoliated g-C3N4, respectively. These results indicate the conduction and valence band edges of bulk g-C3N4 shifted in opposite directions after exfoliation (Figure 1d), which are attributed to the quantum effect of a few-layer structure of g-C3N4 nanosheets.
Figure 2 displays the marked influence of g-C3N4 structure on their photocatalytic behavior during the phenol degradation. It was observed that phenol degradation over both g-C3N4 photocatalysts was fitted satisfactorily with a pseudo-first-order kinetic process (Figure S2 in the SI). The apparent rate constant for phenol photocatalytic degradation increased significantly from 0.18 h-1 on bulk g-C3N4 to 0.57 h-1 on the exfoliated g-C3N4 nanosheets. Control experiments in the absence of any catalysts or oxygen showed negligible phenol degradation, clearly indicating that O2 was required for phenol photocatalytic degradation. According to the classical photo-catalytic theory,2,20 O2 molecules can capture photogenerated electrons (e-) on g-C3N4, resulting in the great improvement of charge separation efficiency (eqs 1-4)
O, + e"
O," + H+
HO, -+H+ + e"
O, + 2H+ + 2e"
(1) (2)
Indeed, the important role ofO2 during photocatalytic reaction over g-C3N4 was further verified by the enhanced phenol degradation in separate experiments with O2 bubbling (Figure 2a,b).
Different ROS trapping experiments were then carried out on both bulk g-C3N4 and g-C3N4 nanosheets to investigate the photocatalytic mechanism of g-C3N4. As shown in Figure 2c,d, hole scavenger TEOA significantly inhibited phenol degrada-
tion, implying that photogenerated holes are critically important for phenol degradation on both g-C3N4 catalysts. On the contrary, the phenol degradation efficiency was reduced by <2 and 15% with the addition of -OH scavenger tert-butyl alcohol over bulk g-C3N4 and g-C3N4 nanosheets, respectively, revealing that -OH radicals play a minor role in phenol degradation. Indeed, according to Figure 1d, because the valence band potential of both g-C3N4 catalysts is more negative than that of -OH/OH- (+1.99 V, vs NHE),21 the photogenerated holes are incapable of oxidizing adsorbed OH-groups into -OH radicals. This suggests -OH radicals generated in both g-C3N4 catalysts may result from molecular O2 reduction by photogenerated electrons. Because H2O2 is an important product of O2 reduction by photogenerated electrons (eqs 1-4), the effect of H2O2 scavenger catalase on the phenol degradation was also investigated. It was found that the phenol degradation efficiency in the presence of H2O2 scavenger was almost equal to that of -OH radical scavenger, implying that -OH was mainly from the decomposition H2O2. It is worth noting that although -OH radicals generated by molecular O2 reduction contributed only slightly to the degradation efficiency, molecular O2 activation process on g-C3N4 surface still significantly influences the following photocatalytic phenol degradation reaction (Figure 2a,b).
The selectively increased contribution of H2O2 and -OH on phenol degradation over g-C3N4 nanosheet implied distinct different molecular O2 activation processes on bulk g-C3N4 and g-C3N4 nanosheets surfaces. As illustrated in eqs 1-4, there are two possible pathways for oxygen reduction by photoexcited electrons: one is a sequential one-electron reduction process to •O2- and H2O2 and the other is a two-electron process to H2O2. To check out the molecular O2 activation pathways on g-C3N4 surfaces, the amount of -O2- and H2O2 produced on g-C3N4 bulk material and nanosheet under illumination was quantified by using an ROS-selective fluorescent or colorimetric probe (Figure 3). According to the aforementioned discussion, the possibility of H2O2 generated via photogenerated holes of g-C3N4 oxidation is ruled out due to the lack of •OH originating from the hole. If the generation pathways of H2O2 are the same via a sequential single-electron reduction of O2 on both g-C3N4 photocatalysts, more H2O2 should be generated from bulk g-C3N4 because more -O2- was detected. Unexpectedly, an interesting observation came from the markedly depressed •O2-generation and greatly enhanced H2O2 production after the exfoliation (Figure 3 and Figure S3 in the SI). This result further implies a significant change of O2 reduction process on the g-C3N4 surface after exfoliation. It can be temporarily assumed that H2O2 is generated by a sequential single-electron reduction pathway (O2 ^-O2- ^ H2O2) on bulk g-C3N4
surface, while one two-electron reduction of O2 to form H2O2 would occur on g-C3N4 nanosheets surface. The generated H2O2 could be converted into -OH radicals by either photogenerated electrons or visible-light irradiation and subsequently participate in the photocatalytic reaction.22 As shown in Figure S4 in the SI, more -OH was detected over g-C3N4 nanosheets because more H2O2 was produced, which accelerated the phenol photodegradation over g-C3N4 nanosheets. This result is consistent with the photocatalytic trapping experiments.
The selective pathway for molecular O2 reduction on the two photoexcited g-C3N4 catalysts was further validated by Raman spectroscopic measurements. As shown in Figure 4a, fresh
Figure 4. Raman spectra of g-C3N samples before and after 5 h of visible-light illumination in an O2 bubbling suspension. Inset shows the structure of 1,4-endoperoxide (intermediate of two-electron O2 reduction).
samples of the two photocatalysts exhibited similar Raman bands, the ones at 706 and 977 cm-1 corresponding to the breathing modes of triazine rings and the one at 750 cm-1 for the out-of-plane bending mode of the graphitic domains.23 After irradiation in a O2 bubbling suspension, a new broad band at 882 cm-1 was observed on g-C3N4 nanosheets, which is assigned to 1,4-endoperoxide.24 It is noted that the concentration of 1,4-endoperoxide in the suspension is low due to the subsequent conversion of this species to H2O2. A long irradiation time is needed to accumulate enough 1,4-endoperoxide for Raman spectroscopic measurements. Photo-excited generation of 1,4-endoperoxide species provides strong evidence of the occurrence of two-electron reduction of O2 on g-C3N4 nanosheet surface. In contrast, no such Raman band
was detected on the bulk g-C3N4. In a previous study, 1,4-endoperoxide was detected by Raman spectroscopy on photoactivated bulk g-C3N4 in the presence of ethanol.24 Presumably, the sacrificial electron donor quenched photogenerated holes and increased the yield of 1,4-endoperoxide. In the present work, 1,4-endoperoxide production was enhanced by exfoliating the photocatalyst without the use of any electron donor. This result confirms the switch of molecular oxygen activation from one-electron transfer to two-electron transfer by exfoliating bulk g-C3N4.
On the basis of the above results, a proposed mechanism of selective O2 activation process on photoexcited g-C3N4 samples is depicted in Scheme 1. On bulk g-C3N4, as reported by Shiraishi et al., without any additional agent or modifiers,24,25 one electron generated from g-C3N4 photoexcitation reduces O2 and produces surface-associated superoxo species, which then gets protonated and dissociates from the surface and converted into HO2- radical. In contrast, on g-C3N4 nanosheets, some superoxo species are reduced very rapidly by another photoelectron to produce a surface-stabilized intermediate, which subsequently convert into H2O2.25 The rapid formation of 1,4-endoperoxide removes two electrons almost simultaneously from g-C3N4 nanosheets and thus stabilizes more holes available for phenol oxidation. On the bulk g-C3N4, one-electron reduction process leads to less electron—hole separation and low H2O2 formation efficiency and subsequently reduces phenol degradation rate. The origin of preferred two-electron reduction of oxygen on g-C3N4 nanosheets is not clear yet, but we hypothesize that the surface energy of g-C3N4 increases after exfoliation, which indicates that g-C3N4 nanosheets may prefer to recombine with oxygen molecule to stabilize their states and thus facilities the formation of 1,4-endoperoxide. It is noted that although one-electron reduction is a much more common process due to the slow kinetics of two-electron reduction, however, the rapid formation of 1,4-endoperoxide on g-C3N4 nanosheets may accelerate the rate of two-electron transfer process and thus give rise to switch the oxygen activation from one-electron reduction to two-electron reduction process by exfoliating bulk g-C3N4. In addition, the 2D nanosheet structure with a thickness of <2 nm provides a much shorter distance than the bulk material for the photogenerated electrons to travel to the catalyst surface and then reduce superoxo before it is released from the surface. This process was further examined by the electrochemical reduction of O2 on g-C3N4 film electrodes (Figure S5 in the SI). On both types of g-C3N4 electrodes negligible current was
Scheme 1. Proposed Two Oxygen Reduction Pathways on Photoexcited Bulk g-C3N4 and g-C3N4 Nanosheet Surfaces
observed in the absence of O2. In an O2-saturated solution, reduction current with an onset voltage of ~80 mV (vs NHE) was observed on the bulk g-C3N4 electrode, indicating that O2 reduction reaction occurred on this surface. Importantly, larger O2 reduction current was observed on the g-C3N4 nanosheet electrode and the onset potential shifted positively by ~40 mV, suggesting the exfoliated g-C3N4 nanosheet greatly promoted O2 reduction. This is consistent with the enhanced ROS generation on g-C3N4 nanosheet due to the efficient two-electron transfer process, which contributes to their enhanced photocatalytic activity.
A cycling photocatalytic degradation experiment proceeded with 15 h of visible light irradiation to evaluate the stability and reusability of the g-C3N4 nanosheet photocatalyst. As shown in Figure S6 in the SI, the high photocatalytic activity of g-C3N4 nanosheet was well maintained during the five repeated cycles. The result demonstrated excellent stability of g-C3N4 nano-sheet for photocatalytic degradation. The high photocatalytic activity and good stability of g-C3N4 nanosheet endow it great potential for practical applications.
In summary, selective reduction of molecular oxygen on bulk g-C3N4 and g-C3N4 nanosheet was demonstrated. The bulk g-C3N4 prefers to reduce O2 to-O2— via one-electron reduction, while the photoexcited g-C3N4 nanosheet promotes the two-electron reduction of O2 to yield H2O2 due to the efficient formation of 1,4-endoperoxide species. The efficient two-electron reduction of O2 on the g-C3N4 nanosheet surface promotes hole generation and subsequently accelerates phenol oxidation degradation. These findings provide some new insights into the molecular O2 reactions on semiconductor surface during photocatalytic reactions, which could help us to design efficient photocatalysts for environmental applications.
■ ASSOCIATED CONTENT [я Supporting Information
Experimental methods, XRD patterns, photodegradation kinetics of phenol over bulk g-C3N4 and g-C3N4 nanosheet under visible light, -OH generation curves on both g-C3N4 samples, current—potential curves of the g-C3N4 electrodes, and cycling runs of g-C3N4 nanosheet for photodegradation of phenol. This material is available free of charge via the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION Corresponding Author
*Phone: 86 10-62849685. E-mail: LHGuo@rcees.ac.cn. Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the National Basic Research Program of China (2011CB936001) and National Nature Science Foundation of China (21321004, 21207146, 21177138, 21377142, and 21477146).
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