Highly Efficient Performance and Conversion Pathway of

Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST

Energy, Environmental, and Catalysis Applications

Highly Efficient Performance and Conversion Pathway of Photocatalytic CH3SH Oxidation on Self-Stabilized Indirect Z-scheme g-C3N4/I3-BiOI Lingling Hu, Huanjunwa He, Dehua Xia, Yajing Huang, Jiarong Xu, Haoyue Li, Chun He, Wenjing Yang, Dong Shu, and Po Keung Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03250 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Highly Efficient Performance and Conversion Pathway of Photocatalytic CH3SH Oxidation on Self-Stabilized Indirect ZScheme g-C3N4/I3--BiOI

Lingling Hu a, Huanjunwa He a, Dehua Xia a,b*, Yajing Huang a, Jiarong Xu a, Haoyue Lia, Chun He a,b *, Wenjing Yang a, Dong Shu c, Po Keung Wong d

a

School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China

b

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou, 510275, CHINA

c

Key Lab of Technology on Electrochemical Energy Storage and Power Generation in Guangdong Universities, School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, China d

School of Life Sciences, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR, China

Corresponding author: School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China. Tel.: +86 20 39332690. Email address: [email protected] (D. Xia); [email protected] (C. He). 1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT A self-stabilized Z-scheme porous g-C3N4/I3--containing BiOI ultrathin nanosheets (g-C3N4/I3--BiOI) heterojunction photocatalyst with I3-/I- redox mediator was successfully synthesized by a facile solvothermal method coupling with light illumination. The structure and optical properties of g-C3N4/I3--BiOI composites were systematically characterized by means of XRD, SEM, TEM, FT-IR, XPS, N2 adsorption/desorption, UV−vis DRS, PL. The g-C3N4/I3--BiOI composites, with heterojunction between porous g-C3N4 and BiOI ultrathin nanosheets, were firstly applied for the photocatalytic elimination of ppm-leveled CH3SH under LED visible light illumination. The g-C3N4/I3--BiOI heterojunction with 10% g-C3N4 showed a dramatically enhanced photocatalytic activity in removal of CH3SH compared with pure BiOI and g-C3N4, due to its effective interfacial charge transfer and separation. The adsorption and photocatalytic oxidation of CH3SH over g-C3N4/I3--BiOI were deeply explored by in situ DRIFTS, and the intermediates and conversion pathways were elucidated and compared. Furthermore, on the basis of reactive species trapping, ESR and Mott-Schottky experiments, it was revealed that the responsible reactive species for catalytic CH3SH composotion were h+, ·O2- and 1O2, thus, the g-C3N4/I3-BiOI heterojunction followed an indirect all-solid state Z-scheme charge transfer mode with self-stabilized I3-/I- pairs as redox mediator, which could accelerate the separation of photo-generated charge and enhance the redox reaction power of charged carriers simultaneously. KEYWORDS: CH3SH conversion pathway, g-C3N4/I3--BiOI, indirect Z-scheme, interfacial charge transfer, self-stabilized I3-/I-

2


Page 2 of 48

Page 3 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Methyl mercaptan (CH3SH), a volatile gas with an odor of rotten or cooked cabbage, has a high toxicity and corrosive with very low odor detection thresholds around 0.4 ppb/v.1 CH3SH is widely produced in urban waste, sewage treatment, industry wastes as well as other energy-related activities.2 Till now, many techniques have been developed to remove CH3SH, including biological treatment,3 adsorption4 and catalytic oxidation.5 However, these technologies are not economically feasible for removal of air pollution at lower parts per million (ppm) levels, due to large occupying area or high operating costs. Recently, the removal of CH3SH through semiconduction-based photocatalytic process, an economical and environment-benign removal technology, has attracted extensive interest.6 In search for effcient and stable visible-light-driven (VLD) photocatalysts, metalfree graphitic carbon nitride (g-C3N4) has become a research hotspot in the field of environmental remediation and energy conversion, owing to its appealing electronic band structure, physicochemical stability, and earth-abundant nature.7-10 More attractively, the modified g-C3N4, synthesized by various strategies, such as inner architecture

modification

(pore

fabrication,

bonds

breaking)

and

surface

functionalization (elemental doping, copolymerization, formation of heterojunctions), was found to exhibit enhanced photocatalytic activity and good adaptability in various application fields.11−15 For instance, soft-template and hard-template method were utilized for pore fabrication, thus higher surface area and more active sites were obtained for g-C3N4;11,

12

g-C3N4 can also hybrid with other semiconductors to

construct heterojunction, thus to efficiently promote the charge separation and enhance the redox potentials.13-16 Particularly, by tailoring porous g-C3N4 with BiOI to form an all-solid-state Z-scheme structure, the composite exhibited increased light

3



Page 4 of 48

harvesting, prolonged lifetime of carriers, and thus enhanced its photocatalytic performance in comparison to that of pristine g-C3N4.17-19 However, this composite is employed scarcely in the photocatalytic removal of odor gas up to now. Based on recent works, hybridized BiOI/g-C3N4 composites were synthesized by a simple deposition method.17, 18, 20 Without fine control, the BiOI components in gC3N4/BiOI composites are always in 4-5 µm hollow flowers with 15 nm thickness.21 Generally, the bulk-BiOI is usually restricted by sluggish charge carrier migration, since photo-induced electrons and holes would spend longer time migrating from deep inside to the surface. To solve this problem, the controllable preparation of ultrathin BiOI nanosheets would be an effective strategy to further modify gC3N4/BiOI composite. For example, Jiang et al. observed that ultrathin BiOI can achieve enhanced oxidation ability than bulk BiOI due to its extended band gap;22 Di et al. has successfully synthesized N-CQDs/atomically-thin BiOI nanosheets nanojunctions and the atomically-thin structure was designed to accelerate carrier transfer among BiOI nanosheet interior.23 Therefore, it can be speculated that tailoring ultrathin BiOI and porous g-C3N4 would also minimize interface mismatch, thus to achieve enhanced interfacial charge transfer between the two components. Despite its promising results, the utilization of supposed photocatalyst is controversial mainly questioning on its stability during photocatalytic reactions. Generally, electron mediators in all-solid-state Z-scheme composites always come across instability, such as oxidized noble metals in Ag2CrO4/Ag/g-C3N4, Pt/CdSTNTAs after reaction and cause great loss of photocatalytic performance.24,

25

Interestingly, the strengthened photocatalysis in the g-C3N4/BiOI is relied on the nonmetal I3−/I− mediator induced indirect Z-scheme charge transfer, in which I− is oxidized to I3− intermediate and I3− is simultaneously reduced back to fulfill the

4


Page 5 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


efficient e--h+ separation during reaction.17, 18 The I3- plays the critical role for charge transfer. When the holes were scavenged, there will be no I3− ions detectable.18 However, the stability of I3−/I− mediator has never been considered in previous work. To design a stabilized I3-/I- mediator in g-C3N4/BiOI may provide an ideal solution for removal of CH3SH at ppm-level in application. Besides, the conversion pathway for pollutant removal over a photocatalyst is a key to understanding the underlying reaction mechanism, estimating the possible generation of toxic intermediates, and optimizing the photocatalyst performance. Although some efficient photocatalysts have been developed for CH3SH removal, little attention has been paid to the conversion route of photocatalytic CH3SH oxidation process.26-28 In situ DRIFTS is an effective tool for gas-phase reaction analysis because signals are observed even for slight changes at the molecular level;2931

thus, it is well suited for investigating the related reaction pathway of photocatalytic

CH3SH oxidation. Here, a self-stabilized Z-scheme porous g-C3N4/I3--containing BiOI ultrathin nanosheets (g-C3N4/I3--BiOI) heterojunction photocatalyst with I3-/I- redox mediator was successfully synthesized by a facile solvothermal method coupling with light illumination, which exhibited substantially high visible-light photocatalytic removal of CH3SH. To reveal the photocatalytic mechanism, the adsorption and photocatalytic oxidation of CH3SH over g-C3N4/I3--BiOI were analyzed by in situ DRIFTS, and the intermediates and conversion pathways were elucidated and compared. Moreover, the charge transfer mode of g-C3N4/I3--BiOI was carefully studied by quenching experiments, electron paramagnetic resonance (EPR) measurement and Mott-Schottky study. Thus, the present work opens numerous opportunities to explore other self-

5



stabilized indirect Z-scheme photocatalyst to control air pollution and presents a novel in situ DRIFTS-based strategy to explore the photocatalytic reaction pathway. 2. EXPERIMENTAL SECTION 2.1. Materials. Melamine (C3H6N6, 99%), Bi(NO3)3•5H2O, KI, tartaric acid (TA), ter-butyl alcohol (TBA), disodium ethylenediaminetetraacetate (EDTA-2Na, C10H18N2Na2O10), ascorbic acid, tartaric acid, K2Cr2O7, sodium azide (NaN3), 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were purchased from Aladdin chemistry Co., Ltd.. 2,2,6,6tetramethylpiperidine (TEMP) and dimethyl sulfoxide (DMSO) were purchased from Tianjin Baishi chemical Reagent Co., Ltd. (China) and Tianjin Zhiyuan Chemical Reagent Co., Ltd. (China), respectively. All reagents used were analytical grade without further purification. Deionized (DI) water was used in all the sample preparation and photocatalytic tests. 2.2. Preparation of photocatalyst. The porous g-C3N4 was synthesized by condensation of tartaric acid (TA) and melamine for the first time. In detail, melamine (10.0 g) and TA (3.0 g) were mixed by grinding in a mortar, then transferred to alumina crucible with a cover. The crucible was heated to 200 °C from room temperature in a muffle furnace at a heating rate of 2 °C/min, and further heated to 550 °C at a rate of 5 °C/min and maintained at 550 °C for 4 h. g-C3N4/I3--BiOI heterojunction was synthesized by a facile solvothermal method and subsequent light illumination. In a typical procedure, gC3N4 was ultrasonically dispersed in 35 mL ethylene glycol for 1 h, subsequently, 1.4553 g Bi(NO3)3·5H2O was added via magnetic stirring. Then, 0.4980 g KI dissolved in 35 mL ethylene glycol was drop-wisely added in the above mixed solution to vigorously stir for 1 h. The solution was transferred to a 100 mL Teflon-

6


Page 6 of 48

Page 7 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lined autoclave for hydrothermal reaction at 160 °C for 24 h. The resulting precipitates were collected, washed with deionized water and ethanol respectively, and dried at 80 °C in oven for overnight. Then the captured samples were immersed with deionized water and irradiated with 500 W Xenon lamp for 0.5 h, to form I3species. I3--BiOI and g-C3N4/BiOI were prepared under the same conditions in the absence of g-C3N4 powder and without light illumination, respectively. As a contrast, mechanical mixed (MM-) g-C3N4/BiOI was fabricated by g-C3N4 and BiOI powders milled for 24 h. Furthermore, the bulk BiOI was synthesized by a simple hydrothermal process. 2.3. Characterization. X-ray diffraction (XRD) analysis of the samples was performed on a Rigaku D/MAX 2500 X-ray diffractometer equipped with a Cu Kα radiation source. Surface electronic states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, KRATOA XSAM800, Japan). The morphology of the samples were examined by scanning electron microscope (SEM) and transmission electron microscope (TEM) analyses operated at 5 kV and a JEM-2010HR microscope (JEOL) transmission electron micro-scope at 200 kV, respectively. Nitrogen adsorption–desorption isotherms of the samples were obtained at 77 K on a Gemini VII 2390 surface area analyzer (MQL). The photoluminescence (PL) spectra of the sample was obtained by a Varian Cary Eclipse spectrometer with an excitation wavelength of 250 nm. UV–vis diffuse reflectance spectrum (DRS) was taken on a Hitachi U-3010 UV–vis spectrometer. The electrochemical impedance spectroscopy (EIS) and photocurrent measurements were obtained on CHI 660E electrochemical workstation equipped with a standard three-electrode system (a platinum foil as the counter electrode and Hg/Hg2Cl2 as the reference electrode. In situ DRIFTS measurements were conducted

7



using a TENSOR II FT-IR spectrometer (Bruker) equipped with in situ diffusereflectance cell (Harrick) and high-temperature reaction chamber (HVC). The diffusereflectance cell contained three windows, including two KBr windows for IR measurements and a quartz window for light irradiation using a KL 2500 LCD lamp (SCHOTT), while the reaction chamber was equipped with three gas ports and two coolant ports. The photocatalyst samples were pretreated at room temperature under He (100 mL/min) for 60 min to remove adsorbed impurities. After the background spectrum was recorded. The samples were then exposed to 70 ppm CH3SH (150 mL/min), which was diluted with N2. When the adsorption-desorption equilibrium was achieved, the white LED lamp was turned on. The DRIFTS spectra of the samples were recorded at room temperature by accumulating 32 scans with a resolution of 4 cm−1 at a given interval. 2.4. Photocatalytic removal of CH3SH. The photocatalytic removal of CH3SH at ppm levels by g-C3N4/I3--BiOI was carried out in a continuous flow reactor at ambient temperature. The photocatalytic reactor with a size of 2.0 cm in diameter and 60 cm height was made of Pyrex glass (Figure 1). A 1.5 m flexible 8 W LED belt (the spectrum is shown in Supporting Information Figure S1) was wrapped around the reactor, which has four color, including white, blue (440~490 nm), green (470~570 nm) and red (610~650 nm). The LED lamp is considered to be the next generation lighting source due to its long lifespan and energy-efficient properties.32 The photocatalysts were immobilized before test: 0.05 g photocatalyst was ultrasonically dispersed in 20 ml ethanol and then loaded on non-woven fabric by drop-casting (5 cm in width, 30 cm in height). For each test, the gaseous CH3SH was premixed in a mixing box and the initial concentration was diluted to be 70 ppm. The gas flow was controlled at 0.15 L min−1

8


Page 8 of 48

Page 9 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and two CH3SH sensors were installed to monitor the inlet and outlet concentration.

Figure 1. Schematic diagram of photocatalytic removal of CH3SH: 1, CH3SH gas cylinder; 2, air gas cylinder; 3, inlet mixing box; 4, glass rod; 5, photocatalyst; 6, visible light source (LED strips); 7, outlet; F1–F3, gas flow meters; S1–S2, gas sensors. 2.5. Reactive species analysis. Ter-butyl alcohol (TBA) was used as an effective hydroxyl radical (•OH) scavenger as it could react with •OH with a high rate (K=6 × 108). Ascorbic acid and NaN3 were used to trap superoxide radical (•O2-) and singlet oxygen (1O2), EDTA2Na and K2Cr2O7 were selected as hole scavenger and electron scavenger, respectively.33 0.050 g photocatalyst was respectively added into 20 mL of trapping agents solution (20 mM of TBA, EDTA-Na, Ascorbic acid, Cr(VI), NaN3) and ultrasonicated for 30 min. The suspension was then loaded on non-woven fabric, followed by drying in air at 60°C until water was completely removed. The next steps were the same as photocatalytic oxidation of CH3SH. Electron spin resonance (ESR) of radicals spin-trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was recorded on a JES FA200 spectrometer. Samples for ESR measurements were prepared by 9



mixing them in a 50 mM DMPO/TEMP solution tank (aqueous dispersion for DMPO•OH, TEMP-1O2 and methanol dispersion for DMPO-•O2−) and irradiated by visible light.34 3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1 Phase structure and chemical composition. To identify the crystalline phase structure and purity of the as-prepared materials, XRD tests were conducted and displayed in Figure 2a. For pure g-C3N4, two typical peaks at 13.1° and 27.5° could be indexed to (100) and (002) diffraction planes (JCPDS-501250). Moreover, the diffraction peaks at 13.1° and 27.5° corresponding to the tri-s-triazine units and the aromatic segments were also observed.35 The XRD pattern of pure BiOI exhibited characteristic diffraction peaks at 29.7°, 31.8°, 45.8° and 55.4°, consistent with the tetragonal BiOI phase (JCPDS No. 10-0445). As for gC3N4/I3--BiOI, it contains all typical peak for BiOI but no patterns of g-C3N4, mainly due to the low crystallinity and well-dispersion of g-C3N4 in composite. This was also observed in other report.36 FT-IR was also utilized to determine the chemical interaction of g-C3N4 and BiOI in the composite. As shown in Figure 2b, the FTIR spectra of g-C3N4/I3--BiOI contains both characteristic peaks of g-C3N4 (triazine units at 810 cm−1 and stretching heterocycles around 1200–1650 cm−1) and BiOI (the asymmetrical stretching vibration of Bi−O bond at 768 cm−1), indicating both components have been coupled together in g-C3N4/I3--BiOI composite.37, 38

10


Page 10 of 48

*

* **

*

* 24%-g-C3N4/I

** *

*

* 16%-g-C3N4/I

* **

*

* 10%-g-C3N4/I

** *

*

*

30

3-

BiOI 1238 1314 1405 1459

24%-g-C3N4/I -BiOI

-BiOI

3-

-BiOI

3-

-BiOI

3-

70

3-

10%-g-C3N4/I -BiOI 3-

4%-g-C3N4/I -BiOI

BiOI

80

1000

1621

g-C3N4

40 50 60 2 Theta (degree)

3-

16%-g-C3N4/I -BiOI

768

4%-g-C3N4/I -BiOI

Transmittance(a.u.)

3-

002

Intensity (a.u.) 20

(b)

810

212

*

* **

.

.

10

(a)

200

* BiOI

. g-C3N4

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


102 110 111

Page 11 of 48

1500

2000

2500

3000

Wavelength (cm-1)

3500

Figure 2. (a) XRD and (b) FTIR patterns of BiOI, g-C3N4, 4%-g-C3N4/I3--BiOI, 10%g-C3N4/I3--BiOI, 16%-g-C3N4/I3--BiOI, and 24%-g-C3N4/I3--BiOI. The chemical structure and composition in 10%-g-C3N4/I3--BiOI are examined by XPS (Figure 3). The XPS spectra in Figure S2 implied that the 10%-g-C3N4/I3-BiOI consisted of C, N, Bi, O, I elements. For C 1s spectra of g-C3N4 and 10%-gC3N4/I3--BiOI, two distinct peaks at 284.7 and 288.4 eV and three low intensity peaks at 285.7, 288.0 and 288.8 eV are observed in Figure 3a. The first peak is due to sp2 C−C bonds, which usually come from adventitious hydrocarbons and is used as a reference, and the second one is sp2 hybridized carbon in N-containing aromatic ring (N−C=N), as well as the last three peaks at around 285.7, 288.0 and 288.8 eV were ascribed to C-O, C=O and O-C=O bonds, respectively. In comparison, the surface C/N atom ratio was increased from 0.76 for g-C3N4 to 1.07 for 10%-g-C3N4/I3--BiOI. This is mainly due to the following two reasons: (1) Since ethylene glycol was used as solvent to prepare BiOI, it resulted in some residual ethylene glycol as carbon impurities on BiOI. This was well confirmed by the identified C1s spectra of pure BiOI (Figure S3); (2) Due to the higher surface area of BiOI/g-C3N4 composite (62.58 m2 g-1) than g-C3N4 (54.01 m2 g-1), it can adsorb more adventitious hydrocarbon like CO2, CO32- in the ambient condition. This was well evidenced by the higher intensity of sp2 C−C bonds (284.7 eV) in the C1s peaks of BiOI/g-C3N4 than g-C3N4 (Figure 11



S3).39 Similar phenomenon of C1s peak was also observed in other references.20, 40, 41, 42

Therefore, these carbon impurities result in the C/N ratio in the BiOI/g-C3N4

composite. Compared with pure g-C3N4, the deconvoluted C 1s peaks have only a slight shift. For N 1s spectra (Figure 3b), three fitted peaks are observed to locate at 398.7, 399.9 and 401.1 eV, respectively. These peaks can be regarded as the sp2 hybridized nitrogen involved in triazine rings (C−N=C), the tertiary nitrogen N−(C)3 groups, and the free amino groups (C−N−H) respectively. By contrast with pure gC3N4, N 1s peaks have about 0.1 negative shifts to higher binding energies, suggesting an increase in the electron density of the N atoms in g-C3N4.43 Especially, the weakest peak at 404.7 eV of N 1s was only observed in g-C3N4 rather than g-C3N4/I3--BiOI, suggesting that I3--BiOI may couple with g-C3N4 via the interaction at π-excitations of g-C3N4 heterocycles.44 Moreover, in contrast with pure BiOI, the binding energies of Bi 4f peaks (Figure 3c) in g-C3N4/I3--BiOI positively shifted from 159.0 and 164.4 eV to 159.4 and 164.8 eV for Bi4f7/2 and Bi4f5/2, respectively, suggesting the electron density of Bi decreased after coupling. Similarly, the three peaks of O 1s in the composite (Figure 3d) also have about 0.1 positive shifts from pure BiOI, in which the peaks at 530.4 eV and 531.8 eV were attributed to the Bi-O bonds in [Bi2O2] slabs and I-O bonds in BiOI component, whereas the peak at 532.8 eV was derived from surface absorbed oxygen species.40 All the shifts for binding energy in the XPS spectra confirmed the closely contacted phases of BiOI and g-C3N4 in the heterojunction. To study the stability of I3-/I- mediator on the surface of composite, the gC3N4/I3--BiOI samples were irradiated with xenon lamp for 10, 30 and 60 min, respectively. The change of I 3d was identified by XPS measurement in Figure 3e. Without light illumination, two bands included I 3d5/2 at 619.34 and I 3d3/2 at 630.76

12


Page 12 of 48

Page 13 of 48

eV were observed, corresponding to I− ions. After illumination, a small amount of I3(deriving from the photocatalytic oxidation of I- ions) at 620.50 and 631.70 eV were appeared at 10 min (ca. 0.725% in Table 1) and then increased at 30 min (ca. 1.267% in Table 1). (b) N 1s

(a) C 1s

288.0

285.7

g-C3N4/I3--BiOI

288.3

g-C3N4/I3--BiOI

Intensity

Intensity

288.8

g-C3N4

g-C3N4

288

286 284 282 Binding Energy (eV)

280

408

159.4

(c) Bi 4f

406

401.1 399.9

404.7

404

402

400

398

Binding Energy (eV)

(d) O 1s

164.8

399.8 401.1

398.7

284.7

286.0

290

398.6

284.7

288.4

531.8

396

394

530.4

532.8 3-

Intensity

g-C3N4/I -BiOI

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


159.0 164.4

g-C3N4/I3--BiOI

530.1

531.5 532.5 BiOI

BiOI 168

166

164

162

160

158

156

538

536

534

532

530

528

Binding Energy (eV)

Binding Energy (eV)

13


526


(e) I 3d 631.70

after cycling

620.50

3-

g-C3N4/I -BiOI-60 min

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 48

3-

g-C3N4/I -BiOI-30 min

3-

g-C3N4/I -BiOI-10 min 630.76

619.34 3-

g-C3N4/I -BiOI

635

630

625

620

Binding Energy (eV)

615

Figure 3. XPS spectra of BiOI, g-C3N4 and 10%-g-C3N4/I3--BiOI composite: (a) C 1s, (b) N 1s, (c) Bi 4f; (d) O 1s; (e) I 3d XPS spectra of 10%-g-C3N4/I3--BiOI composite irradiated for 10 min, 30 min, 60 min and used for 5 cycles. However, no obvious change in the amount of I3- for sample is observed after 60 min irradiation(ca. 1.269% in Table 1), presumably suggesting I3- become stable. This may demonstrate that g-C3N4/I3--BiOI can obtain self-stability after 30 min light irradiation, which is beneficial to maintain the stable charge transfer in the composite. Table 1 Composition (atom%) of I species in 10%-g-C3N4/I3--BiOI according to XPS analysis Sample

I3-

I-

10%-g-C3N4/I3--BiOI

0

10.490%

10%-g-C3N4/I3--BiOI (after 10 min)

0.725%

9.775%


1.267%

9.255%


1.269%

9.252%

10%-g-C3N4/I3--BiOI (after 5 cycles)

1.268%

9.251%

3.1.2 Morphology and formation mechanism.

14


Page 15 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The SEM images in Figure 4a, b showed that pure BiOI was a flower-like microsphere with an average diameter of 3-4 µm, which was assembled by ultrathin nanosheets with thickness of around 5-7 nm, consistent with Jiang et al.’s work.22 Figure S4 indicated that g-C3N4 exhibited a typically aggregated morphology. For the g-C3N4/I3--BiOI heterojunction, it is in sphere-like structure and the introduced gC3N4 is found to uniformly cover on the BiOI ultrathin nanosheets (Figure 4c). Moreover, the increasing content of g-C3N4 can further enhance its coverage on the ultrathin nanosheet of BiOI (Figure 4d, e). However, the sphere-like structure of gC3N4/I3--BiOI was broken when g-C3N4 content reached to 24% (Figure 4f). This suggests too much g-C3N4 will overweigh the support of ultrathin BiOI nanosheet, thus destructing the microstructure of BiOI.

15



Figure 4. SEM of BiOI (a, b), 4%-g-C3N4/I3--BiOI (c), 10%-g-C3N4/I3--BiOI (d), 16%- g-C3N4/I3--BiOI (e) and 24%-g-C3N4/I3--BiOI (f).

Moreover, the SEM equipped elemental mapping images (Figure 5 and Figure S5) indicate Bi, I, O, C and N elements were co-existed and homogeneous dispersed, confirming g-C3N4 and BiOI were in close contact.

16


Page 16 of 48

Page 17 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure. 5. EDS elemental mapping in 10%-g-C3N4/I3--BiOI composite Further insights into the interior structure of g-C3N4/I3--BiOI were obtained by TEM (Figure 6). BiOI was ultrathin nanosheet consisted solid structured sphere (Figure S6), while g-C3N4 presented layer structure with abundant pores (Figure 6a-b). The formation of porous structures in g-C3N4 could be ascribed to the following two reasons: (1) Tartaric acid (TA) plays the role as template: melamine first connect with TA to form supramolecular through the aggregation of NH…O hydrogen bonds,45 and then the TA in supramolecular began to decompose into CO2 to develop pores during heating; (2) TA reacts with partial amino groups in the melamine, thus to alleviate their subsequent thermal condensation.37 The porous fabrication of this method is

17



facial and environment-benign, better than previous works to utilize hazardous reagents and complex process.11, 12

Figure 6. TEM and HRTEM images of pure g-C3N4 (a, b) and 10%-g-C3N4/I3--BiOI (c, d); SAED patterns of g-C3N4 (e) and 10%-g-C3N4/I3--BiOI (f). Moreover, the HRTEM in Figure 6c-d depicted 10%-g-C3N4/I3--BiOI was in sheet-on-sheet flake-like structure, and an obvious interface can be observed between g-C3N4 layer and BiOI nanosheet. The electron diffraction spots in Figure 6f can be well indexed to (110) and (102) of BiOI, further evidencing the well-defined crystallization of BiOI in 10%-g-C3N4/I3--BiOI. In contrast, no obvious lattice fringe of g-C3N4 was observed in Figure 6b-6e, confirming the amorphous structure of g-

18


Page 18 of 48

Page 19 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C3N4. Moreover, N2 adsorption-desorption isotherm of as-prepared 10%-g-C3N4/I3-BiOI in Figure S7 confirms its porous structure. Interestingly, the value of BET surface area of 10%-g-C3N4/I3--BiOI (62.58 m2 g-1) was between that of BiOI (78.05 m2 g-1) and g-C3N4 (54.01 m2 g-1), suggesting the effective bridging of BiOI in the pores of g-C3N4. 3.1.3 Optical properties, Charge separation and transfer. The optical properties of the as-prepared photocatalysts were measured via UVvis DRS technique. As shown in Figure 7a, the adsorption edges of BiOI and g-C3N4 were extended to 690 nm and 460 nm, respectively. In contrast, the adsorption intensity of 10% g-C3N4/I3--BiOI composite was strengthened and the absorption edge red shifted to 670 nm. However, the DRS spectra of I3--BiOI kept almost the same with BiOI (Figure S8), implying I3- species had no obvious effect on optical properties. The calculated band gap values of BiOI (1.80 eV), g-C3N4 (2.60 eV) and 10% gC3N4/I3--BiOI (1.70 eV) were compared as shown in inset of Figure 7a.38 It indicated that all samples can be excited by visible light. Especially, the extended band gap (1.8 eV, 1.7 eV for bulk BiOI) of BiOI confirms its ultrathin structure, consistent with previous reports.22 The separation and transfer efficiency of photogenerated electron-holes in different samples were investigated by photoelectrochemical tests. As shown in Figure 7b, all the samples exhibited a quick response to the light either on or off. Obviously, the photocurrent of ultrathin BiOI (denoted as BiOI) was much higher than that of bulk BiOI, suggesting that the ultrathin-layer was formed and advantageous for the separation of photogenerated charge carriers.22, 23 In addition, 10% g-C3N4/BiOI composite displayed a higher photocurrent than pure BiOI and gC3N4, suggesting more efficient separation and longer life time of photogenerated

19



electron-hole pairs in the g-C3N4/BiOI interface. Interestingly, the photocurrents were found to significantly increase when I3- species were involved in g-C3N4/I3--BiOI heterojunctions, which confirmed that I3- species played an important role in photoinduced charge separation of g-C3N4/I3--BiOI. However, I3- species had no effect on charge transfer of pure BiOI as photocurrent responses of BiOI and I3--BiOI are almost the same. The same result was also obtained by the electrochemical impedance spectroscopy (EIS). As displayed in Figure 7c, g-C3N4/I3--BiOI exhibited the smallest semicircle diameter among of all the samples, implying that this sample possessed the optimal carrier transfer efficiency and the best electronic conductivity. In general, the semicircle in the high frequency region could be ascribed to charge-transfer resistance (Rct), showing the charge transfer rate through the electrode/electrolyte interface.46 Besides, the photoluminence (PL) spectra of the 10% g-C3N4/I3--BiOI composite photocatalysts at an excitation wavelength of 250 nm were measured and presented in Figure 7d. The composite displayed a weakest intensity in the emission, indicating that the most efficient separation of photogenerated electron-hole pairs.29 But for I3-BiOI, the main emission peak presented similar PL intensity with BiOI (Figure S9), implying I3- species cannot accelerate charge transfer of BiOI, which was consistent with photocurrent responses and EIS studies. 25

1.6 (a)

1.0

0.6 BiOI g-C3N4

0.4

3-

10%-g-C3N4/I -BiOI

0.2 0.0

0.8

20

0.8

1.70

1.5

1.80

2.0

2.60

2.5

3.0

3.5

4.0

4.5

5.0

Energy (eV)

BiOI g-C3N4 4%-g-C3N4/I3--BiOI

Current (uA/cm-2)

1.2

0.4

(b)

1.2

(Abs) 0.5 (a.u) 0.5 (ev) 0.5

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 48

15

3g-C3N4/I -BiOI g-C3N4/BiOI

10

MM-g-C3N4/BiOI 3I -BiOI BiOI Bulk-BiOI g-C3N4

5

10%-g-C3N4/I3--BiOI 16%-g-C3N4/I3--BiOI

0.0 200

24%-g-C3N4/I3--BiOI

300

400

500

600

700

Wavelength (nm)

800

0 40

900

80

120

160

Time (s)

20


200

240

Page 21 of 48

10

(d)

(c)

3-

10% g-C3N4/I -BiOI

8 6 3-

g-C3N4/I -BiOI g-C3N4/BiOI

4

MM-g-C3N4/BiOI 3-

I -BiOI BiOI Bulk-BiOI g-C3N4

2

BiOI g-C3N4

Intensity (a.u.)

-Z"(ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 0

2

4

6

8

10

400

Z' (ohm)

450

500

550

600

Wavelength (nm)

650

700

Figure 7. DRS spectra (a), Photocurrent responses (b), EIS Nyquist plots (c) and PL spectra (d) of g-C3N4, bulk BiOI, BiOI, I3--BiOI, MM-g-C3N4/BiOI, g-C3N4/ BiOI and g-C3N4/I3-BiOI composites.

3.2. Visible-light-driven photocatalytic performance of g-C3N4/I3--BiOI. The removal efficiency of CH3SH was used to evaluate photocatalytic activity of as-prepared photocatalysts upon LED irradiation and the results were shown in Figure 8a. Obviously, ultrathin BiOI (denoted as BiOI) displayed a higher CH3SH removal efficiency (76%) than bulk BiOI does (66.9%). The g-C3N4/BiOI composite (= the optimum 10%-g-C3N4/BiOI in Figure S10) displayed a better photocatalytic performance for CH3SH removal (87.35%) than that of pure BiOI (76%) and g-C3N4 (68%), while the mechanically mixed MM-g-C3N4/BiOI only displayed a slight enhancement to 77%. Other x-g-C3N4/BiOI (x = 4, 16, 24%) also exhibited an enhanced performance (Figure S10). All of the degradation data also fits well with the pseudo first-order kinetics and the reaction rate constant k of g-C3N4/BiOI (0.0032 s-1) was also determined to be larger than that of MM-g-C3N4/BiOI (0.0024 s−1) (Figure S11). This demonstrates the solvothermal treatment can efficiently strength the interaction between porous g-C3N4 and ultrathin BiOI, thus to enhance its

21



photocatalytic activity. In particular, all the g-C3N4/I3--BiOI-x (different irradiation time of x = 10, 30, 60 min) exhibit superior activity compared to g-C3N4/BiOI and gC3N4/I3--BiOI at 30 min reaches an unprecedented high CH3SH removal ratio of 94.17%, suggesting the important role of I3- species in accelerating charge transfer. Thus, a facile photoreduction method has been developed to prepare highly efficient g-C3N4/I3--BiOI and the optimized preparation conditions are confirmed. Based on the XPS data (Figure 3), the surface C/N atom ratio increases from 0.76 to 1.07 when BiOI is introduced to g-C3N4, which may have impact on its photocatalytic activity based on some reports. 46, 47 To identify the photocatalytic role of varied C/N ratio in 10%-gC3N4/I3--BiOI, the as-prepared 10%-g-C3N4/I3--BiOI materials were washed for longer time to maximum remove the residual ethylene glycol (denoted as 10%-g-C3N4/I3-BiOI-1#) and then immediately applied for the photocatalytic elimination of CH3SH. As shown in Figure S12, the CH3SH removal efficiency of 10%-g-C3N4/I3--BiOI-1# was 94.12%, which has negligible difference with that of 10%-g-C3N4/I3--BiOI (94.17%) and the reaction rate constant k of 10%-g-C3N4/I3--BiOI-1# (0.0032 s-1) is the same with 10%-g-C3N4/I3--BiOI, suggesting the increased ratio of C/N has little effect on the photocatalytic activity of g-C3N4/I3--BiOI. Moreover, the cyclic experiment (Figure 8b) also revealed that reduction in the photocatalytic activity of reused g-C3N4/I3--BiOI was negligible after 5 cycles. The fifth photocatalytic efficiency was still maintained at 93%, demonstrating the high stability of the gC3N4/I3--BiOI composite.

22


Page 22 of 48

Page 23 of 48

3g-C3N4/I -BiOI (60 min) 3g-C3N4/I -BiOI (30 min)

(a)

1.0

1.0

3g-C3N4/I -BiOI (10 min)

0.8

0.8

g-C3N4/BiOI MM-g-C3N4/BiOI 3I -BiOI

0.6

(b)

0.6

C/C0

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BiOI Bulk-BiOI g-C3N4

0.4

0.4

0.2

0.2 0.0

0.0 0

200

400

600

800

0

1000

15

Time (s)

30

45

Time (min)

60

75

Figure 8. (a) Removal of CH3SH using g-C3N4, bulk BiOI, BiOI, I3--BiOI, MM-gC3N4/BiOI, g-C3N4/BiOI and g-C3N4/I3--BiOI; (b) Removal of CH3SH in multiple runs using 10%-g-C3N4/I3--BiOI under white LED irradiation. To support the finding that the reaction proceed through light absorption within g-C3N4/I3--BiOI composite, the dependence of the removal efficiency on the wavelength of incident light (different colored LEDs) was investigated. As shown in Figure 9c, the removal efficiency of g-C3N4/I3--BiOI slightly decreased from 94.7% to 92.9% as the wavelength of the light source increased from blue to red, matching well with that of the absorption in the optical spectra of Figure 7a. Similarly, the calculated degradation rate slightly decreases from 0.0032 to 0.0026 min−1 with the increasing wavelength of the irradiated light (Figure S13c). Generally, the photon energy of a blue LED lamp is higher than that of other colored lamps.32 Interestingly, even the light with the longest wavelength (610−650 nm, Red) was able to activate g-C3N4/I3-BiOI composite and maintain good performance, suggesting that the photocatalyst could effectively use the full spectrum of VL for CH3SH degradation. In contrast, both pure BiOI and g-C3N4 exhibited the same patterns (lower performance with longer wavelength light), and almost total inhibited photocatalytic performance under red light (43.09% of BiOI and 5% of g-C3N4, Figure 9a, b). This further supports the effective light adsorption and energy utilization in g-C3N4/I3--BiOI composite. 23



(a)

1.0 BiOI 0.8

C/C0

0.6 0.4

Blue White Green Red

0.2 0.0 0

200

400

600

Time (s)

800

1000

1.0

(b)

C/C0

0.8 0.6

g-C3N4

0.4

Blue White Green Red

0.2 0.0 0

200

400

600

800

1000

Time (s) (c)

1.0 0.8

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 48

10%-g-C3N4/I3--BiOI

0.6 0.4 Blue White Green Red

0.2 0.0 0

200

400

600

800

1000

Time (s)

Figure 9. Removal of CH3SH by pure BiOI (a), g-C3N4 (b) and 10%-g-C3N4/I3--BiOI (c) under different singlet colored LED irradiation.

3.3. Role of reactive species on g-C3N4/I3--BiOI composite. Generally, the photocatalysis involves the surface reactions of both photogenerated holes and electrons, which may also produce oxidizing species, such 24


Page 25 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


as •O2−, •OH and 1O2 to further proceed the oxidation reactions.48 The photogenerated radical species can be directly determined by ESR spin-trapping technique. As shown in Figure 10a, the distinct characteristic peaks of DMPO-·O2- (relative intensities of 1:1:1:1) for g-C3N4 were observed, while that for BiOI was negligible. This is because the CB potential of g-C3N4 (-0.8 V) was negative enough to reduce O2 into ·O2- (E0 = −0.33 eV/vs. NHE) but that of BiOI (0.3 V) was too positive.18,

20

After I3--BiOI

coupling, g-C3N4/I3--BiOI displayed much stronger DMPO-·O2- signal than pure gC3N4, implying the enhanced reduction activity of g-C3N4/I3--BiOI for •O2− generation. The results clearly indicated that the photoexcited electrons would accumulate on the CB of g-C3N4, rather than the CB of BiOI in g-C3N4/I3--BiOI composites, thus to intensify the reduction of O2 into •O2− radicals. Moreover, Figure 10b indicates the intensity of DMPO-·O2- ESR signals initially enhanced and became stable at 5 min, suggesting the increasing generation of ·O2- species over time.9 However, no well-defined ESR peaks for DMPO-·OH were observed in the three photocatalytic processes (Figure S14), suggesting the limited generation of ·OH in the photocatalytic reaction. Similar phenomena were also observed by Liu et al.49 The role of 1O2 is always neglected in photocatalytic process because its short life span. TEMP was selected as the spin trap to detect 1O2 in this work. As shown in Figure 10c, the large production of 1O2 were clearly observed for pure BiOI and gC3N4/I3--BiOI with occurrence of three distinctive lines of TEMP-1O2 (relative intensities of 1:1:1), while the peak intensities for g-C3N4 was much weaker since the VB position of g-C3N4 was not positive enough to oxidize O2 or ·O2- to form 1O2.18, 50 Similarly, the intensity of 3-fold TEMP-1O2 peaks g-C3N4/I3--BiOI was reached at 10 min (Figure 10d).

25



(a)

Intensity (a.u.)

g-C3N4/I -BiOI

Intensity (a.u.)

(b)

10 min

3-

-

DMPO-·O2

g-C3N4

-

DMPO-·O2

5 min

1 min BiOI

0 min 3440

3480

3520

3560

Magnetic Field (10-4 T) 1

TEMP- O2

3450

3480

3510

3540

Magnetic Field (10-4 T)

3570

(d)

(c) 10 min

3-

Intensity

g-C3N4/I -BiOI

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 48

BiOI

1

TEMP- O2

5 min

1 min

g-C3N4 0 min

3450

3480 3510 3540 Magnetic Field (10-4 T)

3570

3450

3480 3510 3540 Magnetic Field (10-4 T)

3570

Figure 10. Superoxide radical species trapped by DMPO/DMSO of BiOI, g-C3N4, and 10%-g-C3N4/I3--BiOI irradiated with 5 min under visible light (a) and g-C3N4/I3-BiOI irradiated within different irradiation time (b); Singlet oxygen species trapped by TEMP of BiOI, g-C3N4, and 10%-g-C3N4/I3--BiOI irradiated with 5 min under visible light (c) and g-C3N4/I3--BiOI irradiated with different irradiation time (d). Considering the fact that several reactive oxygen species were generated by gC3N4/I3--BiOI, it is essential to explore the role of each species for CH3SH removal through scavenger study. Simultaneously, photogenerated holes and electrons, as the precursors of ROSs, were also analyzed. As shown in Figure 11, CH3SH removal efficiency of pure BiOI decreased to some extent by adding NaN3 (1O2 scavenger) while CH3SH removal was obviously inhibited when EDTA-2Na (h+ scavenger) was

26


Page 27 of 48

added. It revealed that h+ was the dominant reactive species for CH3SH removal over BiOI. Addition of ascorbic acid (·O2- scavenger) clearly depressed CH3SH removal efficiency on g-C3N4, suggesting that ·O2- played a critical role in CH3SH removal, while the addition of EDTA-2Na (h+ scavenger) slightly inhibited CH3SH removal efficiency. As for g-C3N4/I3--BiOI, the efficient free radicals in the photocatalytic removal of CH3SH over g-C3N4/I3--BiOI were h+, ·O2- and 1O2 and the order of contribution to photocatalytic removal of CH3SH was h+ > ·O2- > 1O2. Obviously, the results indicate the coupling of g-C3N4 with BiOI leads to the different photocatalytic mechanism.

(a)

1.0

BiOI 0.8

C/C0

0.6 No scavenger TBA EDTA-2Na Ascorbic acid Cr (VI) NaN3

0.4 0.2 0.0 0

200

400

600

800

1000

Time (s)

(b)

1.0 0.8

g-C3N4

0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

No scavenger TBA EDTA-2Na Ascorbic acid Cr (VI) NaN3

0.2 0.0 0

200

400

600

800

Time (s)

27


1000


(c)

1.0

g-C3N4/I3--BiOI

0.8 0.6

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 48

0.4

No scavenger TBA EDTA-2Na Ascorbic acid Cr (VI) NaN3

0.2 0.0 0

200

400

600

800

1000

Time (s)

Figure 11. Photocatalytic removal of CH3SH with different scavengers by BiOI (a), g-C3N4 (b) and 10%-g-C3N4/I3--BiOI (c). 3.4. Charge transfer mode of g-C3N4/I3--BiOI. The band edge positions of composites played an important role in reduction and oxidation reactions occurring at photocatalyst surface. UV-vis DRS (Figure 7a) and Mott-Schottky plots (Figure 12) were used to exploit the band alignment of BiOI and g-C3N4. As shown in Figure 12, g-C3N4 has an n-type character with a positive slope, while BiOI was a p-type semiconductor with a negative slope. The flat potential of g-C3N4 (equal to CB band in n-type semiconductor) was located at -0.8 V and the flat potential of BiOI (equal to VB band in p-type semiconductor) was measured at 2.0 V. Combing the result of Mott-Schottky plots and the band-gap values speculated by UV-vis DRS, the staggered band alignment in heterojunctions was constructed shown in Figure 12c, d. However, an issue regarding the charge transfer mode of g-C3N4/I3--BiOI in the system still persisted. In the previous studies,36 it was suggested that g-C3N4/I3--BiOI composites followed a conventional double-transfer mechanism, which meant that the photoinduced electrons transfer occurs from CB of g-C3N4 to that of BiOI, leaving holes survived on the VB of BiOI move to that of gC3N4. In this case, the photogenerated electron would accumulate on the CB of I3-BiOI and result in the dominant reactive species of h+ rather than •O2-/1O2, which is

28


Page 29 of 48

inconsistent with the experimental result. Only when the charge transfer at the interfacial phases follow the Z-scheme transfer mode in g-C3N4/I3--BiOI, the electron can enrich on the CB of g-C3N4, thereby retaining sufficient capacity to reduce O2 species to •O2- and 1O2. 50

(a)

(b)

30

40 25

30

C-2/106 F2

C-2/106 F2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BiOI 20 10

20

g-C3N4

15 10

0

5 -0.8 eV

2.0 eV

-10 1.2

1.6

2.0

0 -1.0

2.4

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential/V vs NHE

Potential/V vs NHE

Figure 12. Mott-Schottky plots of BiOI (a) and g-C3N4 (b) at a frequency of 1000 Hz in aqueous solution of Na2SO4; Two models of charge separation proposed for gC3N4/I3--BiOI under visible irradiation: (c) conventional donor-acceptor charge transfer mode and (d) Z-scheme electron transfer mode. It was well known that Z-scheme bridge (noble Ag, graphene) or redox mediator (Fe2+/Fe3+, IO3-/I-) usually played an important role in Z-scheme photocatalytic mechanism.51, 52 In g-C3N4/I3--BiOI system, I 3d XPS spectra (Figure 3e) displayed that self-stability I3- species could be obtained by light illumination and the extremely

29



reversibility of I3-/I- pair was further proved by cyclic voltammetry (CV) tests (Figure 13). It was supposed that the I3-/I- pair would act as redox mediators for Z-scheme gC3N4/I3--BiOI photocatalyst system. To verify this hypothesis, the g-C3N4/I3--BiOI composite mixed with EDTA-2Na (eliminating photogenerated holes) was used to remove CH3SH for 15 min under visible light illumination and the used g-C3N4/I3-BiOI composite was measured by XPS. As shown in Figure S15, the peaks of I3- ions in the I 3d spectra disappeared, indicating that the hole scavenger EDTA blocked the generation of I3- ions, thus, I3- ions really acted as electron carriers when the photoinduced electron transfer across p-n junctions. Under light irradiation, I− ions in g-C3N4/I3--BiOI was oxidized to I3− ions by the photogenerated holes of g-C3N4 and simultaneously, the I3− ions could be reduced to I− by the photogenerated electrons from BiOI, resulting in a steady I3- level in g-C3N4/I3--BiOI composite. This result was further proved by XPS spectra (Figure 3e), which showed no increase of I3- ions formation in g-C3N4/I3--BiOI composite after 5 cycling runs.

1.2 0.9 Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 48

2I3-→ 3I2+2e-

0.6 0.3

3I-→I3-+2e-

0.0 -0.3 -0.6 0.2

I3-+ 2e-→ 3I-

0.4

0.6

3I2+2e-→ 2I3-

0.8

1.0

1.2

Potential/V vs SCE

Figure 13. Cyclic voltammograms for g-C3N4/I3--BiOI/FTO.

30


Page 31 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Based on the above results, the charge transfer mode of g-C3N4/I3--BiOI was presented in Figure 12d. Under visible light irradiation, both g-C3N4 and BiOI could be excited. Then, the internal static electric fields of BiOI resulted in that the e- of BiOI transfer to the interfacial phase through the intermediate I3-/I- redox mediators. The electron located in the CB of g-C3N4 (-0.8 eV/vs. NHE) possessed higher reducing power to reduction of O2 to ·O2-. Meanwhile, the higher level of VB of BiOI (2.0 eV/vs. NHE) provided stronger oxidizing power for the hole to form 1O2, thus enabling higher oxidation ability to remove CH3SH. Moreover, the charge transmission route over g-C3N4/I3--BiOI can be deduced, which involved Eqs. (1)–(4), in which the step 2 was accelerated. g-C3N4/I3--BiOI + visible light → h+ + e− (1) Acceleration: e− + O2 → •O2− (2) h+ + •O2− / O2→ 1O2 (3) h+/•O2−/1O2 + CH3SH→ CH3SO3-/SO42- + CO32-/CO2 + H2O (4)

3.5. In Situ FT-IR investigation of CH3SH Adsorption and Transformation Pathway. To identify the pathway of photocatalytic CH3SH removal, in situ DRIFTS is carried out to monitor time-dependent evolution of the reaction intermediates and products over g-C3N4/I3--BiOI surface. Under dark condition, a number of adsorption peaks appeared after CH3SH and air were introduced onto g-C3N4/I3--BiOI surface (Figure 14a), implying the strong interaction between CH3SH and g-C3N4/I3--BiOI composite.53 The adsorption and interaction of CH3SH cause characteristic δs-CH and

δas-CH stretch (2948 and 3015 cm-1), CH3 o-o-p bend (1443 cm-1), δs-CH3 and δasCH3 bend (1374 and 1497 cm-1), and C-H vibrations (844 cm-1) peaks to appear.54 Especially, the detected δa CH3S- (1283 cm−1) and dimethyl sulfide (CH3S-SCH3,

31



1004 cm-1) peaks are attributed to the deformation of CH3SH in the presence of catalyst.55 Some studies reported that metal M/M+ could react with mercaptans or other sulfur compounds to break S-H bond and the deprotonated methyl mercaptan (CH3S-) tend to form into dimethyl sulfide (CH3S-SCH3), so did the same role of metal bismuth to cause only negligible peak of S-H stretch (2605 cm-1) in this study.56 With prolonged adsorption time, some peaks assigned to deprotonated sulfonate acid (CH3SO3-) at 1169 cm-1 and sulfate (SO42-) at 1374 cm-1 were observed, suggesting the adsorbed CH3SH could be partially oxidized and even C-S bond cleaved. This is mainly due to the active two-coordinated N atoms of g-C3N4 or Bi metal of BiOI that facilitate the formation of activated oxygen species and then enhance the oxidation capacity of the surface oxygen species for CH3SH oxidation.57, 58 Moreover, other absorption bands developed progressively can be assigned undissociated H2O (3253 cm-1), mainly generated from the purging air.59 The normalized absorbance curve clearly shows that the amounts of the adsorbed CH3SH accumulated on the photocatalyst surface increased with time, and an outstanding intermediate of CH3Sformed during CH3SH adsorption (Figure 14b). The adsorption equilibrium is achieved in 12 min. Once the adsorption equilibrium was achieved, the LED light was applied to initiate the photocatalytic reaction as shown in Figure 14c. The “baseline” spectrum was the same as that of “CH3SH 12 min” in Figure 14a. After illumination for 2 min, the adsorption bands at 963, 1446, 1464 and 2840~3000 cm−1 corresponding to CH3SH species sharply decreased or disappeared over time, indicating the great consumption of accumulated CH3SH during adsorption.56 Correspondingly, some new bands of S element related to sulfonic acid (CH3SO3−) at 1169 cm−1 and sulfate species (SO42-) at 1325 cm−1 significantly increase, indicating C-S bonds were cleaved

32


Page 32 of 48

Page 33 of 48

and S elements were efficiently oxidized into SO42− over the catalyst. Especially, it was noted the CH3SO3− (1169 cm−1) fluctuated, while sulfates (1325 cm-1) gradually increased and then approached to a steady level with prolonged illumination time, suggesting the final product of sulfates (1325 cm-1) progressively accumulate on gC3N4/I3--BiOI. These accumulated products can be easily removed by water washing and the photocatalyst can be regenerated via this facile method.60 In parallel, some new peaks of C elements were also greatly intensified, corresponding to the νas(COO−) at 1636 cm−1 and ν(C=O) at 1766 cm−1 of carboxylic acids, as well as the νs(COO−) at 1546 cm−1 of bidentate carbonate, evidencing the C elements in the cleaved CH3SH are further converted into carboxylic acids and carbonate.61,

62

Interestingly, the peaks intensity of COO- and carbonate species increased sharply in the initial, then decreased and even disappeared. This suggests C elemental converse into carbonate and may release out as CO2. Therefore, the peaks intensity of CO2 at 2358 cm-1 dynamically stayed at a high level because generated CO2 have desorption on the surface of g-C3N4/I3--BiOI.63 Similarly, the normalized absorbance of intermediates (CH3SO3-) and final product (SO42− and CO2) are illustrated in Figure 14d, which were greatly boosted in the tendency of species evolution. This further indicates g-C3N4/I3--BiOI could efficiently adsorb and oxidize CH3SH to final products under visible-light illumination. 1497

(a)

1283 1004 938

1169

1438 1374

1668

1112

Normalized absorbance (a.u.)

CH3SH 10 min CH3SH 8 min CH3SH 6 min CH3SH 4 min CH3SH 2 min

Background

900

1200

0.0036

CH3S δs−CH3

0.0030

CH3S-SCH3

3253 2605 2948 3015

844

CH3SH 12 min

1500

1800

3000 3500

(b)

δas−CH3

3500

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2-

SO4

0.0024

CH3SO3

0.0018 0.0012 0.0006 0.0000 2

4

6

8

Time (min)

-1

Wavenumber (cm )

33


10

12


1325 1169

1450 1546 1421

1636

2358

(c)

0.08

3253

Normalized absorbance (a.u.)

2-

Light on 10 min Light on 8 min

(d)

CO2

Light on 12 min

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 48

Light on 6 min

Light on 4 min Light on 2 min Ads. Equil. Background

1200

1400

1600

1800 2400 3200 3600

Wavenumber (cm-1)

CO3

COO

0.06

2-

SO4

-

CH3SO3

0.04

0.02 Adsorp. Equil.

0.00 0

2

4

6

8

10

12

Time (min)

Figure 14. In situ DRIFTS spectra of CH3SH (a) and species evolution on photocatalysts surface (b) with 10%-g-C3N4/I3--BiOI during CH3SH adsorption process; In situ DRIFTS spectra of CH3SH (c) and species evolution on photocatalysts surface (d) with 10%-g-C3N4/I3--BiOI during photocatalytic CH3SH oxidation process. Based on the results, the conversion pathways for the CH3SH adsorption and photocatalytic oxidation processes on g-C3N4/I3--BiOI were proposed for the first time, as depicted in Figure 15.

Figure 15. Conversation pathway of adsorption and photocatalytic oxidation of CH3SH 34


Page 35 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To further reveal the photocatalytic process of g-C3N4/I3--BiOI, the products CH3SO3− and SO42- of CH3SH degradation were quantitatively measured through IC analysis, which each plot was acquired by repeating separated experiments in which the illumination stopped at a given time (Figure S16). It was found the amount of SO42- increased with reaction time and the concentration reached up to 4.11 µmol till 15 min, revealing the continuous production of SO42- during the photocatalytic process. Meanwhile, the amount of CH3SO3− increased at the initial 5 min, then keeps almost constant at 0.186 µmol regardless of the prolonged illumination. Notably, the amount of produced CH3SO3− was much lower than that of produced SO42-, indicating that the major final product was SO42- for the removal of CH3SH over g-C3N4/I3-BiOI, consistent with the in situ FT-IR investigation. These results provided strong evidence for the mineralization of CH3SH over g-C3N4/I3--BiOI sample.

3. CONCLUSIONS In summary, g-C3N4/I3--BiOI with self-stabilized I3-/I- redox mediator was successfully synthesized by a facile solvothermal method coupling with light illumination, in which g-C3N4 was firstly prepared by condensation of melamine-TA supramolecular aggregates. Possible reaction mechanism of CH3SH removal was clearly revealed by in situ DRIFTS and chromatography (IC). Furthermore, the active species trapping experiments and ESR studies confirmed that the responsible reactive species for CH3SH removal were h+, ·O2- and 1O2 and further study revealed that an indirect all-solid state Z-scheme charge transfer mode of g-C3N4/I3--BiOI using selfstabilized I3-/I- as redox mediator was demonstrated, which could accelerate the separation of photo-generated charge carriers and enhance the redox reaction power of charged carriers. The present work could provide promising perspectives in purification of odor gas.

35



ACKNOWLEDGEMENT The authors wish to thank the National Natural Science Foundation of China (No. 51578556, 21673086, 41603097), Natural Science Foundation of Guangdong Province (No. 2015A030308005, S2013010012927, S2011010003416), Science and Technology Research Programs of Guangdong Province (No. 2014A020216009) for financially supporting this work. Dr. Xia was also supported by the Start-up Funds for High-Level Talents of Sun Yat-sen University (38000-18821110).

ASSOCIATED CONTENT Supporting Information Available: Additional characterization and visible-light-driven photocatalytic performance

REFERENCES (1) He, C.; Li, X.Z.; Sharma, V.K.; Li, S. Elimination of Sludge Odor by Oxidizing Sulfur-Containing Compounds with Ferrate(VI). Environ. Sci. Technol. 2009, 43, 5890-5895. (2) Sun, J.; Hu, S.; Sharma, K.R.; Ni, B.; Yuan, Z. Degradation of Methanethiol in Anaerobic Sewers and its Correlation with Methanogenic Activities. Water Res. 2015,

69, 80-89. (3) Vellingiri, K.; Kim, K.H.; Kwon, E.E.; Deep, A.; Jo, S.H.; Szulejko, J.E. Insights into the Adsorption Capacity and Breakthrough Properties of a Synthetic Zeolite Against a Mixture of Various Sulfur Species at Low ppb Levels. J. Environ. Manage.

2016, 166, 484-492. (4) Ryzhikov, A.; Hulea, V.; Tichit, D.; Leroi, C.; Anglerot, D.; Coq, B.; Trens, P. 36


Page 36 of 48

Page 37 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Methyl Mercaptan and Carbonyl Sulfide Traces Removal through Adsorption and Catalysis on Zeolites and Layered Double Hydroxides. Appl. Catal. A: Gen. 2011,

397, 218-224. (5) Zeng, J.; Hu, L.; Tan, X.; He, C.; He, Z.; Pan, W.; Hou, Y.; Shu, D. Elimination of Methyl Mercaptan in ZVI-S2O82− System Activated with in-Situ Generated Ferrous Ions from Zero Valent Iron. Catal. Today 2017, 281, 520-526. (6) Sano, T.; Koike, K.; Hori, T.; Hirakawa, T.; Ohko, Y.; Takeuchi, K. Removal of Methyl Mercaptan with Highly-Mobile Silver on Graphitic Carbon-Nitride (g-C3N4) Photocatalyst. Appl. Catal. B: Environ. 2016, 198, 133-141. (7) Tan, G.; She, L.; Liu, T.; Xu, C.; Ren, H.; Xia, A. Ultrasonic Chemical Synthesis of Hybrid mpg-C3N4/BiPO4 Heterostructured Photocatalysts with Improved Visible Light Photocatalytic Activity. Appl. Catal. B: Environ. 2017, 207, 120-133. (8) Jiang, G.; Li, X.; Lan, M.; Shen, T.; Lv, X.; Dong, F.; Zhang, S. Monodisperse Bismuth Nanoparticles Decorated Graphitic Carbonnitride: Enhanced Visible-LightResponse Photocatalytic NO Removal and Reaction Pathway. Appl. Catal. B:

Environ. 2017, 205, 532-540. (9) Xia, D.; Wang, W.; Yin, R.; Jiang, Z.; An, T.; Li, G.; Zhao, H.; Wong, P. K. Enhanced Photocatalytic Inactivation of Escherichia Coli by a Novel Z-Scheme gC3N4/m-Bi2O4 Hybrid Photocatalyst under Visible Light: The Role of Reactive

37



Oxygen Species. Appl. Catal. B: Environ. 2017, 214, 23-33. (10) Dong, G.; Yang, L.; Wang, F.; Zang, L.; Wang, C. Removal of Nitric Oxide through Visible Light Photocatalysis by g-C3N4 Modified with Perylene Imides. ACS

Catal. 2016, 6, 6511-6519. (11) Zhang, M.; Xu, J.; Zong, R.; Zhu, Y. Enhancement of Visible Light Photocatalytic Activities via Porous Structure of g-C3N4. Appl. Catal. B: Environ.

2014, 147, 229-235. (12) Kang, Y.; Yang, Y.; Yin, L.; Kang, X.; Wang, L.; Liu, G.; Cheng, H. Selective Breaking of Hydrogen Bonds of Layered Carbon Nitride for Visible Light Photocatalysis. Adv. Mater. 2016, 28, 6471-6477. (13) Cui, W.; Li, J.; Dong, F.; Sun, Y.; Jiang, G.; Cen, W.; Lee, S. C.; Wu, Z. Highly Efficient Performance and Conversion Pathway of Photocatalytic NO Oxidation on SrO-Clusters@Amorphous Carbon Nitride. Environ. Sci. Technol. 2017, 51, 1068210690. (14) Yang, X.; Qian, F.; Zou, G.; Li, M.; Lu, J.; Li, Y.; Bao, M. Facile Fabrication of Acidified g-C3N4/g-C3N4 Hybrids with Enhanced Photocatalysis Performance under Visible Light Irradiation. Appl. Catal. B: Environ. 2016, 193, 22-35. (15) Xu, H.; Yan, J.; Xu, Y.; Song, Y.; Li, H.; Xia, J.; Huang, C.; Wan, H. Novel Visible-Light-Driven AgX/Graphite-Like C3N4 (x = Br, I) Hybrid Materials with

38


Page 38 of 48

Page 39 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Synergistic Photocatalytic Activity. Appl. Catal. B: Environ. 2013, 129, 182-193. (16) Jiang, Z.; Qian, K.; Zhu, C.Z.; Sun, H.; Wan, W.; Xie, J.；Li, H.; Wong, P.K.; Yuan, S. Carbon Nitride Coupled with CdS-TiO2 Nanodots as 2D/0D Ternary Composite with Enhanced Photocatalytic H2 Evolution: A Novel Efficient ThreeLevel Electron Transfer Process. Appl. Catal. B: Environ. 2017, 210, 194-204. (17) Bai, Y.; Ye, L.Q.; Wang, L.; Shi, X.; Wang, P.; Bai, W.; Wong, P.K. gC3N4/Bi4O5I2 Heterojunction with I3−/I− Redox Mediator Forenhanced Photocatalytic CO2 Conversion. Appl. Catal. B: Environ. 2016, 194, 98-104. (18) Wang, J.; Yao, H.; Fan, Z.; Zhang, L.; Wang, J.; Zang, S.; Li, Z. Indirect ZScheme BiOI/g-C3N4 Photocatalysts with Enhanced Photoreduction CO2 Activity under Visible Light Irradiation. ACS Appl. Mater. Interfaces 2016, 8, 3765-3775. (19) Jiang, Z.; Wan, W.; Li, H.; Yuan, S.; Zhao, H.; Wong, P.K. A Hierarchical Z Scheme α-Fe2O3/g-C3N4 Hybrid for Enhanced Photocatalytic CO2 Reduction. Adv.

Mater. 2017, 30, 1706108-1706116. (20) Ye, L.; Liu, J.; Jiang, Z.; Peng, T.; Zan, L. Facets Coupling of BiOBr-g-C3N4 Composite Photocatalyst for Enhanced Visible-Light-Driven Photocatalytic Activity.

Appl. Catal. B: Environ. 2013, 142-143, 1-7. (21) Zhang, X.; Li, B.; Wang, J.; Yuan, Y.; Zhang, Q.; Gao, Z.; Liu, L.; Chen, L. The Stabilities and Electronic Structures of Single-Layer Bismuth Oxyhalides for

39



Page 40 of 48

Photocatalytic Water Splitting. Phys. Chem. Chem. Phys. 2014, 16, 25854-25861. (22) Jiang, Z.; Liang, X.; Liu, Y.; Jing, T.; Wang, Z.; Zhang, X.; Qin, X.; Dai, Y.; Huang, B. Enhancing Visible Light Photocatalytic Degradation Performance and Bactericidal Activity of BiOI via Ultrathin-Layer Structure. Appl. Catal. B: Environ.

2017, 211, 252-257. (23) Di, J.; Xia, J.; Ji, M.; Xu, L.; Yin, S.; Chen, Z.; Li, H. Bidirectional Acceleration of Carrier Separation Spatially via N-CQDs/Atomically-Thin BiOI Nanosheets Nanojunctions for Manipulating Active Species in a Photocatalytic Process. J. Mater.

Chem. A 2016, 4, 5051-5061. (24) Gong, Y.; Quan, X.; Yu, H.; Chen, S. Synthesis of Z-scheme Ag2CrO4/Ag/g-C3N4 Composite

with

Enhanced

Visible-Light

Photocatalytic

Activity

for

2,4-

dichlorophenol Degradation. Appl. Catal. B: Environ. 2017, 219, 439-449. (25) Zhu, Y.; Chen, Z.; Gao, T.; Huang, Q.; Niu, F.; Qin, L.; Tang, P.; Huang, Y.; Sha, Z.; Wang, Y. Construction of Hybrid Z-Scheme Pt/CdS–TNTAs with Enhanced Visible-Light Photocatalytic Performance. Appl. Catal. B: Environ. 2015, 163, 16-22. (26) Chen, D.; He, D.; Lu, J.; Zhong, L.; Liu, F.; Liu, J.; Yu, J.; Wan, G.; He, S.; Luo, Y. Investigation of the Role of Surface Lattice Oxygen and Bulk Lattice Oxygen Migration of Cerium-Based Oxygen Carriers: XPS and Designed H2-TPR Characterization. Appl. Catal. B: Environ. 2017, 218, 249-259.

40


Page 41 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Liu, T.; Li, X.; Li, F. AgNO3-Induced Photocatalytic Degradation of Odorous Methyl

Mercaptan

in

Gaseous

Phase:

Mechanism

of

Chemisorption

and Photocatalytic Reaction. Environ. Sci. Technol. 2008, 42, 4540-4545. (28) He, D.; Chen, D.; Hao, H.; Yu, J.; Liu, P.; Lu, J.; Wan, G.; He, S.; Li, K.; Luo, Y. Enhanced Activity and Stability of Sm-Doped HZSM-5 Zeolite Catalysts for Catalytic Methyl Mercaptan (CH3SH) Decomposition. Chem. Eng. J. 2017, 317, 60-69. (29) Tan, T. H.; Scott, J. A.; Ng, Y. H.; Taylor, R. A.; Aguey-Zinsou, K.-F.; Amal, R. Plasmon Enhanced Selective Electronic Pathways in TiO2 Supported Atomically Ordered Bimetallic Au-Cu Alloys. J. Catal. 2017, 352, 638-648. (30) Yi, D.; Huang, H.; Meng, X.; Shi, L. Adsorption-Desorption Behavior and Mechanism of Dimethyl Disulfide in Liquid Hydrocarbon Streams on Modified Y Zeolites. Appl. Catal. B: Environ. 2014, 148-149, 377-386. (31) Li, H.; Song, H.; Chen, L.; Xia, C. Designed SO42−/Fe2O3-SiO2 Solid Acids for Polyoxymethylene Dimethyl Ethers Synthesis: The Acid Sites Control and Reaction Pathways. Appl. Catal. B: Environ. 2015, 165, 466-476. (32) Xia, D.; Shen, Z.; Huang, G.; Wang, W.; Yu, J.C.; Wong, P.K. Red Phosphorus: An Earth-Abundant Elemental Photocatalyst for “Green” Bacterial Inactivation under Visible Light. Environ. Sci. Technol. 2015, 49, 6264-6273. (33) Xia, D.; Liu, H.; Jiang, Z.; Ng, T.W.; Lai, W. S.; An, T.; Wang, W.; Wong, P. K.

41



Visible-Light-Driven Photocatalytic Inactivation of Escherichia Coli K-12 over Thermal Treated Natural Magnetic Sphalerite: Band Structure Analysis and Toxicity Evaluation. Appl. Catal. B: Environ. 2018, 224, 541-552. (34) Xia, D.; Ng, T.W.; An, T.; Li, G.; Li, Y.; Yip, H.Y.; Zhao, H.; Lu, A.; Wong, P.K. A Recyclable Mineral Catalyst for Visible-Light-Driven Photocatalytic Inactivation of Bacteria: Natural Magnetic Sphalerite. Environ. Sci. Technol. 2013,

47, 11166-11173. (35) Yang, L.; Huang, J.; Shi, L.; Cao, L.; Yu, Q.; Jie, Y.; Fei, J.; Ouyang, H.; Ye, J. A Surface Modification Resultant Thermally Oxidized Porous g-C3N4 with Enhanced Photocatalytic Hydrogen Production. Appl. Catal. B: Environ. 2017, 204, 335-345. (36) Di, J.; Xia, J.; Yin, S.; Xu, H.; Xu, L.; Xu, Y.; He, M.; Li, H. Preparation of Sphere-Like g-C3N4/BiOI Photocatalysts via a Reactable Ionic Liquid for VisibleLight-Driven Photocatalytic Degradation of Pollutants. J. Mater. Chem. A 2014, 2, 5340-5351. (37) He, F.; Chen, G.; Yu, Y.; Zhou, Y.; Zheng, Y.; Hao, S. The Sulfur-Bubble Template-Mediated Synthesis of Uniform Porous g-C3N4 with Superior Photocatalytic Performance. Chem. Commun. 2015, 51, 425-427. (38) Qiu, P.; Xu, C.; Chen, H.; Jiang, F.; Wang, X.; Lu, R.; Zhang, X. One Step Synthesis of Oxygen Doped Porous Graphitic Carbon Nitride with Remarkable

42


Page 42 of 48

Page 43 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Improvement of Photo-Oxidation Activity: Role of Oxygen on Visible Light Photocatalytic Activity. Appl. Catal. B: Environ. 2017, 206, 319-327. (39) Schobing, J.; Tschamber, V.; Brillard, A.; Leyssens, G. Impact of Biodiesel Impurities on Carbon Oxidation in Passive Regeneration Conditions: Influence of the Alkali Metals. Appl. Catal. B: Environ. 2017, 226, 596-607. (40) Chang, C.; Zhu, L.; Wang, S.; Chu, X.; Yue, L. Novel Mesoporous Graphite Carbon Nitride/BiOI Heterojunction for Enhancing Photocatalytic Performance under Visible-Light Irradiation. ACS Appl. Mater. Interfaces 2014, 6, 5083-5093. (41) Wang, K.; Zhang, G.; Li, J.; Li, Y.; Wu, X. 0D/2D Z-Scheme Heterojunctions of Bismuth Tantalate Quantum Dots/Ultrathin g-C3N4 Nanosheets for Highly Efficient Visible Light Photocatalytic Degradation of Antibiotics. ACS Appl. Mater. Interfaces

2017, 9, 43704-43715. (42) Wang, Q.; Wang, W.; Zhong, L.; Liu, D.; Cao, X.; Cui, F. Oxygen Vacancy-Rich 2D/2D BiOCl-g-C3N4 Ultrathin Heterostructure Nanosheets for Enhanced VisibleLight-Driven Photocatalytic Activity in Environmental Remediation. Appl. Catal. B:

Environ. 2018, 220, 290-302. (43) Yang, Z.; Xu, X.; Liang, X.; Lei, C.; Cui, Y.; Wu, W.; Yang, Y.; Zhang, Z.; Lei, Z. Construction of Heterostructured MIL-125/Ag/g-C3N4 Nanocomposite as an Efficient Bifunctional Visible Light Photocatalyst for the Organic Oxidation and Reduction

43



Reactions. Appl. Catal. B: Environ. 2017, 205, 42-54. (44) Liu, J.; Zhang, T.; Wang, Z.; Dawson, G.; Chen, W. Simple Pyrolysis of Urea into Graphitic Carbon Nitride with Recyclable Adsorption and Photocatalytic Activity.

J. Mater. Chem. 2011, 21, 14398-14401. (45) Su, F.; Xu, C.; Yu, Y.; Zhang, W. Carbon Self-Doping Induced Activation of nπ* Electronic Transitions of g-C3N4 Nanosheets for Efficient Photocatalytic H2 Evolution. Chem. Cat. Chem. 2016, 8, 3527-3535. (46) Zhang, Z.; Zhang, Y.; Lu, L.; Si, Y.; Zhang, S.; Chen, Y.; Dai, K.; Duan, P.; Duan, L.; Liu, J. Graphitic Carbon Nitride Nanosheet for Photocatalytic Hydrogen Production: The Impact of Morphology and Element Composition. Appl. Surf. Sci.

2017, 391, 369-375. (47) Xu, Q.; Jiang, C.; Bei, C.; Yu, J. Enhanced Visible-Light Photocatalytic H2Generation Activity of Carbon/g-C3N4 Nanocomposites Prepared by Two-Step Thermal Treatment. Dalton Trans. 2017, 46, 10611-10619. (48) Xia, D.; Hu, L.; He, C.; Pan, W.; Yang, T.; Yang, Y.; Shu, D. Simultaneous Photocatalytic Elimination of Gaseous NO and SO2 in a BiOI/Al2O3-Padded Trickling Scrubber under Visible Light. Chem. Eng. J. 2015, 279, 929-938. (49) Liu, C.; Zhu, H.; Zhu, Y.; Dong, P.; Hou, H.; Xu, Q.; Chen, X.; Xi, X.; Hou, W. Ordered Layered N-Doped KTiNbO5/g-C3N4 Heterojunction with Enhanced Visible

44


Page 44 of 48

Page 45 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Light Photocatalytic Activity. Appl. Catal. B: Environ. 2018, 228, 54-63. (50) Nosaka, Y.; Nosaka, A. Y. Generation and Detection of Reactive Oxygen Species in Photocatalysis. Chem. Rev. 2017, 117, 11302-11336. (51) Ou, M.; Wan, S.; Zhong, Q.; Zhang, S.; Song, Y.; Guo, L.; Cai, W.; Xu, Y. Hierarchical Z-Scheme Photocatalyst of g-C3N4@Ag/BiVO4 (040) with Enhanced Visible-Light-Induced Photocatalytic Oxidation Performance. Appl. Catal. B:

Environ. 2018, 221, 97-107. (52) Yan, J.; Wu, H.; Chen, H.; Zhang, Y.; Zhang, F.; Liu, S. Fabrication of TiO2/C3N4 Heterostructure for Enhanced Photocatalytic Z-Scheme overall Water Splitting. Appl.

Catal. B: Environ. 2016, 191, 130-137. (53) Romão, B.M.V.; Diniz, M.F.; Azevedo, M.F.P.; Lourenço, V.L.; Pardini, L.C.; Dutra, R.C.L.; Bruel, F. Characterization of the Curing Agents Used in Epoxy Resins with TG/FT-IR Technique. Polimeros. 2006, 16, 94-98. (54) Kim, D.J.; Lee, H.I.; Yie, J.E.; Kim, S.J.; Kim, J.M. Ordered Mesoporous Carbons: Implication of Surface Chemistry, Pore Structure and Adsorption of Methyl Mercaptan. Carbon 2005, 43, 1868-1873. (55) Tang, X.; Hu, T.; Li, J.; Wang, F.; Qing, D. Desulfurization of Kerosene by the Electrochemical Oxidation and Extraction Process. Energ. Fuel. 2015, 29, 2097-2103. (56) Liu, T.; Li, X.; Li, F. AgNO3-Induced Photocatalytic Degradation of Odorous

45



Methyl Mercaptan in Gaseous Phase: Mechanism of Chemisorption and Photocatalytic Reaction. Environ. Sci. Technol. 2008, 42, 4540-4545. (57) Zhu, X.; Yu, J.; Jiang, C. Cheng, B. Catalytic Decomposition and Mechanism of Formaldehyde over Pt–Al2O3 Molecular Sieves at Room Temperature. Phys. Chem.

Chem. Phys. 2017, 19, 6957-6963. (58) Fang, D.; Liao, X.; Zhang, X.; Teng, A.; Xue, X. A Novel Resource Utilization of the Calcium-Based Semi-Dry Flue Gas Desulfurization Ash: As a Reductant to Remove Chromium and Vanadium from Vanadium Industrial Wastewater. J. Hazard.

Mater. 2018, 342, 436-445. (59) Solis-Garcia, A.; Louvier-Hernandez, J.F.; Almendarez-Camarillo, A.; FierroGonzalez, J.C. Participation of Surface Bicarbonate, Formate and Methoxy Species in the Carbon Dioxide Methanation Catalyzed by ZrO2-Supported Ni. Appl. Catal. B:

Environ. 2017, 218, 611-620. (60) Jiang, G.; Li, X.; Lan, M.; Shen, T.; Lv, X.; Dong, F.; Zhang, S. Monodisperse Bismuth Nanoparticles Decorated Graphitic Carbonnitride: Enhanced Visible-LightResponse Photocatalytic NO Removal and Reaction Pathway. Appl. Catal. B:

Environ. 2017, 205, 532-540. (61) Zhou, Y.; Zhao, Z.; Wang, F.; Cao, K.; Doronkin, D.E.; Dong, F.; Grunwaldt, J.D. Facile Synthesis of Surface N-Doped Bi2O2CO3: Origin of Visible Light

46


Page 46 of 48

Page 47 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Photocatalytic Activity and in Situ DRIFTS Studies. J. Hazard. Mater. 2016, 307, 163-172. (62) Nie, L.; Wang, J.; Yu, J. Preparation of a Pt/TiO2/Cotton Fiber Composite Catalyst with Low Air Resistance for Efficient Formaldehyde Oxidation at Room Temperature. RCS Adv. 2017, 7, 21389-21397. (63) Yang, X.; Yu, X.; Lin, M.; Ge, M.; Yao, Z.; Wang, F. Interface Effect of Mixed Phase Pt/ZrO2 Catalysts for HCHO Oxidation at Ambient Temperature. J. Mater.

Chem. A 2017, 5, 13799-13806.

47



Graphical abstract:

48


Page 48 of 48

Highly Efficient Performance and Conversion Pathway of

Recommend Documents