Ag2SO4 Nanocomposite as a Direct Z

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Solid-Solution-like o-C3N4/Ag2SO4 Nanocomposite as A Direct Z-Scheme Photocatalytic System for Photosynthesis of Active Oxygen Species chuanbao xiong, Shujuan Jiang, shaoqing Song, Xi Wu, Jinghua Li, and Zhang-Gao Le ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02241 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Sustainable Chemistry & Engineering

Solid-Solution-like o-C3N4/Ag2SO4 Nanocomposite as A Direct Z-Scheme Photocatalytic System for Photosynthesis of Active Oxygen Species Chuanbao Xiong1, Shujuan Jiang1,3, Shaoqing Song1,3*, Xi Wu1,3, Jinghua Li2*, Zhanggao Le1* 1

Laboratory of Atmospheric Environment, Key Laboratory of Nuclear Resources and

Environment (Ministry of Education), East China University of Technology, 418 Guanglan Road, Nanchang 330013, China 2

Department of Materials Science and Engineering, Northwestern University,

Evanston IL 60208, USA 3

School of Materials Science and Chemical Engineering, Ningbo University, 818

Fenghua Road, Ningbo 315211, PR China E-mail: [email protected] (S. Song); [email protected] (Z. Le); and [email protected] (J. Li)

ABSTRACT: Efficient photosynthesis of active oxygen species (e.g., .OH, .O2-, and H2O2) is of cardinal significance for environmental science and biochemistry. We report a system of o-C3N4/Ag2SO4 with solid-solution-like structure synthesized by coordinating 5s orbit of Ag+ with surface 2p lone electrons of o-C3N4. The as-synthesized o-C3N4/Ag2SO4 demonstrates unique electronic structure, as illuminated high light absorption, perfect redox potentials, large BET specific area, abundant active sites, and efficient interfacial charge transfer. Electron paramagnetic resonance spectra, and Mott-Schottky measurements confirm that o-C3N4/Ag2SO4 1

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photocatalysts demonstrate highly efficient activity for yielding .OH .O2-, and H2O2 species with the Z-scheme photocatalytic principle. Experimental and DFT calculation reveal that the transferred electrons on CB made up of Ag 5s were favorably shifted into VB composed of N 2p with the formed coordination under irradiation, greatly promoting electron transportation in the Z-scheme photocatalytic system. (o-C3N4)2/(Ag2SO4)1 with the mass ratio of 2 to 1 possesses highest photocatalytic rate of .OH (45 µmol∙L-1h-1), .O2- (87 µmol∙L-1h-1), and H2O2 (480 µmol∙L-1h-1). The work provides a new approach to design advanced photocatalysts to efficiently yield active oxygen species for environmental purification. Keywords:

Active

oxygen

species;

Photocatalysis;

Z-scheme

principle;

Solid-solution-like structure; Interfacial charge transfer

INTRODUCTION Active oxygen species (AOS), such as superoxide (.O2-), hydroxyl (.OH), and hydrogen peroxide (H2O2), are of great significance for environmental science and biochemistry.1,2 It is known that AOS will be evolved over semiconductor under irradiation with photo-induced electrons (e-) on conduction band (CB) reducing O2 to .O2-, and holes (h+) on valence band (VB) oxidizing H2O and/or -OH into .OH.3,4 In order to promote photocatalytic AOS evolution, the absorbed photon energy (hν) should be larger than energy gap (Eg) of semiconductor, and redox potentials of O2/.O2-, and H2O, -OH/.OH should locate between the potentials of CB and VB.5 While the former factor requires the narrow band-gap energy, the latter factor suggests 2


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the lower CB potential and higher VB potential to meet the requirement for thermodynamical condition for O2 reduction and H2O oxidation.6 The two factors are mutually exclusive. Therefore, it is difficult to simultaneously achieve strengthened light-absorption capability and strong redox potentials over photocatalyst with single component, which can nevertheless be achieved over Z-scheme photocatalytic system.5,7-10 Under light irradiation, excitons can be spatially separated over Z-scheme photocatalytic system (noted as PSI-PSII), and lower reduction potential over CB of PS I and higher oxidation potential over VB of PS II can be also demonstrated.8,9 Therefore, Z-scheme photocatalytic system is beneficial to photocatalytically evolve .O2- and .OH,11-16 which is of great significance to environmental purification and sustainable development. Taking graphitic carbon nitride (g-C3N4) as example, g-C3N4 is a type of visible-light-response photocatalyst with a bandgap of ~2.7 eV and CB potential of -1.23 V (vs.NHE at pH=7).17-20 Therefore g-C3N4 is suitable to be used as PS I for producing .O2-, and a large number of g-C3N4 (PS I)-based Z-scheme photocatalysts have

been

constructed,8

such

Bi3TaO7/g-C3N4,21

as

g-C3N4/Ag/MoS2,22

PTCDI/g-C3N4,23 β-Bi2O3/g-C3N4,24 LaFeO3/g-C3N4,25 g-C3N4@Ag/BiVO4,26 and Ag2CrO4/Ag/g-C3N4.27 Studies have revealed that efficient transfer of charges across the interface between PS I and PS II is the key for converting light energy into chemical energy.28,29 On one hand, noble metals (Ag, Au, and Pt) and/or graphene were utilized as electron mediators to enhance interfacial charge transfer.21,25-27,30 For example, owing to the Schottky barrier effect, CB potential over Ag3PO4 (PS II) is 3



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more negative than that of Ag Fermi level. However, the Fermi level of Ag is more positive than the VB of g-C3N4, thus CB electrons of Ag3PO4 ultimately ﬂowed into VB of g-C3N4 (PS I) by Ag as electron mediator.31 On the other hand, Yu et al. utilized g-C3N4 nanosheets (PS I) as template to grow MnO2 nanosheets (PS II) to construct 2D/2D Z-scheme photocatalyt.32 Due to the face-to-face contact, the 2D/2D Z-scheme photocatalyt efficiently promotes the charge transfer across the interface, resulting in the yield of .O2- and .OH for the degradation of dye and the removal of phenol. Actually, interfacial charge transfer has been enhanced by using these strategies because of lowering electric resistance of contact interface, which can also be solved by interface structure matching. It is known that g-C3N4 is composed of C and N by sp2 hybridization, and residual lone electrons of N remain on pz orbital.33 According to the crystal field theory, electrons on pz orbital can coordinate unoccupied orbital of semiconductor by the static coulomb effect, which will result in close contact interface and low electric resistance. Herein, we report a solid-solution-like o-C3N4/Ag2SO4 nanocomposite built by coordinating 5s orbit of Ag+ with surface lone electrons of o-C3N4. The as-constructed o-C3N4/Ag2SO4 presented excellent light absorption, suitable redox potentials, full interface contact, and efficient interfacial charge transfer. Under irradiation, o-C3N4/Ag2SO4

photocatalysts

demonstrated

highly

efficient

activity

for

evolving .OH, .O2-, and H2O2 species, and oxygen-trapping EPR spectra and Mott-Schottky measurement confirmed the Z-scheme photocatalytic principle over o-C3N4/Ag2SO4 for the evolution of these active oxygen species. Accordingly, the 4


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work provides new insights and a simple route for obtaining efficient Z-scheme photocatalyst for environmental science and biochemistry.

EXPERIMENTAL SECTION Preparation of o-C3N4/Ag2SO4 photocatalysts. Firstly, 10 g carbamide was heated at 520°C for 2 h in a muffle furnace with a ramp rate of 5 oC/min, and the as-prepared sample was ground into powder in an agate mortar. The powder-like sample was thermally treated at 480 oC in a semi open tubular furnace with a ramp rate of 8 oC/min, and the as-prepared sample was referred to as o-C3N4. Subsequently, 0.4 g o-C3N4 was dispersed into 100 mL of aqueous AgNO3 (1.2 mmol.L-1) under magnetic stirring followed by ultrasonic for 40 min. 100 mL of Na2SO4 solution (0.6 mmol.L-1) was then dropped slowly into the mixture solution with vigorous stirring. After stirring for 30 min, the product was filtered and washed for 3 times using deionized water. Afterwards, the samples were dried in oven at 80 oC for 2 h, and the obtained sample was noted as (o-C3N4)2/(Ag2SO4)1. When the concentrations of AgNO3/Na2SO4 were increased into 2.5/1.28 mmol.L-1, and 5.12/2.56 mmol.L-1, respectively, the corresponding samples were noted as (o-C3N4)1/(Ag2SO4)1 and (o-C3N4)1/(Ag2SO4)2. In the meanwhile, pristine Ag2SO4 was synthesized with using the preparation method of (o-C3N4)2/(Ag2SO4)1 but without the introduction of o-C3N4. The preparation process was demonstrated by Scheme S1 of Supporting Information (SI). g-C3N4 was prepared by directly heating urea at 520 oC for 2 h, and the (g-C3N4)2/(Ag2SO4)1 sample was also synthesized according to the preparation 5



method of (o-C3N4)2/(Ag2SO4)1. The characterization methods, DFT calculation details and the photocatalytic process of HCHO and MO elimination were elaborated in S1-3 of SI. Photocatalytic evolution of active oxygen species. Active oxygen species yielded under visible-light irradiation were characterized by Electron Paramagnetic Resonance (EPR) measurement with the aid of 5,5-Dimethyl-l-pyrroline N-oxide (DMPO). In a typical procedure, 10 mg as-prepared photocatalysts were dispersed homogeneously in 1.04 mL aqueous DMPO solution (including 0.04 mL DMPO and 1 mL H2O). Afterwards, the stirred-mixed solution was illuminated with using a 300 W Xe Lamp for 120 s. EPR spectrometer was used to detect the signal of DMPO-.O2-. Moreover, the characterization process was also applied to detect DMPO-.OH when CH3OH was mixed with DMPO instead of H2O. Photocatalytic evolution amounts of .O2- and .OH were measured by investigating nitroblue tetrazolium (NBT) absorbance and terephthalic acid (TA) photoluminescence over AuCN samples. According to the molar ratio of 1:4, NBT molecular reacts with .O2- to form an efficiently fluorescent NBT-.O2- which displays an absorbance peak at 259 nm, thus evolution amounts of .O2- was achieved by detecting the content variation of NBT. TA reacts with .OH with a molar ratio of 1:1 to form TA-OH which reflects an obvious fluorescence at 425 nm, and evolution amounts of .OH was calculated by measuring the content change of TA. Moreover, photocatalytic H2O2 evolution was also investigated. In the photocatalytic process, 40 mg sample was placed into Schlenk flask (100 mL) 6


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including 30 mL H2O and 3 mL IPA, and the reactor was ultrasonicated for 10 min. Afterwards, O2 was introduced into the mixture solution along with stirring for 60 min before irradiation, and the reactor was then illuminated using the same lamp with an optical filter (λ ≥ 350 nm). H2O2 evolution amount was detected and calculated with using by typical titration chromogenic reaction between H2O2 and acidified KMnO4, and the consumed content of KMnO4 was equivalent to H2O2 evolution amount.

RESULTS AND DISCUSSION Figure 1 shows X-ray diffraction XRD patterns of Ag2SO4 nanosheets, o-C3N4 nanosheets, and o-C3N4/Ag2SO4 nanocomposites. The as-prepared Ag2SO4 nanosheets were in orthorhombic structure (JCPDS NO. 27-1403).34 Angles at 22.2, 28.0, 31.1, 33.8, 35.4, and 37.0o correspond to diffraction facets of (220), (040), (311), (022), (202), and (331), respectively.35 Peaks of o-C3N4 at 27.1o can be assigned to (002) diffraction facet,36 which reflects the stacking of π-conjugated tri-s-triazine layers of g-C3N4.33,37-39 It should be noted that the characteristic peak intensities of o-C3N4 and Ag2SO4 present a weakening trend in comparison with pristine o-C3N4 and Ag2SO4. The results suggest that o-C3N4 should homogeneously hybrid Ag2SO4 and thus form solid-solution-like structures due to the coordination of o-C3N4 surface electrons with Ag+. In order to reveal the coordination effect between 5s orbit of Ag+ and 2p lone electrons of o-C3N4, DFT calculation was carried out to probe the coordination location and surface charge distribution of o-C3N4 and Ag+. In Figure 2, when 5s orbit of Ag+ coordinates with 2p lone electrons of o-C3N4 at site 1 (Figure 2A) and site 2 7



(Figure 2B), the formation energy is -2148.8421 (a.u.) and -2148.8329 (a.u.) respectively. This clearly suggests that site 1 is energetically favorable for coordinating 5s orbit of Ag+, which results in lessening the structure defects of s-triazine.33 Moreover, natural bond orbital population analysis demonstrate that the coordination of Ag+ on site 1 gives rise to the increase of N electronegativity around Ag+ and the enhancement of Ag+ electropositivity (Table S1-2 of SI), which promotes the electron transfer from o-C3N4 to Ag2SO4 by the coordination between 5s orbit of Ag+ and 2p lone electrons of N. Moreover, surface chemical composition and valence state of samples were investigated with XPS. All binding energies in these XPS spectra have been calibrated by C 1s at 284.8 eV. In Figure 3A, Ag 3d spectrum of pristine Ag2SO4 exhibits two signals at 367.8 and 373.8 eV classifying into Ag 3d5/2 and Ag 3d3/2,34,40 respectively. Nevertheless, for the pattern of (o-C3N4)2/(Ag2SO4)1, two binding energies lowered to 368.3 and 374.3 eV. In Figure 3B, O 1s peaks of Ag2SO4 are located at 531.5 and 532.5 eV, corresponding to lattice oxygen from Ag2SO4 and adsorbed oxygen species.18,33,41 For o-C3N4, only adsorbed oxygen species can be detected. For (o-C3N4)2/(Ag2SO4)1, both adsorbed oxygen species and lattice oxygen can be observed. Furthermore, the peak of lattice oxygen lowered to 531.3 eV. N 1s peaks of o-C3N4 and (o-C3N4)2/Ag2SO4 are demonstrated in Figure 3C. N 1s signals for o-C3N4 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),38,42 respectively. When Ag2SO4 is combined with o-C3N4, the N 1s 8


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binding energy of C=N-C and C3-N slightly shifted into higher value. The results suggest formation of hybridization between o-C3N4 and Ag2SO4 by flowing electrons from o-C3N4 to Ag2SO4 for achieving a thermodynamic balance, confirming coordinating Ag+ with surface lone electrons of o-C3N4.32 C 1s spectra for o-C3N4 and (o-C3N4)2/(Ag2SO4)1 are presented in Figure 3D. The high-resolution XPS profile of S 2p confirms the existence of SO42- in the samples (Figure S1 of SI). The features of the prepared (o-C3N4)2/(Ag2SO4)1 were observed by field-emission scanning electron microscopy (FESEM)

and transmitting electron microscope

(TEM), and the morphology results are shown in Figure 4 and Figure S2-4 of SI. The oxygen-treated g-C3N4 displays gauze-shaped layers with smooth and transparent characteristics in Figure S2, and the pristine Ag2SO4 is composed of the layered nanosheets with a size of 200-300 nm in Figure S3. When o-C3N4 combined with Ag2SO4, o-C3N4/Ag2SO4 samples with uniform flocculation were obtained. Taking (o-C3N4)2/(Ag2SO4)1 as an example, o-C3N4 and Ag2SO4 fully dissolved together and formed solid-solution-like structure, which is demonstrated as the fluffy flocculation with layers of 150-500 nm (Figure S4). TEM image exhibits the structure of integration in Figure 4A, and the EELS elements reveal a homogeneous distribution of Ag, S, O, C, and N without any phase separation (Figure 4B-F), which suggests a close integration between o-C3N4 and Ag2SO4.43 The as-formed solid-solution-like structure of(o-C3N4)2/(Ag2SO4)1 can not only supply rich active sites but also promote electron transfer of interface due to the full contact between o-C3N4 and Ag2SO4.44 N2 adsorption/desorption isotherms and the derived pore structures of Ag2SO4, 9



o-C3N4, and (o-C3N4)2/(Ag2SO4)1 samples are displayed in Figure 5A&B. All samples demonstrate type IV isotherms with H3 hysteresis loops, which represents the feature of slit-shaped mesoporous.45 Owing to the effect of gauze-shaped layers of o-C3N4, (o-C3N4)2/(Ag2SO4)1 sample show higher adsorption quantity in comparison with pristine Ag2SO4. BET specific surface areas are 13, 106, and 150 m2.g-1 for Ag2SO4, (o-C3N4)2/(Ag2SO4)1, and o-C3N4, respectively. Pore size distributions by calculating with BJH method center 8-17, 11-25, and 15-27 nm corresponding for Ag2SO4, (o-C3N4)2/(Ag2SO4)1, and o-C3N4. The detailed data of BET specific surface areas and pore sizes for other samples are listed in Table S3 of SI. It is seen that BET specific surface areas and pore sizes for Ag2SO4 samples enhanced with increasing o-C3N4 content, which is beneficial for promoting reactants adsorption and photocatalytic reaction. UV-vis spectra of Ag2SO4, o-C3N4, and o-C3N4/Ag2SO4 samples were investigated to study the optical characteristics, and results are shown in Figure 6A. The light-absorption verges of o-C3N4, and Ag2SO4 are 352, and 415 nm. When Ag2SO4 was grown on o-C3N4, the o-C3N4/Ag2SO4 hybrid displays obviously a red shift of light and corresponding strengthened light intensity, indicating that the as formed solid-solution-like structure is beneficial for the absorption of light.43,44 The light of photon energy vs. (αhν)1/2 and Mott-Schottky plots were shown in Figure S5 of SI. The band-gap energies (Eg) are 2.67 (o-C3N4), and 3.31 (Ag2SO4) eV (Figure S5A of SI). Positive slopes of tangent lines for Mott-Schottky plots show that potentials of CB are -1.35 (o-C3N4) and -0.25 V (Ag2SO4) vs. NHE. As a result, VB potentials were 10


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calculated to be 1.32, and 3.06 V corresponding to o-C3N4 and Ag2SO4. Accordingly, the CB potential over o-C3N4 and VB potential over Ag2SO4 are conducive to yield .O2- and .OH species, respectively. Owing to the coordination between 5s orbit of Ag+ and N 2p lone electrons of o-C3N4, electrons will effectively transmit between Ag2SO4 and o-C3N4. In the EIS Nyquist plot, the intermediate frequency pattern arises from charge transfer efficiency on the interface between electrode and electrolyte.46 In Figure 6B, the (o-C3N4)2/(Ag2SO4)1 with the solid-solution-like structure presents smaller electric resistance (62 Ω) in comparison with o-C3N4 (600 Ω) and Ag2SO4 (75 Ω), as reflected by a smaller semicircle in the EIS Nyquist patterns, indicating solid-solution-like (o-C3N4)2/(Ag2SO4)1 with full interface efficiently favors electron transfer.

Time-resolved

fluorescence

decay

patterns

of

o-C3N4,

and

(o-C3N4)2/(Ag2SO4)1 samples were fitted to a biexponential decay function (Figure 6C). Fast and long 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.33 Fluorescence decay results demonstrate that the short lifetime (τ1) is 0.78 and 1.2 ns, and the long lifetime (τ2) is 4.55, and 6.70 ns for o-C3N4, and (o-C3N4)2/(Ag2SO4)1, respectively. Compared with those over o-C3N4, time for fast fluorescene decay is prolongated, and percentage of non radiation decreases over (o-C3N4)2/(Ag2SO4)1, suggesting the advantage of solid-Solution-like structure for light absorbance and energy conversion over (o-C3N4)2/(Ag2SO4)1. Simultaneously, the increased lifetime for long fluorescence decay and normal radiation percentage indicates an increase in 11



the effective utilization of photo-induced carriers over (o-C3N4)2/(Ag2SO4)1, revealing that the photo-induced charge can easily arrive at the surface of (o-C3N4)2/(Ag2SO4)1. In Figure 6D, EPR signals of o-C3N4, Ag2SO4, and (o-C3N4)2/(Ag2SO4)1 samples are at g = 2.000, which are assigned to electrons trapped in oxygen defects or absorbed O2 over these samples.47,48 Furthermore, (o-C3N4)2/(Ag2SO4)1 exhibits more obvious EPR spectrum, and the result suggests that electrons are preferential to being trapped by oxygen defects or absorbed O2 of (o-C3N4)2/(Ag2SO4)1.9,47 Figure 6E demonstrates I-t patterns for o-C3N4, Ag2SO4, and (o-C3N4)2/(Ag2SO4)1 with four on-off recycles of intermittent irradiation. As can be seen, photocurrent signal rapidly descends to 0 when light irradiation is turned off, and the current recovers to a constant value with the on-again light irradiation. Under light irradiation, photo-induced electrons are transferred into back contact over photocatalysts to thus yield current.49,50 With the prolonging of irradiation, photocurrent curves decay from the anodic photocurrent spike at the initial time to the constant current, and the evolution of photocurrent decay suggest that holes on surface of photocatalysts competitively trap photo-induced electrons instead of being captured by reduced species in the electrolyte. After the balance between competitive separation and recombination of photo-induced charges, photocurrent achieves a constant value.51,52 Notably, for (o-C3N4)2/(Ag2SO4)1 sample, the anodic photocurrent intensity is much higher than that of o-C3N4 and Ag2SO4, further, photocurrent is almost increasing until the light is turned off. Owing to the coordination between 5s orbit of Ag+ and N 2p lone electrons of o-C3N4, photo-irradiation electrons will efficiently transfer between Ag2SO4 and 12


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o-C3N4 in the solid-solution-like structure. Surface photovoltage spectra (SPS) signal arises from the photo-induced charge separation by a diffusion process on the surface of photocatalyst.53 When O2 was drawn into SPS system, photogenerated electrons are easily captured by O2 molecular, which thus impels hole diffusing on the surface of electrode.54 Accordingly, intensity of SPS signal is proportional to charge transfer and use efficiency over photocatalyst.55 (o-C3N4)2/(Ag2SO4)1 shows much highest efficiency

for

charge

transfer

(Figure

6F).

moreover,

the

surface

of

(o-C3N4)2/(Ag2SO4)1 possesses abundant active sites for adsorbing O2, which was confirmed by O2-temperature-programmed desorption (O2-TPD in Figure S6 of SI). Thus, (o-C3N4)2/(Ag2SO4)1 demonstrates the most obvious SPS signal. Under irradiation, the active oxygen species over (o-C3N4)2/(Ag2SO4)1 were detected by EPR. DMPO was used to react with ·O2- and ·OH, yielding productions of DMPO-·O2- and DMPO-·OH in aqueous and methanol solution,56-58 respectively. Figure

7A displays

EPR

curves

of

DMPO-·O2-

over

o-C3N4,

Ag2SO4,

(o-C3N4)2/(Ag2SO4)1 under irradiation for 120 s, strongest DMPO-·O2- characteristic signals were obtained over (o-C3N4)2/(Ag2SO4)1, and o-C3N4 brought about a relatively obvious EPR signals nevertheless much poorer signals were detected over Ag2SO4, which suggests that ·O2- species were also preferably yielded over (o-C3N4)2/(Ag2SO4)1 under light irradiation. Results of ·OH species-trapping experiments are exhibited in Figure 7B. Similar strongest DMPO-·OH characteristic peak was shown over (o-C3N4)2/(Ag2SO4)1. However, for Ag2SO4, the peak is low, and for o-C3N4, EPR signal is much weaker so that it can be negligible. Therefore, it 13



is clear that the efficiency of (o-C3N4)2/(Ag2SO4)1 for producing ·OH species is higher than that of individual o-C3N4 and Ag2SO4. Consequently, it is concluded that .O2and .OH as highly active oxygen species can be achieved over (o-C3N4)2/(Ag2SO4)1. However, for individual o-C3N4 and Ag2SO4, CB potential of Ag2SO4 is -0.25 V vs. NHE which is much higher than the reduction potential from O2 to .O2- (-0.28 V vs. NHE); besides, VB potential of o-C3N4 is much lower than the oxidation potential of .OH/H2O,OH-. The CB and VB potentials for the single o-C3N4 and Ag2SO4 cannot simultaneously satisfy thermodynamic requirement of .O2- and .OH. Therefore, we consider a Z-scheme photocatalytic principle for yielding .O2- and .OH (Figure 7C). DFT have revealed that the top of VB of Ag2SO4 is comprised of Ag 4d and O 2s/2p orbitals, and CB bottom is made up of Ag 5s and O 2s/2p orbitals.34 For o-C3N4, CB is composed of N 2p and C 2p orbitals, while VB mainly consists of N 2s and N 2p orbitals.49 Under light irradiation, both o-C3N4 (PS I) and Ag2SO4 (PS II) favorably generate photo-induced e- and h+, and e- transfer from VB to CB. The transferred electrons on Ag 5s of CB were favorably shifted into N 2p of VB through the formed coordination and thus delete the corresponding h+ over VB of o-C3N4. As a result, eon the CB of o-C3N4 reduces O2 to produce .O2-, and h+ oxidizes H2O/-OH to generate .OH. In the light of the reacting relationship between NBT and .O2- (molar ratio of 1:4), .O2- evolution efficiency is 38, 87, 57, 25, and 0 µmol∙L-1h-1 over o-C3N4, (o-C3N4)2/(Ag2SO4)1, (o-C3N4)1/(Ag2SO4)1, (o-C3N4)1/(Ag2SO4)2, and Ag2SO4, as shown in Figure 7D, respectively. Terephthalic acid photoluminescence probing technique (TA-PL) method was used to detect .OH evolution amount when .OH reacts 14


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with TA in equal ratio to yield 2-hydroxyterephthalic acid,59 and .OH evolution efficiency in Figure 7E can be only obtained over (o-C3N4)2/(Ag2SO4)1, (o-C3N4)1/(Ag2SO4)1, (o-C3N4)1/(Ag2SO4)2, and Ag2SO4, and corresponding amounts are 45, 30, 21, and 9 µmol∙L-1h-1. Absorption spectra of NBT-.O2- at 259 nm decrease, and fluorescence spectra of TA-.OH signals strengthen with the irradiation time over (o-C3N4)2/(Ag2SO4)1 (Figure S7&8 of SI), indicating the consecutive yield of .O2and .OH. The derived plots of .O2- and .OH contents vs. irradiation time is shown in Figure 7F. The solid-solution-like o-C3N4/Ag2SO4 samples show high efficency for photocatalytically producing .O2- and .OH active species. The good performance of the o-C3N4/Ag2SO4 photocatalysts mainly arises from i) o-C3N4 possesses high specific surface area and abundant active sites; ii) thus o-C3N4 provides a good platform for the formation of the solid-solution-like o-C3N4/Ag2SO4 through the coordination of 5s orbit in Ag+ with N 2p lone electrons in o-C3N4; iii) solid-solution-like structure exhibits a full interface contact, which thus facilitates the efficient interfacial charge transfer in the photocatalytic process. However, these advantages are not presented over g-C3N4 and (g-C3N4)2/(Ag2SO4)1. g-C3N4 obtained by direct heating polymerization of urea has bulk structure with small specific surface area and insufficient active site exposure, which results in the poor interface contact between g-C3N4 and Ag2SO4. Therefore, (g-C3N4)2/(Ag2SO4)1 show an inferior performance in the photocatalytic evolution of active oxygen species in comparison with (o-C3N4)2/(Ag2SO4)1 (Figure 7D&E, and Figure S9-14 of SI). Accordingly, these photocatalysts were also used to eliminate gaseous HCHO 15



and/or aqueous MO pollutant. As shown in Figure S15 of SI, o-C3N4/Ag2SO4 samples demonstrate higher performance than o-C3N4 and Ag2SO4, and (o-C3N4)2/(Ag2SO4)1 shows the best activity for photocatalytically eliminating HCHO and MO. In 5 recycles, (o-C3N4)2/(Ag2SO4)1 keeps a high effeciency, and XPS and TEM characterizations reveal the stability of the photocatalysts after recycles (Figure S16), indicating the solid-solution-like structure enhances the stability and inhibits the photocorrosion of o-C3N4/Ag2SO4 photocatalysts by facilitating the interfacial charge transfer. When O2 is bubbled into H2O, H2O2 yield over these samples gradually increases with continuous illumination. Under light irradiation, higher H2O2 yield is achieved over series o-C3N4/Ag2SO4 samples than over o-C3N4 and/or Ag2SO4 samples in Figure 8A. After 60 min of illumination, 115, 105, 180, 310, and 480 μmol∙L-1h-1 of H2O2 were obtained over o-C3N4, Ag2SO4, (o-C3N4)1/(Ag2SO4)2, (o-C3N4)1/(Ag2SO4)1, and (o-C3N4)2/(Ag2SO4)1, respectively. Moreover, the recycling experiments of (o-C3N4)2/(Ag2SO4)1 were also investigated, and the photocatalytic H2O2 evolution over (o-C3N4)2/(Ag2SO4)1 is not clearly decreasing after 8 recycles (Figure S17), suggesting that the activity over (o-C3N4)2/(Ag2SO4)1 is very stable for photocatalytic evolution of active oxygen species. In order to reveal photocatalytic principle of active oxygen species over (o-C3N4)2/(Ag2SO4)1, a series of active oxygen species trapping tests were operated in the photocatalytic evolution of H2O2 in Figure 8B. Firstly, 99.999% N2 was bubbled into H2O to remove O2, and H2O2 yield over (o-C3N4)2/(Ag2SO4)1 was not entirely vanished, indicating that O2 is not the only 16


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source for H2O2 evolution. On one hand, h+ of (o-C3N4)2/(Ag2SO4)1 can evolve more O2, and on the other hand, h+ can also oxidize H2O into .OH, which thus leads to H2O2 by combining with each other. When H2PtCl6 and TBA was introduced into the reactor system to remove e- and .OH, respectively. It is seen that decrease trends for H2O2 yield can be presented over (o-C3N4)2/(Ag2SO4)1, however still maintaining H2O2 generation, indicating that H2O2 has been photocatalytically yielded over (o-C3N4)2/(Ag2SO4)1 by multi-path mechanism with .OH and .O2- participation. Based on these observations, the photocatalytic evolution of active oxygen species over (o-C3N4)2/(Ag2SO4)1 follows these steps in Scheme 1.

CONDLUSION In summary, we have established an efficient photocatalytic system of solid-solution-like o-C3N4/Ag2SO4 by coordinating 5s orbit of Ag+ with N 2p lone electrons of o-C3N4 for photosynthesis of .OH, .O2-, and H2O2 species. The coordination-induced solid-solution-like structure exhibits heightened light absorption, enlarged BET specific area, and promoted interfacial charge transfer. Under light irradiation, the photo-induced electrons on CB comprised of Ag 5s were favorably transferred into VB consist of N 2p with the aid of the formed coordination, which thus results in the construction of the Z-scheme photocatalytic system. The Z-scheme g-C3N4/Ag2SO4

photocatalysts displayed

highly

efficient

performance

for

producing .OH, .O2-, and H2O2 species, and photocatalytic rate of .OH, .O2-, and H2O2 can be achieved to be 45, 87 and 480 µmol∙L-1h-1 over (o-C3N4)2/(Ag2SO4)1 with a 17



mass ratio of 2 to 1. The work may shed light on a new perspective to develop novel and advanced photocatalysts to efficiently produce .OH, .O2-, and H2O2 for environmental protection and control.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Characterization methods, DFT calculation details, Photocatalytic test procedures, Scheme S1, Figure S1-S17, and Table S1-S3.

ACKNOWLEDGMENTS This work was partially supported by Natural Science Foundation of China (51462002, and 11765002), and the Foundation of State key Laboratory Breeding Base of Nuclear Resources and Environment (AE1603, Z201408 and Z1604).

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Figure 1. XRD patterns of Ag2SO4 (a), (o-C3N4)1/(Ag2SO4)2 (b), (o-C3N4)1/(Ag2SO4)1 (c), (o-C3N4)2/(Ag2SO4)1 (d), and o-C3N4 (e).

28


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EFormation energy=-2148.8421 (a.u.)

EFormation energy=-2148.8329 (a.u.)

Figure 2. Formation energies and natural population analysis of Ag coordination at site 1 and 2 over o-C3N4.

29



Figure 3. X-ray photoelectron spectroscopy (XPS) spectra of (A) Ag 3d, (B) O 1s, (C) N 1s, and (D) C 1s for o-C3N4, Ag2SO4, and (o-C3N4)2/(Ag2SO4)1.

30


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Figure 4. Transmission electron microscope (TEM) and element mapping images for (o-C3N4)2/(Ag2SO4)1.

31



Figure 5. N2 adsorption/desorption isotherms (A), and pore size distribution (B) for Ag2SO4, o-C3N4, (o-C3N4)2/(Ag2SO4)1.

32


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Figure 6. UV-vis. spectra (A), electrochemical impedance spectra (B), time-resolved photoluminescence of o-C3N4 and (o-C3N4)2/(Ag2SO4)1

(C), electron paramagnetic

resonances (D), photocurrent spectra (E), surface photovoltage spectra (F) for the samples.

Ag2SO4

(a),

(o-C3N4)1/(Ag2SO4)2

(b),

(o-C3N4)2/(Ag2SO4)1 (d), and o-C3N4 (e).

33


(o-C3N4)1/(Ag2SO4)1

(c),


Figure 7. EPR spectra of (A) DMPO-.O2- and (B) DMPO-.OH with irrdiation for 120 s in methanol and aqueous solution in the presence of o-C3N4, Ag2SO4, and (o-C3N4)2/(Ag2SO4)1. The Z-Scheme for electron transfer mechanism (C), .O2- (D) and .OH species (E) yields over photocatalysts, and .O2- and .OH species yields vs. Irradiation time (F) over (o-C3N4)2/(Ag2SO4)1.

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Figure 8. The photocatalytic yield of H2O2 over o-C3N4/Ag2SO4 samples (A), and scavengers effect on H2O2 yield (H2PtCl6, KI, TBA, N2 for eliminating e-, h+, .OH, O2) (B).

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Scheme 1. Photocatalytic evolution mechanism for active oxygen species.

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Solid-solution-like o-C3N4/Ag2SO4 synthesized by coordinating Ag+ 5s orbit with N 2p electrons of o-C3N4 demonstrate high activity for yielding active oxygen species with Z-scheme principle.

1


Ag2SO4 Nanocomposite as a Direct Z

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