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A Metal-Free Photocatalyst with Visible-LightDriven Post-Illumination Catalytic Memory Qi Zhang, Hua Wang, Zhangliang Li, Cong Geng, and Jinhui Leng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 14, 2017

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ACS Applied Materials & Interfaces

A Metal-Free Photocatalyst with Visible-Light-Driven Post-Illumination Catalytic Memory Qi Zhang1, Hua Wang*1, 2, 3, Zhangliang Li2, Cong Geng1, Jinhui Leng1 1

School of Fisheries and Life Science, Dalian Ocean University, Dalian 116023, China. 2 Fujian Provincial Key Laboratory of Ecology-Toxicological Effects & Control for Emerging Contaminants, Putian 351100, China. 3 Key Laboratory of Mariculture & Stock Enhancement in North China’s Sea, Ministry of Agriculture, Dalian 116023, China.

Abstract A novel metal-free photocatalyst with post-illumination catalytic memory was fabricated by the graphitic carbon nitride (g-C3N4), carbon nanotubes (CNTs), and graphene (Gr), in which g-C3N4 acts as an efficient photocatalyst and the CNTs and Gr act as supercapacitors. The removal of phenol was achieved in the dark by post-illumination catalytic memory because the photocatalyst could store a portion of its photoactivity via photogenerated electrons in the CNTs and Gr under visible light illumination and then release the electrons again in the dark. Therefore, this metal-free photocatalyst is capable of operation in the dark for a broad range of applications. Keywords: metal-free; photocatalyst; catalytic memory; visible light; environmental applications

1 Introduction Because photocatalysis can convert solar energy into chemical energy for potential applications in environmental control technologies, the development of photocatalysts has received considerable attention in recent years.1-5 However, traditional photocatalysts have an inherent drawback in that they only function under 1

ACS Paragon Plus Environment


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light illumination. In the dark, many practical applications have been hampered by the fact that photocatalysts lose their activity. Therefore, the design of a photocatalyst that remains active even without light illumination is highly desirable. Recently, Li et al. discovered a photocatalyst based on PdO/TiO2 that exhibited catalytic capability in the dark,6 and the researchers named this catalytic capability post-illumination catalytic memory.7 In the mechanism of this memory, photogenerated electrons are trapped by PdO under light illumination and then released when the light illumination is turned off. To date, several photocatalysts including Cu2O/TiO2,8 Cu2O/SnO2,9 iodine-modified TiO2,10 and Bi nanoparticles11 have been found to exhibit post-illumination catalytic memory. However, these reported photocatalysts with post-illumination catalytic memory are metal-based semiconductors, so alternative metal-free photocatalysts are being pursued. Graphitic carbon nitride (g-C3N4) has been reported as a distinguished metal-free photocatalyst for the elimination of organic and inorganic environmental pollutants based on its cost effectiveness and solar energy utilization.12-14 In a previous study, we successfully fabricated an atomic single layer g-C3N4 and discovered that this metal-free photocatalyst exhibited excellent photocatalytic capability for the removal of ammonia from water under visible light irradiation.15 Unfortunately, g-C3N4 alone does not display post-illumination catalytic memory. Continuing with the development of carbon nanomaterials, graphene and carbon nanotubes (CNTs) are often referred to as supercapacitors due to their high electron-storage capacities and good electrical conductivity.16-18 Recently, Yu and Dai fabricated a self-assembled 2


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graphene/CNT hybrid film with an outstanding supercapacitive performance.19 Additionally, Kongkanand and Kamat reported that CNTs can accept and store electrons from photoirradiated TiO2.20 Motivated by the above findings, we predicted that a metal-free photocatalyst with post-illumination catalytic memory could also be formed from g-C3N4 and CNTs/graphene because the atomic single layer g-C3N4 could beneficially attach to the CNTs/graphene, which could store and shuttle photogenerated electrons. To date, there have been no reports on the fabrication of a metal-free photocatalyst with efficient post-illumination catalytic memory. Therefore, in the present work, a g-C3N4/CNTs/graphene (CN-CNT-Gr) photocatalyst was prepared by a hydrothermal method and the post-illumination catalytic memory was evaluated.

2 Experimental section 2.1 Synthesis of metal-free memory photocatalyst Graphene oxide was fabricated based on the Hummers’ method.21 Single layer g-C3N4 was prepared based on our previous report.15 Multi-walled CNTs were obtained from Sigma-Aldrich and acid-oxidized before use. Typically, a 10 mL suspension of CNTs (15 mg•mL−1) was mixed with a 10 mL of suspension of graphene oxide (5 mg•mL−1), denoted as CNT-Gr, and the mixture was treated under ultrasonication for 30 min. A series of CN-CNT-Gr catalysts were synthesized by mixing the as-prepared single layer g-C3N4 and CNT-Gr in a mass ratio of 1:1, 2:3 and 1:2 (denoted as CN-CNT-Gr1, CN-CNT-Gr2 and CN-CNT-Gr3, respectively). Next, the aforementioned mixtures were added to a solution of water and isopropanol 3



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(v:v = 60:40). The mixed solution was sonicated and transferred in a 100 mL of Teflon-lined autoclave, followed by the hydrothermal reaction at 150 °C for 24 h. Finally, the obtained products were washed and freeze-dried overnight. Scheme 1 presents the fabrication process of CN-CNT-Gr.

Scheme 1. Illustrative preparation process of CN-CNT-Gr.

2.2 Characterization The properties of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, specific surface area, X-ray diffraction (XRD) patterns, Raman spectra, Fourier transform infrared (FTIR) spectra, X-ray photoelectron spectroscopy (XPS) spectra, UV-vis absorption spectra, diffused reflectance spectroscopy (DRS) spectra, photoluminescence (PL) spectra, Mott-Schottky plots and Electrochemical impedance spectroscopy (EIS) were characterized on the basis of our previous reports.15, 22 2.3 Photocatalytic removal of phenol The photocatalytic removal of phenol was carried out in a cylindrical quartz container (volume of 100 mL). Typically, 40 mg of catalyst and 100 mL of 5 mg•L-1 phenol was added in quartz container for 1 h in the dark before illumination. Then, the quartz container was placed in front of a Xe lamp (CHF-XM35) with a cut filter of 4


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visible light (λ ≥ 400 nm). The light source provided an incident light intensity of 100 mW•cm-2, which was measured by a radiometer (model FZ-A). 2.4 Memory effect evaluation The memory capability of the CN-CNT-Gr nanostructures was tested for the degradation of phenol without light irradiation. Briefly, CN-CNT-Gr2 was illuminated by lamp for 1, 3, 5 and 7 h. Then the CN-CNT-Gr2 samples were used to carry out phenol degradation experiments in the dark. About 1 mL of the reaction solution was withdrawn every 10 min. The concentration of phenol was measured by high-performance liquid chromatography (HPLC, Waters). In addition, the memory capability of catalysts (CN-CNT-Gr1, CN-CNT-Gr2 and CN-CNT-Gr3) that were irradiated for 5 h was also evaluated via the degradation of phenol in the dark.

3 Results and discussion (b)

(a) g-C3N4

CNTs

g-C3N4

Gr

CNTs Gr

500 nm

2µm

5



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(c)

(d)

Figure 1. (a) SEM and (b) TEM images of CN-CNT-Gr; (c) pore-size distribution curve. Inset: N2 adsorption isotherm recorded for CN-CNT-Gr; (d) XRD of CN-CNT-Gr.

The SEM image in Figure 1a demonstrates that CN-CNT-Gr possesses a porous, wrinkled and fluffy microstructure that is orderly stacked by bridging CNTs. The pores are highly interconnected, and the pore wall is composed of randomly oriented single layer g-C3N4 and Gr. Moreover, Figure 1a illustrates that the single layer g-C3N4 is tightly attached to Gr by CNTs, forming a “sheet-CNT-sheet” structure. The TEM image in Figure 1b revealed a well-connected porous network composed of CNT-Gr coupled with single layer g-C3N4. Meanwhile, Figure 1b further demonstrated that the single layer g-C3N4, Gr and CNTs that constitute the CN-CNT-Gr frameworks are transparent flakes approximately several micrometers in size, and some corrugation and scrolling is observed. Note also that the characteristic structural features of g-C3N4 and Gr bridged by CNTs are well preserved in the CN-CNT-Gr nanocomposite. As depicted in Figure 1c, two peaks at 2.1 and 3.7 nm were observed for CN-CNT-Gr in the pore-size distribution curve, which are likely attributable to the 6


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inner cavity diameter of the CNTs.23 In addition, CN-CNT-Gr exhibited a surface area of 168 m2•g-1 (see the inset of Figure 1c). As illustrated in Figure 1d, the XRD spectrum of CN-CNT-Gr exhibited a sharp peak at 26.2° corresponding to the (002) reflection of CNT.24 A shoulder peak at 27.4° was also observed, which indicated the preservation of highly crystalline g-C3N4.25 A small diffraction peak at approximately

10.8°,

corresponding

to

the

(002) reflection

of

Gr,

was

significantly weakened in CN-CNT-Gr.26 In addition, a diffraction peak appeared at approximately 42.5°, which corresponded to the (100) reflection of functionalized CNT.

(a)

(b)

Figure 2. (a) Raman and (b) FTIR spectra of CN-CNT-Gr.

Figure 2a presents the Raman spectra of functionalized CNTs, Gr and CN-CNT-Gr. The Raman spectrum of the functionalized CNTs exhibited a D band at 1321 cm-1 and a G band at 1575 cm-1.27 The spectrum of Gr contained D and G bands located at 1345 cm−1 and 1589 cm−1, respectively,28 and their intensities were approximately equal. For CN-CNT-Gr, although D and G bands were observed, their peak positions were shifted to a lower frequency as well as an increment in the D/G 7



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intensity ratio. Figure 2b exhibits the FTIR spectra of functionalized CNT, Gr, g-C3N4 and CN-CNT-Gr. The FTIR spectrum of g-C3N4 presented three characteristic absorption bands at < 1000, 1200-1650 and > 3000 cm-1, which are attributed to the breathing mode of s-triazine, the stretching mode of the N─H bond and heterocycles, respectively.29 In the case of Gr, an intense band at approximately 3460 cm−1 corresponds to O─H stretching vibrations. The other peaks at 1724, 1629, 1464, 1256 and 1072 cm-1 are attributed to the C═O, C─O, O─H, epoxy groups and C─O, respectively.30, 31 The typical bands in g-C3N4 were preserved after coupling Gr and functionalized CNT. In particular, the appearance of the peaks at 1675 and 1543 cm-1 implies the formation of covalent bonds between Gr and g-C3N4, which favors the stability of CN-CNT-Gr. Note that several characteristic bands exhibited blue shifts compared to those in g-C3N4; the peak at 1390 cm-1 shifted to 1371 cm-1, and the peak at 1538 cm-1 shifted to 1522 cm-1, which can be ascribed to interactions between the single layer g-C3N4 and CNTs. Meanwhile, the peak at 1630 cm-1 in CN-CNT-Gr that corresponds to the C═C group was confirmed in the CNTs. A new band appeared at 1730 cm-1 can be attributed to the carboxyl groups on the surface of the functionalized CNTs. These –COO– groups on the CNTs can interact with the single layer g-C3N4 and Gr through electrostatic interactions, which, in combination with the π-π stacking interaction, is responsible for the self-assembly of the CN-CNT-Gr architecture.

8


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(a)

(b)

(c)

(d)

Figure 3. XPS spectrum of (a) CN-CNT-Gr and high-resolution of (b) C 1s, (c) N 1s and (d) O 1s.

Figure 3a depicts the XPS spectrum of CN-CNT-Gr containing strong coupling between the single layer g-C3N4, Gr and CNTs, in which the presence of C, N and O is observed. The C 1s curve of CN-CNT-Gr (Figure 3b) was deconvoluted into six peaks based on Gaussian curve fitting, and these peaks correspond to the sp2 C─C bond (284.9 eV), C─O bond (286.8 eV) and sp2-bonded carbon in the N-containing aromatic rings (N─C═N, 288.1 eV), and the C═C (284.6 eV), C─OH (285.7 eV), and COOH (289.1 eV) species in the functionalized CNTs.32 The peaks at 284.9 and 288.1 eV were also present in the C 1s curve of g-C3N4 and corresponded to the major carbon species in g-C3N4.33 The middle peak at 286.8 eV was not present in g-C3N4 9



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and is attributed to the residual C─O bond from Gr after the hydrothermal process.29 The high-resolution N 1s spectra of CN-CNT-Gr was separated into two major peaks centered at 398.3 and 399.8 eV (Figure 3c). The peak at 398.3 eV corresponds to the sp2-bonded N atoms in the triazine rings; this peak split to form a new shoulder peak at 397.6 eV, which was caused by the interaction between the functionalized CNTs and the N atoms in the g-C3N4.34 The peak at 399.8 eV is attributed to the bridging N atoms or the N atoms bonded to H atoms. Another peak at 403.3 eV resulted from the protonation of g-C3N4.35 The two peaks at 398.3 and 399.8eV, together with the aforementioned sp2-bonded C (N─C═N), confirm the existence of the heptazine heterocyclic ring (C6N7) units, which is the elementary building block of g-C3N4. The presence of the C─O bond was further confirmed by the peak (531.4 eV) in the O 1s XPS spectrum (Figure 3d).

(b)

(a)

Figure 4. (a) DRS and (b) PL spectra of CN-CNT-Gr samples.

The DRS in Figure 4a indicates that pristine g-C3N4 contains an absorption edge of 430 nm, which corresponds to an optical bandgap of 2.90 eV.36 The CN-CNT-Gr samples exhibited a similar absorption edge as g-C3N4, which suggests 10


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that the intrinsic band gap of CN-CNT-Gr originated from the electron transitions from the valence band to the conduction band of g-C3N4. Meanwhile, an enhanced background absorption was observed from 420 to 700 nm, and the absorption intensity increased with increasing CNT-Gr content in the CN-CNT-Gr samples. Figure 4b depicts the PL spectra of the CN-CNT-Gr samples. All of the PL spectra of the CN-CNT-Gr samples are significantly lower in intensity than those of g-C3N4, which indicates that the recombination of photogenerated electron-hole pairs was inhibited.37 CNT and Gr are known to be good shuttles for electrons and can serve as a “bridge” for the photogenerated electron transfer. Moreover, the degree of inhibition gradually increased with increasing CNT-Gr content in CN-CNT-Gr.

Figure 5. Current-voltage (I-V) characteristics of g-C3N4, CN-CNT-Gr1 and CN-CNT-Gr2 measured with a micromanipulator manual probe station. Inset: I–V characteristics of the CNTs and CN-CNT-Gr3.

The electrical characteristics of the CN-CNT-Gr heterojunction were determined 11



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using a micromanipulator manual probe station,38 as shown in Figure 5. As depicted in the inset of Figure 5, the I–V plot of the CNTs was a straight line that passed through the zero point, which indicated that the CNTs were good conductors. The current of g-C3N4 was almost zero, whereas the I–V plot of CN-CNT-Gr3 displayed the same tendency as the CNTs since the CNT-Gr content increased, resulting in the disappearance of the interface barrier and the creation of a low-resistance ohmic contact between CNT-Gr and g-C3N4. However, the shape of the I–V plot changed from a straight line to an asymmetric curve for the CN-CNT-Gr2 sample. The asymmetry of the I–V plot for the CN-CNT-Gr2 sample demonstrated that a heterojunction

had been

formed

between

g-C3N4

and

CNT-Gr.

The

obvious asymmetry of the I–V plot was also observed for the CN-CNT-Gr1 sample. Note also that the space charge region of the heterojunction could provide an additional function of reducing the recombination of photogenerated electrons and holes.39 Furthermore, the separated electrons were easily transferred to CNT-Gr. To further evaluate the electrical performance, the specific capacitance (C) values were calculated from the CV curves using the equation (1):40

=

dV (1) 2 ∆V

where C is the specific capacitance of the active materials (F•g-1), dV the integrated area of the CV curve, v is the scan rate (V•s−1), ∆V is the absolute value of the potential window (V), and m is the total mass of the active material (g). According to equation (1), the specific capacitance values of CN-CNT-Gr1, CN-CNT-Gr2 and CN-CNT-Gr3 were 18.9, 31.8 and 38.4 F•g-1, respectively (Table 1). The specific 12


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capacitance value of the CN-CNT-Gr samples clearly increased with increasing CNT-Gr content. Based on the report of Yu and Dai,19 a graphene/carbon nanotube nanocomposite exhibited a specific capacitance value of 120 F•g-1 even at an exceedingly high scan rate of 1 V/s. This value is higher than our CN-CNT-Gr nanocomposites.

In

addition,

Niu

et

al.41

reported

that

supercapacitor

electrodes prepared from catalytically grown multiwalled CNTs exhibited a maximum specific capacitance of 113 F•g-1. Meanwhile, Xue et al.42 reported that GO–rGO patterned polyethylene terephthalate electrodes demonstrated capacitances of up to 141.2 F•g-1. Hence, both CNTs and Gr have good specific capacitance. Table 1. Specific capacitance values of the CN-CNT-Gr samples.

Sample

Specific capacitance (C) (F•g-1)

CN-CNT-Gr1

18.9

CN-CNT-Gr2

31.8

CN-CNT-Gr3

38.4

(a)

(b)

Figure 6. (a) Mott-Schottky plots of g-C3N4 and CN-CNT-Gr2; (b) electrochemical impedance spectroscopy plots of g-C3N4 and CN-CNT-Gr2.

13



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To compare the charge carrier density of CN-CNT-Gr2, Mott-Schottky plots were measured. The charge carrier density ND was calculated according to the equation (2):

= 2⁄( ) (2) where e is the elementary electron charge, is the dielectric constant, is the permittivity in vacuum, and k is the slope of the C-2 versus potential plot. The slopes (k values) corresponding to the linear portions of the Mott-Schottky plots (Figure 6a) for g-C3N4 and CN-CNT-Gr2 were 4.06×105 and 1.38×105, respectively. These results indicates that the carrier density of CN-CNT-Gr2 was 2.9 times greater than that of g-C3N4. This high carrier density in CN-CNT-Gr2 would facilitate charge transport. The charge transport resistance was characterized by electrochemical impedance spectroscopy (EIS). The EIS plots of g-C3N4 and CN-CNT-Gr2 are displayed in Figure 6b. The arcs of both g-C3N4 and CN-CNT-Gr2 are observed to be very large in the dark. The radii of the semicircles under visible light irradiation are significantly smaller than those in the dark. Further, Figure 6b reveals that the semicircle for CN-CNT-Gr2 is smaller than for g-C3N4 under light irradiation, which indicates that the electron transfer resistance of CN-CNT-Gr2 is lower than that of g-C3N4.

14


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Figure 7. Photocatalytic degradation of phenol (C0 = 5 mg•L−1) under visible light irradiation.

The photocatalytic activities of the as-prepared CN-CNT-Gr nanocomposites with different ratios were evaluated using phenol as a target under visible light irradiation. Approximately 8.6% of the phenol was removed by absorption to CN-CNT-Gr2 in the dark. As illustrated in Figure 7, the concentration of phenol remained nearly unchanged under light irradiation, suggesting that the photolysis of phenol was negligible under the present experimental conditions. After irradiation for 60 min, 12% of the phenol was degraded by the simple g-C3N4 catalyst. The degradation efficiency of phenol in the photocatalytic process was greater than 43% after 60 min when using the CN-CNT-Gr2 catalyst, whereas 22% and 30% of the phenol was removed using the CN-CNT-Gr1 and CN-CNT-Gr3 catalysts, respectively. Under the same experimental conditions, the degradation efficiencies of phenol with the CN-CNT-Gr nanocomposites were all higher than that of the simple g-C3N4 catalyst. This is because after the hole-electron pairs are photogenerated, they are separated by the internal electrostatic field in the heterojunction region between 15



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g-C3N4 and CNT-Gr, and the separated electrons are easily transferred from g-C3N4 to CNT-Gr; thus, the recombination of the photogenerated charge carriers is minimized. Moreover, the degradation efficiency of phenol by the CN-CNT-Gr2 catalyst was 2.0 and 1.4 times greater that of the CN-CNT-Gr1 and CN-CNT-Gr3 catalysts, respectively. Hence, for the given experimental conditions, there is an optimum CNT-Gr content in the CN-CNT-Gr nanocomposites for photocatalytic applications. As the mass ratio of g-C3N4 to CNT-Gr reached 0.67, the photocatalytic activity of CN-CNT-Gr nanocomposites gradually increased with increasing CNT-Gr content. However,

further

increasing

the

CNT-Gr

content

clearly

decreased

the

photocatalytic activity, because the overloaded CNT-Gr may cover the surface and prevent the active sites on g-C3N4 from efficiently utilizing the visible light.43

(a)

(b)

Figure 8. Phenol (C0 = 5 mg•L−1) removal in the dark (a) with CN-CNT-Gr2 after prior illumination for 1, 3, 5 and 7 h; (b) with g-C3N4 and different CN-CNT-Gr samples after prior illumination for 5 h.

In this experiment, the post-illumination catalytic capability of CN-CNT-Gr2 for the removal of phenol in the dark was investigated. The CN-CNT-Gr2 catalyst was 16


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first illuminated for 1, 3, 5 and 7 h. Next, the illuminated CN-CNT-Gr2 catalyst was used to conduct catalytic phenol degradation in the dark under the same experimental setup. Figure 8a depicts the phenol removal by CN-CNT-Gr2 in the dark after varying periods of illumination. All of the pre-illuminated CN-CNT-Gr2 catalysts removed phenol in the dark. After 60 min in the dark, the degradation efficiency of phenol was approximately 25.5% for the CN-CNT-Gr2 catalyst that had been illuminated for 5 h, whereas 16.7%, 21.2% and 25.9% degradation was observed for illumination periods of 1, 3 and 7 h, respectively. Moreover, the removal of phenol by the photocatalyst that was irradiated for 7 h was only slightly higher than that of the photocatalyst that was irradiated for 5 h, meaning that excess illumination above 5 h did not markedly improve the post-illumination catalytic capability. Although the degradation efficiency in the dark was lower than that under visible light illumination, phenol was removed in the dark using CN-CNT-Gr2 as a catalyst. This observation clearly demonstrates that the post-illumination catalytic capability of CN-CNT-Gr in the dark relied on its “memory” of the visible light illumination. In the CN-CNT-Gr catalysts, photogenerated electron storage and release occurred when the visible light illumination was on and off, respectively. The supercapacitor property of CNT-Gr allowed it to store the electrons generated by g-C3N4 under light illumination and then release them to the surface of CN-CNT-Gr in the dark. Figure 8b presents the removal of phenol by the simple g-C3N4 catalyst and different CN-CNT-Gr samples after the same period of illumination (5 h). No post-illumination effect was observed for the simple g-C3N4 catalyst in the dark. Compared to simple g-C3N4, the addition of 17



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CNT-Gr enhanced the “memory” capability of CN-CNT-Gr. The removal of phenol by the CN-CNT-Gr2 catalyst that had been illuminated for 5 h was approximately 25.3%, which was 1.2 and 1.1 times greater than that of CN-CNT-Gr1 (20.6%) and CN-CNT-Gr3 (22.9%), respectively. This result is consistent with the above I-V characterization. The CN-CNT-Gr2 catalyst that was illuminated for 5 h appears to contain an optimum content of CNT-Gr. When the CNT-Gr content was further increased, a drop in the phenol removal occurred. This indicates that too high of a CNT-Gr content is harmful to the removal of phenol. Because the overloaded CNT-Gr may cover the surface of g-C3N4 and shade the photocatalyst from light, the “memory” activity may also decrease. In addition, the reusability of the CN-CNT-Gr2 catalyst that was irradiated 5 h was evaluated based on phenol degradation. After six consecutive cycles, no obvious deactivation of the post-illumination catalytic capability was observed, which indicates that the CN-CNT-Gr2 possesses sufficient stability.

18


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Figure 9. Phenol (C0 = 5 mg•L−1) removal in the dark with CN-CNT-Gr2 that was illuminated for 5 h upon the addition of of AgNO3.

Generally, photogenerated holes and electrons can transfer to the surface of a catalyst to participate in the surface reaction of the photocatalytic process. Based on previous reports, the standard redox potential of g-C3N4 is more negative than that of •OH/OH−,44 meaning that the hydroxyl ion (OH−) cannot be oxidized by the holes from the valence band of g-C3N4 to form •OH. However, •OH may be produced from the photogenerated electrons in the conduction band of g-C3N4. Thus, a possible pathway for the photocatalytic oxidation of phenol is an indirect oxidation process by •OH that was formed from photogenerated electrons. To further illustrate the degradation mechanism of CN-CNT-Gr2 in the dark, a radical trapping experiment was performed to explore the reactive radical species that is involved in the removal of phenol by the irradiated (5 h) CN-CNT-Gr2 catalyst. Silver nitrate (AgNO3) was selected as an electron scavenger in this study. As indicated in Figure 9, when AgNO3 was added, the post-illumination catalytic memory capability was remarkably inhibited. This result implies that photogenerated electrons were the crucial active species in the removal of phenol in the dark. Incorporation of CNT-Gr on single layer g-C3N4 resulted in electrons from g-C3N4 that were photogenerated with visible light being stored in CNT-Gr and then released in the dark. The crucial •O2− radicals are generated by reduction of O2 via the separated electrons. Subsequently, •OH radicals are generated from the •O2− radicals.45

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Scheme 2. Schematic diagram illustrating the removal of phenol in the dark by the CN-CNT-Gr2 catalyst that was illuminated for 5 h.

To clearly elucidate the role of CNT-Gr within CN-CNT-Gr in the removal of phenol, a catalytic mechanism is proposed. As illustrated in Scheme 2, the incident light from irradiation of the CN-CNT-Gr catalyst with visible light can be directly absorbed by CN-CNT-Gr. Next, the single layer g-C3N4 component is excited to produce electrons in the CB (Eq. (3) and (4)). Subsequently, the photogenerated electrons transfer to the CNT-Gr surface (Eq. (5)). These separated electrons are easily trapped by O2 to produce superoxide radicals (•O2−) (Eq. (6)). g– C N ( ! !"#) → g– C N ( % & )

(3)

g– C N ( % & ) → ′

(4)

g– C N ( % ) CNT– Gr → g– C N CNT– Gr( % )

(5)

In these equations, % and & are electrons and holes, and and ′ are the energies of absorbed and emitted photons, respectively (h: Planck’s constant). 20


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Interestingly, a portion of the photogenerated electrons are trapped and stored into the CNT-Gr. When the visible light is switched off, these stored electrons are released again to the catalyst surface, where they can react with oxygen/water to produce •OH radicals (Eq. (7)) by the following reaction: CNT– Gr( % ) O, → • O% ,

(6)

2 • O,% 2H& → 2 • OH O,

(7)

Therefore, the formation of •OH could effectively degrade phenol.

4 Conclusion Here, a metal-free CN-CNT-Gr photocatalyst with post-illumination catalytic “memory” capability was prepared using CNT-Gr as a supercapacitor. The CN-CNT-Gr photocatalyst exhibited post-illumination catalytic “memory” capability for phenol removal. This function was attributed to the storage of photogenerated electrons in CNT-Gr following transfer from g-C3N4 under light irradiation and subsequent release of the electrons in the dark to maintain the catalytic activity without light illumination. Therefore, it is desirable to design other metal-free photocatalyst systems that can store a portion of their photocatalytic activity as “memory” under visible light irradiation so that the catalysts can remain active for an extended period of time in the dark.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. 21



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Including detailed experimental procedures of CN-CNT-Gr.

Author Information Corresponding Authors *E-mail address: [email protected] (H. Wang)

Notes The authors declare no competing financial interest.

Acknowledgements We greatly appreciate the support of the Scientific Research Project of Liaoning Provincial Department of Education (L201603) and the Open Foundation of Fujian Provincial Key Laboratory of Ecology-Toxicological Effects & Control for Emerging Contaminants (PY16005).

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Decorated Graphitic Carbon Nitride as an Efficient Metal-Free Photocatalyst for Phenol Degradation. Appl. Catal., B 2016, 180, 656–662.

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