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Plasmonic Graphene-like Au/C3N4 Nanosheets with Barrier-Free Interface for Photocatalytically Sustainable Evolution of Active Oxygen Species Shujuan Jiang, Chuanbao Xiong, Shaoqing Song, and Bei Cheng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04338 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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ACS Sustainable Chemistry & Engineering

Plasmonic Graphene-like Au/C3N4 Nanosheets with Barrier-Free Interface for Photocatalytically Sustainable Evolution of Active Oxygen Species Shujuan Jiang†, Chuanbao Xiong†, Shaoqing Song†‡*, Bei Cheng‡* School of Material Science and Chemical Engineering, Ningbo University, Fenghua

†

Road 818, Ningbo, 315211, PR China State Key Laboratory of Advanced Technology for Materials Synthesis and

‡

Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070, P. R. China *E-mail: [email protected] (S. Song), and [email protected] (B. Cheng)

ABSTRACT: Graphene-like carbon nitride supported plasmonic Au NPs with physical barrier-free interface (Au/C3N4) was in situ synthesized by one-step polymerization of the homogeneous mixture of HAuCl4 and urea. The plasmonic graphene-like structure of Au/C3N4 with the physical barrier-free interface enhances the visible-light capturing capability, increases the redox potentials, and facilitates the directional transfer of electrons from N 2p of C3-N species in g-C3N4 to Au in the photocatalytic procedure, which greatly promotes the activity of Au/C3N4 for the evolution of ∙O2-, ∙OH, and H2O2 species. The optimal Au/C3N4 sample provides the highest photocatalytic efficiency of active oxygen species, obtaining 31 (∙OH), 68 (∙O2-), and 990 µmol.L-1h-1 (H2O2) without scavenger under visible light, respectively. The work supplies a new approach to design efficient graphene-like structure with 1

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physical barrier-free interface for photocatalytically enhancing sustainable solar conversion. KEYWORDS: Carbon nitride, Plasmonic graphene-like structure, Active oxygen species, Barrier-free interface, Directional transfer

INTRODUCTION Active oxygen species (AOS), e.g., superoxide (.O2-), hydroxyl (.OH), and hydrogen peroxide (H2O2), as efficiently active and green oxygenants, are of great importance for environmental, and biological chemistry.1,2 It is known that AOS can be basically yielded during photocatalytic process, i.e., photo-generated electrons (e-) reduce O2 to .O2-, and holes (h+) oxidize H2O and/or -OH into .OH under light excitation.1,3-5 However, the activities of photocatalysts are generally restricted by following factors: (a) the absorbed photon energy ( hν ) should be larger than energy gap ( Eg) of photocatalyst; (b) the redox potentials of O2/.O2-, and H2O,-OH/.OH should be located between potentials of conduction band (CB) and valence band (VB).6,7 The former factor proposes that narrowed band-gap energy will enhance efﬁcient absorption and conversion of solar light, nevertheless, the latter factor suggests that lower CB potential and higher VB potential can thermodynamically promote the reduction and oxidation of O2, and H2O, respectively.8,9 The require will enlarge gap energy, which results in reducing light absorption ability. It can be seen that these requires are mutually exclusive. Consequently, it is scarcely possible to simultaneously have good light absorption ability and strong redox capability over 2


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single component photocatalyst.10 Graphitic

carbon

nitride

(g-C3N4),

a

typical

visible-light-response

bulk

photocatalyst containing π-conjugated tri-s-triazine layers, has been used to photocatalytically yield AOS and eliminate pollutants.11-20 Recently, Wang reported that photo-excited electrons reacted with O2 to yield .O2-, which improved its photocatalytic efficiency for elimination of Escherichia coli (E. coli) cells, and organic pollutants.21 Guo reported that the photo-induced electrons on CB were captured with O2 to form .O2-, and O2- whereafter reacted with H2O to ultimately yield H2O2.12 Kim and Choi et al. found H2O2 yield efficiency was enhanced over g-C3N4 modified with anthraquinone.22 Zhao et al. reported that photocatalytic rate for H2O2 yield was obviously increased over g-C3N4 with using SiW11 as cocatalysts.23 However, g-C3N4 suffers from low specific surface area, poor visible absorption, high recombination rate of photo-induced charges, and a low amount of surface active sites.24 It has been suggested that the layered bulk g-C3N4 by thermal polymerization of nitrogenous organic solid compounds arises from hydrogen bonding between amine groups on polymeric melon units.25 When interlayer interaction of g-C3N4 is weakened, g-C3N4 will be in the form of polymeric melon with a graphene-like layered structure, which leads to the exposure of basal planes.24 Such graphene-like g-C3N4 is beneficial to photon energy harvesting and reactants diffusion.26 Therefore, researchers have devoted to suppressing bulk growth and promoting the formation of graphene-like g-C3N4 by interrupting hydrogen bonds to weaken interlayer interactions.25,27 Currently, important strategies (e.g., oxygen and/or ammonia etching, 3



and strong acid exfoliation are utilized to solve challenge for synthesis of graphene-like g-C3N4,28 and the as-prepared g-C3N4 samples present inhomogeneous layer sizes and a large number of defects, which weakens the effective transfer and utilization of photo-excitation electrons.26 Moreover, as the layer size of g-C3N4 becomes smaller, the quantum size effect will result in enlarged transition energy of electron by the oppositely shifting LUMO and HOMO,25 which weakens the photon absorption of g-C3N4. Plasmonic metal nanoparticles (NPs) with surface plasmon resonance (SPR) can broaden intensity and wavelength from visible to near-infrared light and photo-charge transfer of g-C3N4.29 Under visible irradiation, the excited electrons on plasmonic NPs experience a collective oscillation, which results in the enhanced electron transfer from valence band (VB) to conduction band (CB).30,31 Therefore, the contact between plasmonic NPs and g-C3N4 plays a key role in the transfer of photo-induced charges, however, chemical preparation generally results in plasmonic NPs with inadequate interface contact.32,33 Therefore, the preparation of g-C3N4 and its physical barrier-free interface with plasmonic NPs are important for the light absorption, photo-excited carriers transport as well as effective utilization. Herein, we in-situ synthesized graphene-like g-C3N4 supported plasmonic Au NPs with barrier-free interface by polymerizing the homogeneous mixture of HAuCl4 and urea, and the synthesis process is shown in scheme 1. On one hand, Au NPs as “pillars” inhibit the superposition of π-conjugated tri-s-triazine layers during polymerization; on the other hand, the released Cl2 by HAuCl4 decomposition may simultaneously break hydrogen bonds and oxidize layers of g-C3N4, resulting in the 4


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formation of plasmonic graphene-like g-C3N4 (Au/C3N4) with physical barrier-free interface. DFT and experimental results show that, as we have conceived, the plasmonic graphene-like structure with physical barrier-free interface favors photo-induced electrons from N 2p of C3-N species in g-C3N4 to Au under visible-light irradiation, and Au/C3N4 samples present high photocatalytic activities for yielding

OH,

.

O2-, and H2O2 species in the photocatalytic process.

.

Oxygen-trapping EPR spectra, Mott-Schottky measurement, and scavenger-effect tests reveal the photocatalytic principle of photo-induced charges for the evolution of these active oxygen species. Accordingly, the work provides new insights and a simple route for obtaining efficient photocatalyst for environmental science and biochemistry.

EXPERIMENTAL SECTION Construction of Plasmonic Graphene-like Au/C3N4 Photocatalysts Plasmonic graphene-like Au/C3N4 was designed by directly heating the homogeneous mixture of carbamide and HAuCl4. Typically, 10 g carbamide was dispersed into 25 mL HAuCl4 solution and stirred for 45 min, and then ultrasonicated for 25 min. The mixture was subsequently dehydrated at 80 oC for 2 h. The dried samples were crushed and placed on the corundum porcelain boat of muffle furnace for 2 hours of heating at 540 oC with 5 oC min-1 rate. The as-prepared samples were named as Au/C3N4-1, Au/C3N4-2, and Au/C3N4-3 corresponding to the concentration of HAuCl4 of 3, 5, and 7 mg/mL, respectively. In the meanwhile, bulk g-C3N4 was 5



synthesized by directly heating carbamide at 540 oC for 2 h. Moreover, 2.5 wt.% Au-supported g-C3N4 (Au/g-C3N4) was synthesized through the deposition precipitation, and the characterization results are shown in Figure S1-4 of Supporting Information. 2.5 wt.% Au-supported graphene (Au/GR) was also synthesized according to previous report.34

Photocatalytic Evolution of Active Oxygen Species Active oxygen species were detected by oxygen-trapping EPR spectra. Typically, 10 mg Au/C3N4 sample was homogeneously distributed in 1.04 mL aqueous DMPO solution (0.04 mL DMPO and 1 mL H2O). After illuminating for 2 min under a 300 W Xe Lamp, EPR spectrometer was utilized to examine DMPO-.OH, and the characterization process was also used to the detection of DMPO-.O2- was also used the same method with using CH3OH instead of H2O. Photocatalytic efficiency of .O2and .OH was measured by probing nitroblue tetrazolium (NBT) absorbance and terephthalic acid (TA) photoluminescence. NBT, showing an obvious absorbance at 260 nm, reacts with .O2- in the molar ratio of 1:4 and therefore determine the amount of .O2-. Moreover, TA reacts with .OH in a molar ratio of 1:1 to form TA-∙OH with strong fluorescence at 425 nm. Photocatalytic H2O2 evolution over Au/C3N4 samples was examined. 40 mg sample was dispersed into Schlenk flask (100 mL) containing 30 mL H2O and 3 mL IPA. Subsequently, O2 was bubbled into the reaction solution for 60 min before irradiation, and the mixture solution was then irradiated (λ ≥ 420 nm). H2O2 evolution amount 6


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was probed through the discoloration titration with the acidic KMnO4 (0.2 mmol L-1) and the consumed amount of KMnO4 was the equal of H2O2 amount. RESULTS AND DISCUSSION Crystal Phase and Morphology In Figure 1, Au/C3N4 photocatalysts demonstrate the diffraction signal characteristics of pristine g-C3N4, i.e., (100) plane at 13.1o and (002) plane at 27.3o.35-37 The former reflects the in-plane repeating units of tri-triazine, and the latter is the interlayer stacking of conjugated tri-s-triazine polymer. Moreover, peaks at 38.1o and 44.5o arise from (111) and (200) diffraction planes of Au with face-centered cubic structure,33,38 respectively. These results suggest that Au/C3N4 samples preserve the π-conjugated tri-s-triazine structure with Au formation. Noticeably, characteristic signals of g-C3N4 at 13.1o remain approximately constant, suggesting that Au NPs are not located in the vacancy or not bonded with N atoms of the conjugated tri-s-triazine vacancy. Furthermore, the peak at 27.3o for (002) diffraction plane of g-C3N4 presents obvious shift to smaller value (i.e., 27.2, 27.1 and 27.0o) with increase of Au content corresponding to Au/C3N4-1, Au/C3N4-2 and Au/C3N4-3, respectively. On the basis of Bragg equation, interlayer spacing was enlarged from 0.326 nm in g-C3N4 to 0.329 nm in Au/C3N4-3, which arises from the following aspects. One is that Au NPs as pillar can prevent superposition of the tri-s-triazine nanosheets; and the other is that the released Cl2 by HAuCl4 decomposition can simultaneously oxidize the nanosheets along with the formation of the nanosheets in urea polymerization, which thus obtains graphene-like g-C3N4 samples. Meanwhile, the half peak width for Au increases, 7



indicating the growth inhibition of Au NPs on g-C3N4 nanosheet. Therefore, it is concluded that Au particles as pillar spatially prevent the superposition of the tri-s-triazine nanosheets, and simultaneously, Au NPs growth is also confined in the formation of nanosheets. These results suggest the good interaction between Au and π-conjugated tri-s-triazine polymer nanosheet,39,40 which is beneficial to electronic transfer under visible-light irradiation. Moreover, chemical states and surface compositions of Au/C3N4 samples were investigated using XPS. In Figure 2A&B, C 1s spectra for Au/C3N4 samples present same signals as that for g-C3N4. N 1s signals for g-CN sample can be fitted into 398.5, 399.7, and 401.2 eV, which are assigned to sp2-hybridized N (C=N-C), graphic-like N (C3-N), and hydrogen-bonding N (-NH2 and/or -NH3), respectively.35,36 For Au/C3N4 samples, the N 1s binding energy of C3-N slightly shifted into higher value, indicating the transition orientation of lone pair state of N in the C3-N.41 In Au 4f spectra of Figure 2C, it is seen that two peaks at 83.5 eV and 87.3 eV originate from Au 4f7/2 and Au 4f5/2, respectively, which is in agreement with binding energy of metallic Au. With the references of Au NPs,42,43 it is seen that Au binding energy shifts to lower value, suggesting the reception inclination of electrons over Au. The result suggests that electrons transfer can be achieved tendencily from N of C3-N to Au for a thermodynamic balance, suggesting close interaction between Au and π-conjugated tri-s-triazine polymer nanosheet. Due to the low adsorption and activation energy for oxygen over Au(111),44,45 therefore, it is necessary to probe the surface oxygen species over Au/C3N4 samples. Asymmetrical O 1s peaks are presented in the XPS spectra (Figure 2D), suggesting various oxygen 8


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species on the surfaces of g-C3N4 and Au/C3N4 samples. Peaks at 532.0, and 534.0 eV are identified as the surface containing-oxygen (N-C-O), and adsorbed O2 molecule, respectively.46,47 For Au/C3N4 samples, it is seen that the oxygen signal for adsorbed oxygen can be detected, and the chemical adsorption properties of oxygen over the conjugated tri-s-triazine polymer of Au/C3N4 samples are improved, which facilitates the transfer of electron from Au/C3N4 to O2 in the photocatalytic evolution of AOS. Meanwhile, morphology evolution of g-C3N4 before and after the in-situ introduction of Au can be observed, as shown in Figure 3A-D. g-C3N4 displays tightly packed nanosheets and particles with irregular shape (Figure 3A), which arises from nanosheet superposition induced by the abundant H bonds in the directly thermal polymerization of urea.24-26 After in-situ thermal polymerization of urea with HAuCl4 participation, gauze-shaped layers with smooth and transparent features are demonstrated in Figure 3B,C&D. AFM measurement demonstrates the gauze-liked structure of the Au/C3N4-3 samples with an average thickness of ~1.5 nm (Figure 3E). Furthermore, it is seen that Au NPs with an average size of ~15 nm were wrapped in gauze-liked structure of Au/C3N4 nanosheets (Figure 3B, C&D), which was also confirmed by TEM image. In Figure 3F, thin layer with the thickness of 1-2 nm in-situ grew and closely enwrapped the bare surface of Au NPs for Au/C3N4-3. HRTEM image presents the obvious lattice fringes with lattice spacing of 0.234 nm, which corresponds to (111) crystal surface of Au NPs. The as-formed Au NPs are fully contacted with graphene-like π-conjugated tri-s-triazine structure of Au/C3N4 samples, which favors a physical barrier-free interface between Au NPs and π-conjugated 9



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tri-s-triazine structure.48

Surface Area and Pore Structure N2 adsorption/desorption isotherms and the derived pore structures of g-C3N4 and Au/C3N4 samples are shown in Figure 4A&B. All isotherms are type IV with H3 hysteresis loops, representing the mesoporous features.49 In the in-situ synthesis process, Au NPs as pillar prevent superposition of the tri-s-triazine nanosheets, resulting in the formation of graphene-like structure and the expansion of interlayer spacing.

Thereby,

Au/C3N4

samples

possess

high

adsorption

quantity.

Brunauer-Emmett-Teller (BET) specific surface areas are 25, 55, 90, and 150 m2.g-1 for g-C3N4, Au/C3N4-1, Au/C3N4-2, and Au/C3N4-3, respectively. The optimal BET specific surface area of Au/C3N4 sample (i.e., Au/C3N4-3) is 6 times as large as that of pristine

g-C3N4.

Moreover,

pore

size

distributions

by

calculating

with

Barrett-Joyner-Halenda (BJH) center 4, 6, 7, and 9 nm for g-C3N4, Au/C3N4-1, Au/C3N4-2, and Au/C3N4-3, respectively.

The enlarged the surface and pore size of

Au/C3N4 samples should be attributed to the formation of graphene-like structure, which facilitates light absorption, and mass diffusion and transfer.

Electronic Structures Figure 5A exhibits UV-Vis absorption patterns of g-C3N4 and Au/C3N4 samples. The absorption edge of g-C3N4 is 422 nm, and the derived band-gap energy is 2.74 eV by Kubelka-Munk function. Nevertheless, a clear blue-shift trend of absorption edge D

10


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from 422 to 415 nm over Au/C3N4 samples reflect an increased bandgap for Au/C3N4-1 (2.82 eV), Au/C3N4-2 (2.87 eV), and Au/C3N4-3 (2.88 eV), respectively (Figure 5B). Blue shifts of absorption edges and the corresponding widened band gaps are attributed to the quantum size effect induced by the ultrathin-layered structure.25,26, A series of the enhanced light absorption in the range of 450-800 nm with the peaks at 580 nm are observed, which is induced by the surface plasmon resonance (SPR) of Au. In principle, the produced Au SPR effect will therefore enhance photon adsorption over graphene-like g-C3N4,29,50 and promote more optical transition and the charge transfer from g-C3N4 to Au. Electrochemical impedance spectroscopy with Mott-Schottky method are shown in Figure 5C, and positive slopes of tangent lines suggest an n-type conductivity feature, and the potentials of conduction band are -1.03, -0.93, -0.85, and -0.67 V vs. NHE for g-C3N4, AuC3N4-1, AuC3N4-2, and AuC3N4-3, respectively. In according with the formulas of band-gap energy (i.e., Eg = EVB - ECB), corresponding VB potentials are 1.71, 1.89, 2.02, and 2.21 V, respectively. Thus, it is seen that AuC3N4 samples possess increased CB and VB potentials in comparison with pristine g-C3N4 (Figure 5D), noticeably, the reduction potential of O2/∙O2- (-0.33 V vs NHE) and oxidation potential of H2O,-OH/∙OH (1.99 V vs. NHE)15 are well between CB and VB of AuC3N4-2 and AuC3N4-3, which can satisfy the thermodynamic conditions for photocatalytic yield of ∙O2- and ∙OH under visible irradiation. Au SPR-induced collective oscillation of the electrons accelerates the transfer of photogenerated electrons over the plasmonic graphene-like g-C3N4, and the Au NPs 11



with the physical barrier-free interface over Au/C3N4 sample can act as an electron reservoir due to its work function, which favors the directional transfer of electrons from g-C3N4 to the Au NP surface. DFT calculation was performed to investigate the electrons state change around Au over plasmonic graphene-like g-C3N4 model, and the results were shown in Figure 6. When Au atom was introduced to the vacancy of g-C3N4 (site 1) and out-of-plane (site 2), energy is -2138.755 (a.u.) and -2138.803 (a.u.), respectively. This clearly suggests that out-of-plane at site 2 is energetically favorable for Au location, which is consistent with XRD result. Moreover, mülliken charge analysis demonstrate that the charge difference between N and C atom around the Au atom becomes smaller, the distribution of electrons tends to be uniform, and electrons are concentrated on the surface of Au, confirming that the directional transfer of electrons from N 2p of of C3-N species in g-C3N4 to Au in the plasmonic graphene-like g-C3N4.

Photocatalytic Performance and Mechanism In the photocatalytic process, AOS over pristine g-C3N4, and series Au/C3N4 samples were detected by oxygen-trapping EPR method. Oxygen-trapping EPR spectra arise from the signals of DMPO-·OH and DMPO-·O2-, which can be detected the evolution of ·OH and ·O2-,7,9,51,52 respectively. Figure 7A displays EPR signals of DMPO-·O2- with the characteristic peaks of 1:1:1:1 over g-CN, and series Au/C3N4 samples, and the increased DMPO-·O2- characteristic peaks are shown over series Au/C3N4 samples. When Au NPs were directly located on g-C3N4 (Figure S2-S5 in SI), 12


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the prepared Au/g-C3N4 also presents DMPO-·O2- characteristic signals, and the signal intensity for Au/g-C3N4 are stronger than that of g-C3N4 however much weaker than that of Au/C3N4 samples. Therefore, it is clear that the increased efficiency over Au/C3N4 samples has been obtained. ·OH species-trapping spectra are demonstrated in Figure 7B. It is noted that DMPO-·OH characteristic signals with peak intensity ratio of 1:2:2:1 were obtained over Au/C3N4-2, and Au/C3N4-3, nevertheless the signals can not be detected over g-C3N4, Au/g-C3N4, and Au/C3N4-1, which suggests that ·OH species are preferably yielded over Au/C3N4-2, and Au/C3N4-3 under light irradiation. Consequently, .O2- and .OH species can be achieved over Au/C3N4-2, and Au/C3N4-3. Absorption spectra of NBT at 259 nm decrease, and fluorescence spectra of TA-.OH signals at 425 nm strengthen with the irradiation time over Au/C3N4-3 (Figure S5&S6 of SI), demonstrating the sustainable evolution of .O2- and .OH. According to NBT absorbance combined with reaction relationship, .O2- yield rate is 13, 35, 8, 25, 40, and 68 µmol.L-1h-1 over g-C3N4, Au/g-C3N4, Au/GR, Au/C3N4-1, Au/C3N4-2, and Au/C3N4-3 (Figure 7C), respectively. In Figure 7D, it is seen that .OH evolution rate is 18 and 31 µmol.L-1h-1 over Au/C3N4-2 and Au/C3N4-3. The derived plots of .O2- and .OH contents vs. irradiation time over Au/C3N4-3 is shown in Figure 7E. Moreover, the photocatalytic evolution of H2O2 over the as-prepared samples were also measured. In Figure 7F, evolution amount of H2O2 is 350 µmol.L-1h-1 over g-C3N4. For series Au/C3N4 samples, H2O2 amount increases from 450 to 520, and 620 µmol.L-1h-1 over Au/C3N4-1 to Au/C3N4-2, and Au/C3N4-3, however, only 150 µmol.L-1h-1 for H2O2 evolution over Au/GR has been obtained. Similar with evolution 13



trends of .O2- and .OH over Au/C3N4 samples, Au/C3N4-3 exhibits optimal performance in the photocatalytic evolution of H2O2. Additionally, H2O2 evolution efficiency over Au/C3N4-3 is extraordinarily higher than that over Au/g-C3N4 (480 µmol.L-1h-1). The recycle tests for H2O2 yield over Au/C3N4-3 were performed, and the performances are demonstrated in Figure S7. The stable performances have been confirmed over Au/C3N4-3 in the photocatalytic process of H2O2 evolution, and the corresponding XRD, XPS, and FESEM image in Figure S8-10 verify the integrity and stability of composition and structure. Compared with the reported representative photocatalytic materials,53-64 Au/C3N4-3 shows great advantages in terms of light source, H2O2 evolution and reaction time (Table 1). Moreover, Au/C3N4 photocatalysts were used to eliminate gaseous HCHO and/or aqueous MO pollutant. As shown in Figure S11 and S12, Au/C3N4 samples demonstrate higher performance than g-C3N4 and Au/g-C3N4, and Au/C3N4-3 shows the best activity for photocatalytically eliminating HCHO, and MO. Furthermore, Au/C3N4 samples also present better performance than g-C3N4 and Au/g-C3N4 in the photocatalytic removal of phenol, and the activity follows the order: Au/C3N4-3> Au/C3N4-2 > Au/g-C3N4 > Au/C3N4-1 > g-C3N4, and the reaction rate over Au/C3N4-3 is 1.42 times of Au/g-C3N4 (Figure S13). To reveal the essence of the high activity over Au/C3N4 samples, series photoelectronic characterizations were carried out. In Figure 8A, the second electron cutoff of Au over Au/C3N4-3 and Au/g-C3N4 is 16.6 and 16.4 eV, therefore the corresponding work function (WF) can be obtained to be 4.6 and 4.8 eV with 14


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subtracting He (I) excitation energy, respectively. The electrostatic potential by DFT calculation for g-C3N4 nanosheet is 4.5 eV.41 Thereby, it is seen that the decreased schottky barrier at the interface has been achieved over Au/C3N4-3 by our supplied in situ synthesis method. Therefore, the apparent barrier-free interface between Au and C3N4 nanosheets by in situ synthesis can promote electron transfer, which is consistent with the role of the matched exposed crystal facets of photocatalyst.65,66 EIS Nyquist spectrum of Au/g-C3N4 exhibits the smaller radius than that of g-C3N4, suggesting the obvious improvement of electron transfer due to Au SPR effect. Graphene-like Au/C3N4-3 presents better electronic conductivity than Au/g-C3N4. Potential barrier between the layers decreases from 32 (bulk g-C3N4) to 7 eV (layered g-C3N4) with the decrease of layers,24 thus graphene-like π-conjugated tri-s-triazine polymer nanosheet promotes the transport of charge carriers. In Figure S14, g-C3N4, Au/g-C3N4, and Au/C3N4-3 samples exhibit EPR signals at g = 2.000, and the signals are assigned to the lone electrons in the π-conjugated structure of g-C3N4.67 As the formation of plasmonic graphene-like structure with the physical barrier-free interface of Au, g signal increases, suggesting the improved π-conjugated structure of g-C3N4. Au introduction induces the uniform distribution of electrons in the plasmonic graphene-like g-C3N4, resulting in the improved π-conjugated structure, also confirmed in the DFT results (Figure 6). Therefore, the improved π-conjugated structure can optimize the electron structure for separating and transferring charge. Time-resolved fluorescence decay patterns of g-C3N4, Au/g-C3N4, and Au/C3N4-3 samples were fitted into a biexponential decay function (Figure 8C). Fast and long 15



fluorescence decay spectra come from nonradiative and radiative processes, respectively. The non-radiation reflects defects of semiconductors, while the radiation attributes to the combination of photo-induced electron and hole.34 Fluorescence decay results demonstrate that the short lifetime (τ1) is 0.75, 0.99, and 1.17 ns, and the long lifetime (τ2) is 4.45, 5.67, and 6.63 ns for g-C3N4, Au/g-C3N4, and Au/C3N4-3, respectively. Compared with those over bulk g-C3N4, and Au/g-C3N4, time for fast fluorescene decay is prolongated, and percentage of non radiation decreases over graphene-like Au/C3N4, suggesting the advantage of plasmonic graphene-like structure for light absorbance and energy conversion over Au/C3N4. Simultaneously, the increased lifetime for long fluorescence decay and normal radiation percentage indicates an increase in the effective utilization of photo-induced carriers over Au/C3N4, revealing that the photo-induced charge can easily arrive at the surface of Au/C3N4 from its basal planes. Utilization of photo-induced carriers is also reflected in the transient photo-current response (Figure 8D) and atmosphere-controlled surface photovoltage spectra (Figure 8E). When sample is irradiated, photocurrent pattern can be obtained, however the current density instantaneously reduced to 0 (Figure 8D), corresponding to the excitation and de-excitation process. For Au/C3N4-3 sample, the anodic photocurrent intensity is 6.7, and 2.0 times higher than that of g-C3N4 and Au/g-C3N4, respectively, suggesting the enhanced charge transfer efficiency over Au/C3N4 sample. Moreover, the same phenomenon is also obtained in SPS test. In Figure 8E, SPS signal reflects the transfer efficiency of photo-induced charge by a diffusion process on the surface of photocatalyst.68 When O2 is introduced into SPS 16


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system, photogenerated electrons react with O2 molecular, thus impelling hole diffusion on electrode.69 Therefore, peak intensity of SPS reflects the efficiency of charge transfer and utilization.8 The plasmonic graphene-like structure with the physical barrier-free interface of Au results in the decreased electronegativity difference between C and N, ultimately giving rise to electron accumulation on Au atom. Combined the oxygen adsorption ability in Figure 2D, Au/C3N4-3 shows much highest signal. Therefore, plasmonic graphene-like Au/C3N4 with physical barrier-free interface plays an important role in the photocatalytic evolution of AOS. In order to clearly probe the photocatalytical principle of AOS over Au/C3N4, a series of AOS-trapping tests were operated in the photocatalytic evolution of H2O2 (Figure 9). High-purity N2 (99.999%) was introduced into the aqueous system to remove the water-soluble O2, nevertheless, H2O2 yield is not completely lessened over g-C3N4 and Au/C3N4-3, confirming that H2O2 yield should not only arise from photocatalytic reduction of water-soluble O2. From both VB potentials of g-C3N4 and Au/C3N4-3 (Figure 8F), photo-induced h+ over g-C3N4 and Au/C3N4-3 oxidizes H2O into O2. Moreover, h+ over Au/C3N4-3 oxidizes H2O,-OH into ∙OH, which then forms H2O2 by ∙OH combination. Moreover, H2PtCl6, and IPA is utilized to delete the photo-generated e-, and ∙OH, respectively. Although H2O2 evolution shows obvious decreasing trend, and a considerable amount of H2O2 is also photocatalytically synthesized over Au/C3N4-3 in the presence of H2PtCl6, and IPA, respectively, suggesting H2O2 evolution is achieved by a multi-path principle along with participation of the yielded ∙OH and ∙O2- over Au/C3N4-3. Therefore, it is concluded 17



that the photocatalytic evolution of active oxygen species over plasmonic graphene-like Au/C3N4 relates to the following steps:

Accordingly, plasmonic graphene-like Au/C3N4 with the physical barrier-free interface of Au promotes to optimize the electron structure, and increases the CB ( V) and VB ( V) potentials. Under light irradiation, photo-induced e- can be migrated from VB to CB over the plasmonic graphene-like Au/C3N4. The migrated e- is preferential to interact with the absorbed O2 to yield ∙O2-, and the h+ can oxidize H2O,-OH to generate ∙OH. The e- can further react with ∙O2-/H+ to form H2O2, meanwhile, the generated ∙OH species combine each other to thus evolve H2O2.

CONCLUSIONS In summary, plasmonic graphene-like g-C3N4 photocatalysts with the physical barrier-free interface of Au were in situ synthesized by one-pot heating of the homogeneous mixture of HAuCl4 and urea. Compared with g-C3N4 and Au/g-C3N4 photocatalysts, Au/C3N4 photocatalysts demonstrate much higher photocatalytic activities in the photocatalytic evolution of ∙O2-, ∙OH, and H2O2 species. The high activity of the photocatalysts principally ascribes the plasmonic graphene-like structure with physical barrier-free interface of Au. The formed plasmonic 18


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graphene-like structure induces i) the increased CB and VB potentials, ii) the enhanced visible-light absorption, iii) the improved π-conjugated structure by uniform distribution of electrons, resulting in efficient-directional charge transfer capability from N 2p of of C3-N species in g-C3N4 to Au. Therefore, plasmonic graphene-like g-C3N4 photocatalysts display the enhanced photocatalytic activities for yielding .OH and .O2-, and H2O2. The results reported herein may provide an efficient strategy to design plasmonic graphene-like photocatalysts with suitable electron structures for sustainable energy-related applications.

Acknowledgments This study was supported by the National Natural Science Foundation of China (51462002, 51662003, 21667003, and 21871155), the General Financial Grant from the China Postdoctoral Science Foundation (2014M562075 and 2015T80849).

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Figure 1. XRD patterns (A) and the shift of (002) diffraction peak of g-C3N4, and Au/C3N4 samples.

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Figure 2. XPS spectra of C 1s (A), N 1s (B), Au 4f (C) and O 1s (D) for g-C3N4 (a), Au/C3N4-1 (b), Au/C3N4-2 (c), and Au/C3N4-3 (d).

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A

B

Au

500 nm

2 µm

C

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Au

Au

Au Au

D Au

Au Au 500 nm

500 nm

E

F 0.234 nm Au (111) 4 nm 0.234 nm

Au (111)

20 nm

Figure 3. FESEM morphological analysis of g-C3N4 (A), Au/C3N4-1 (B), Au/C3N4-2 (C), Au/C3N4-3 (D), AFM of Au/C3N4-3 sample (E), and TEM image of Au/C3N4-3 with HRTEM image (inset).

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Figure 4. N2 adsorption-desorption isotherms of g-C3N4 and Au/C3N4 samples (A), and the corresponding pore size distribution curves (B).

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Figure 5. Optical absorption spectra (A), plots of (αhυ)1/2 vs. photon energy (hυ) (B), electrochemical Mott-Schottky plots (C) of of g-C3N4 and Au/C3N4 samples, and band structures of g-C3N4 and Au/C3N4-3 (D).

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Figure 6. Energies of Au atom at site 1 (A) and 2 (B) of g-C3N4 and mülliken charge analysis of g-C3N4 (C) and plasmonic graphene-like g-C3N4 (D).

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Figure 7. EPR spectra of DMPO-.O2- (A) and DMPO-.OH under irradiation for 120 s in deionized water, and methanol solution, respectively. Photocatalytic yield of .O2(C), .OH (D), and H2O2 (F) over Au/C3N4 samples, Au/g-C3N4, and Au/GR. .O2and .OH evolution curves over Au/C3N4-3 vs. Irradiation time (E).

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Au/C3N4-3

Au/C3N4-3

Au/g-C3N4

Au/g-C3N4 g-C3N4

g-C3N4

Figure 8. Secondary-electron cutoff region of Au/C3N4-3 and Au/g-C3N4, (A), EIS Nyquist plots (B), time-resolved fluorescence decay spectra (C), transient photo-current response (D),and surface photovolt-age spectra (E) over g-C3N4, Au/g-C3N4, and Au/C3N4-3, and scheme for the band structure of Au/C3N4-3 and redox potentials of active species vs. NHE (F).

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Figure 9. The scavengers effect on H2O2 yield over g-C3N4 and Au/C3N4-3 (H2PtCl6, KI, IPA, N2 for eliminating e-, h+, .OH, O2).

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Table 1. Performance comparison for H2O2 evolution over Au/C3N4 and the reported photocatalysts.

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Plasmonic graphene-like Au/C3N4 with physical barrier-free interface promotes sustainable evolution of active oxygen species by efficiently directional transfer of electron.


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C - ACS Publications - American

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