Removal of Nitric Oxide through Visible Light

Aug 24, 2016 - acid rain, and ozone depletion.1−3 The concentration of NO in the atmosphere has greatly increased over the past decades because of t...
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Removal of Nitric Oxide through Visible Light Photocatalysis by g-C3N4 Modified with Perylene Imides Guohui Dong, Liping Yang, Fu Wang, Ling Zang, and Chuanyi Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01657 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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Removal of Nitric Oxide through Visible Light Photocatalysis by

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g-C3N4 Modified with Perylene Imides

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Guohui Dong ∗, † , # , Liping Yang †, ‡, #, Fu Wang †, Ling Zang *, § and Chuanyi Wang *, †

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Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of

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Sciences, Urumqi 830011, China

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Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry;

The Graduate School of Chinese Academy of Science, Beijing, 100049, China

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§ Nano

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City, Utah 84112, United States.

Institute of Utah and Department of Materials Science and Engineering, University of Utah, Salt Lake

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#

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RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required

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according to the journal that you are submitting your paper to)

These authors contributed equally to this work.

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To whom correspondence should be addressed. E-mail: [email protected]; [email protected]; [email protected], Phone: +86-0991-3835879; Fax: +86-0991-3838957. 1

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Abstract: For photocatalytic removal of nitric oxide (NO), two major issues need to be addressed,

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incomplete oxidation of NO and the deactivation of photocatalyst. In this study, we aimed to solve

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these two problems by constructing an all-solid-state Z-Scheme heterojunction (PI-g-C3N4) consisting

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of g-C3N4 surface-modified with perylene imides (PI). PI-g-C3N4 exhibits significant enhancement in

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photocatalytic activity (compared to pristine g-C3N4) when examined for NO removal. More

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importantly, the Z-scheme charge separation within PI-g-C3N4 populates electron and hole into the

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increased energy levels, thereby enabling direct reduction of O2 to H2O2 and direct oxidation of NO

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to NO2. H2O2 can further oxidize NO2 to NO3- ion at a different location (via diffusion), thus

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alleviating the deactivation to catalyst. The results presented may shed light on the design of visible

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photocatalysts with tunable reactivity for application in solar energy conversion and environmental

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sustainability.

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Keywords: Nitric oxide removal; g-C3N4; Z-Scheme; PTCDI; Molecular oxygen activation

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Introduction

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As a common gaseous pollutant, nitric oxide (NO) causes environmental problems, such as haze,

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photochemical smog, acid rain, and ozone depletion.1-3 The concentration of NO in the atmosphere

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has greatly increased over the past decades because of the increasing automobiles and industrial

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activities.4,

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atmospheric NO has become a global concern. Although some methods including thermal catalysis

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reduction, physical or chemical adsorption have been adopted to remove NO from atmosphere, most

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of them suffer from low efficiency and may produce secondary pollution.6-9 As an efficient catalytic

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process that can be operated under mild conditions (e.g., without high temperature or addition of

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strong oxidizing or reducing reagents), semiconductor photocatalysis has recently been recognized

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as an attractive alternative technology for NO removal.10 Under ambient light irradiation (with

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Therefore, developing efficient and economical technologies to eliminate the

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energy equal or higher than the bandgap of the semiconductor), an electron (e−) is excited from the

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valence band (VB) into the conduction band (CB), leaving a hole (h+) in the valence band. The

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photogenerated electrons and holes migrate to the photocatalyst surface and initiate subsequent

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redox reactions.11 Through the redox reactions, NO can be eliminated effectively. Utilization of solar

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energy to treat environment problems, like those via photocatalysis, has been considered as the most

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energy efficient and cost effective approach.

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It was reported that metal oxide semiconductor like TiO2, SiO2, and Al2O3 could function as

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effective photocatalysts for complete removal of NO under UV light irradiation.12-14 However, the

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wide band gap of these materials limits the photoresponse to UV region, with no use of visible light,

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which accounts for about five times higher intensity than UV light in solar spectrum. Therefore, it is

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desired to develop visible light sensitive photocatalysts for the practical application in NO removal.

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To this regard, many visible light photocatalysts, such as BiOBr, InVO4, Bi2MoO6, (BiO)2CO3 and

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graphitic carbon nitride (g-C3N4), have been developed and utilized for NO removal.15-20 Among

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these catalysts, g-C3N4 is a metal-free polymeric semiconductor material, which becomes

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increasingly promising for photocatalysis under visible light mainly due to its desirable band gap of

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2.7 eV, strong robustness, and low cost.21-23 Our previous work demonstrated that g-C3N4 could

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oxidize NO to NO2 under visible light irradiation.24 However, NO2 is a more toxic gas, which is

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detrimental to lung and increases the risk of bronchitis and pulmonary fibrosis. Recently, we found

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that modification of g-C3N4 with noble metals could change the photocatalytic reaction of NO from

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producing NO2 to NO3-.25 However, NO3- ions occupy the surface active sites and cause the

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deactivation of catalysis. Therefore, it is of the utmost interest, though a great challenge, to design a

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modification method which makes g-C3N4 can deeply oxidize NO to NO3- but with minimal

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deactivation in catalytic activity. In nature, photosynthesis with Z-scheme charge transfer process 3

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can separate the photogenerated electrons and holes into two photosystems through the electron

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mediator.26 Inspired by the advantage of photosynthesis, we aimed to design an all-solid-state

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Z-Scheme heterojunction structured photocatalyst based on g-C3N4, with which the photogenerated

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electrons and holes can be separated into two different phases, helping spatially isolate the oxidation

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and reduction reaction sites,26 and thus minimizing the catalytic deactivation.

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Molecules of perylene tetracarboxylic diimide (PTCDI) represent a unique class of n-type

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organic semiconductor, with strong thermal and photo-stability.27 PTCDI materials have been

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extensively used in bulk heterojunction (BHJ) solar cells because of their strong visible light

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absorption, good charge transportation properties, and durable photostability.27-29 PTCDI can be

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synthesized through a one-step reaction between perylene tetracarboxylic dianhydride (PTCDA) and

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the primary amines (-NH2). Since the edges of g-C3N4 are full of -NH2 groups, PTCDI can be

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modified onto g-C3N4 simply by reacting PTCDA with g-C3N4 in a solution. In this study, we

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provide an all-solid-state Z-Scheme heterojunction consisting of PI and g-C3N4 (PI-g-C3N4) for the

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photocatalytic removal of NO under visible light. The structure of the PI-g-C3N4 heterojunction was

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characterized by various experimental methods. Compared to the single-phase g-C3N4, PI-g-C3N4

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demonstrated much improved photocatalytic efficiency for NO removal. The photocatalysis

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mechanism of the heterojunction structure was deeply explored as presented below.

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Experimental Section

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Preparation of Photocatalysts. All chemicals were purchased in analytical grade and used without

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further purification. g-C3N4 was synthesized by direct heating melamine following a protocol

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developed in our previous work.30 Typically, a given amount of melamine was placed in a covered

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crucible, and heated to 520 °C at a heating rate of 20 °C/min and kept at 520 °C for 4 hours.

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In order to modify the surface of g-C3N4 with PTCDI, PTCDA was chosen to react with g-C3N4 4

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via the condensation reaction as illustrated in Scheme 1. Briefly, PTCDA (0.0355 g), g-C3N4 (0.71 g),

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and imidazole (2.5 g) were placed in a 100 mL three-neck round-bottom flask, followed by heating

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at 140 °C for 5 h in nitrogen atmosphere. Then the reaction mixture was cooled to room temperature

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and 50 mL ethanol was added into the reaction vessel under stirring. The solution was then

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transferred to a 250 mL flask containing 150 mL 2M hydrochloric acid (HCl). After stirring for 12 h,

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the mixture was centrifuged and washed thoroughly with methanol and deionized water. The

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resulting red solid was transferred to a 100 mL round bottom flask containing 50 mL potassium

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carbonate aqueous solution (K2CO3, 10%), which was refluxed in an oil bath at 100 °C for 1 h. After

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cooled to 50 °C, the solution was centrifuged and washed with 300 mL of K2CO3 solution (10%) for

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3 times,

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thoroughly with methanol and deionized water until the pH of the rinsed water became neutral. The

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collected solid (PI-g-C3N4) were dried in vacuum at 80 °C for 12 h.

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Sample Characterization. UV-vis absorption spectra of g-C3N4 and PI-g-C3N4 were recorded on a

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Solid Spec-3700 DUV spectrophotometer using BaSO4 as reference and were converted from

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reflection to absorption by the Kubelka-Munk method. The powder X-ray diffraction (XRD)

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measurements were recorded on a Bruker D8 diffractometer with monochromatized Cu Kα radiation

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(λ = 1.5418 Å). Fluorescence spectra were monitored with a fluorescence spectrophotometer

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(Hitachi, Model F-7000) equipped with a PC recorder. Electron paramagnetic resonance (EPR)

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spectra were recorded on a Bruker ElexsysE500 spectrometer by applying an X-band (9.43 GHz, 1.5

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mW) microwave with sweeping magnetic field at 110 K in cells that can be connected to a

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conventional high-vacuum apparatus (residual pressure 420 nm) to investigate the effect of the heterojunction between PTCDI and g-C3N4

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on the catalysis efficiency. Figure 5a shows the relative change of NO concentration (C/C0) as a

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function of irradiation time tested over the three different catalysts, PI-g-C3N4, g-C3N4 and PTCDA.

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As a control test, the removal of NO was negligible under the same visible light irradiation for 50

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min, indicating the high photostability of NO against visible light. As shown in Figure 5a, 37% NO

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was removed over pure g-C3N4 after 35 min of irradiation, while 47% was removed in only 10 min

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when PI-g-C3N4 was used as catalyst under the same irradiation, indicating much improved catalysis

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efficiency (We selected an optimal relative content ratio of PI to g-C3N4 in the present work. The

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photoactivities for NO removal of other ratio samples can be found in Figure S1.). In contrast,

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PTCDA did not demonstrate any catalysis for NO removal under the same photo-irradiation.

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In addition to monitoring the decrease in NO concentration, the concentration of NO2 produced

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during the photocatalysis was also monitored (Figure 5c). With g-C3N4 as the catalyst the amount of

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NO2 produced increased gradually and reached a plateau value after about 40 min of irradiation,

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clearly indicating that the main product of the photocatalysis is NO2. In sharp comparison, when

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PI-g-C3N4 was used as catalyst instead, the generation of NO2 quickly reached its equilibrium

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(within ca. 5 min) and the accumulated amount of NO2 was one order of magnitude lower than that

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in the case of g-C3N4. This implies that most of NO2 was further converted (oxidized) to NO3-.

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Such conversion was confirmed by FTIR spectral measurement. The PI-g-C3N4 sample after

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photocatalytic tests was collected and analyzed by FTIR. It can be seen that the used PI-g-C3N4

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exhibits new bands at 1457 and 1419 cm-1 (Figure 5d), which are ascribed to the antisymmetric

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stretching vibration modes of NO3- groups, implying that the major product in PI-g-C3N4 is NO3-. It

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was reported that NO3- can occupy the surface active sites, causing the deactivation of g-C3N4.17, 25

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Therefore, the stability and recyclability of PI-g-C3N4 was examined and compared with that of

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g-C3N4 and Pd-g-C3N4 (g-C3N4 was modified by the nano-palladium. The main product of NO

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removal over it is NO3-.25 The synthetic process can be found in the supporting information.) by

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running the same photocatalytic removal experiment continuously in multiple cycles. The results

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evince that the activity of PI-g-C3N4 does not decline after eight cycles of NO removal under the

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visible light irradiation (Figure 6a). However, the NO removal on g-C3N4 and Pd-g-C3N4 decreased

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about 10% and 67%, respectively (Figure 6c and 6e). The activity decrease of g-C3N4 and Pd-g-C3N4

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should be attributed to the occupation of active site caused by the NO3-. Although the activity of

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g-C3N4 did not significantly decline, it could produce large amount of NO2 in each cycle of 10

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photocatalytic NO removal (Figure 6d). Compare with g-C3N4 and Pd-g-C3N4, PI-g-C3N4 can not

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only greatly alleviate the deactivation of NO removal but also effectively inhibit the second pollution

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due to the production of NO2. The catalytic activity of PI-g-C3N4 remained about the same after

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eight cycles of test, indicating that formation of NO3- may occur at a position segregated from the

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catalyst.

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Generally, the photocatalytic removal of NO involves the surface reactions of both

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photogenerated holes and electrons, which may also produce oxidizing species, such as ·O2-, H2O2

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and ·OH to further the oxidation reactions.10, 33 In this study, DMPO spin-trapping ESR technique

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was employed to characterize the ·O2- species generated during photocatalysis. As shown in Figure

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7a, four characteristic peaks of DMPO-·O2- were clearly observed in methanol suspensions of

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g-C3N4, whereas only trace level of DMPO-·O2- could be detected for the PI-g-C3N4 under the same

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conditions. However, the PI-g-C3N4 system produced about 5 times amount of H2O2 than the g-C3N4

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system (Figure 7b). Since in semiconductor photocatalysis H2O2 is normally generated through

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direct reduction (O2 → H2O2) or multistep reaction (O2 → ·O2- → H2O2) from oxygen,31 the ESR

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results suggest that H2O2 is likely produced via the direct reduction way in PI-g-C3N4 suspension, in

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comparison to the multistep reaction route in g-C3N4.

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The photocatalytic reactions of NO removal over g-C3N4 and PI-g-C3N4 were further explored

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through a series of control experiments. With addition of potassium iodide (KI, a hole scavenger34)

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the NO removal was significantly depressed for both g-C3N4 and PI-g-C3N4 (Figure 8a and 8b),

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suggesting that the photogenerated holes paly a critical role in NO removal in both cases. On the

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other hand, when potassium dichromate (K2Cr2O7, an electron scavenger35) was added, the NO

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removal was inhibited over g-C3N4 (Figure 8a), whereas the NO removal over PI-g-C3N4 remained

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little changed. These observations imply that photogenerated electrons are indispensable for NO 11

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removal over g-C3N4 but not PI-g-C3N4. Moreover, the production of NO2 during photocatalysis of

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PI-g-C3N4 was significantly increased in the presence of K2Cr2O7 (Figure 8d), indicating that the

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photogenerated holes (increased due to scavenging of electrons) are primarily responsible for

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oxidizing NO to NO2 in PI-g-C3N4 system. Considering the fact that conduction band electrons can

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be captured by O2 to produce active oxygen species, it is essential to explore the role of these species

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in conversion of NO. Comparative investigations were performed by using varying scavengers such

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as tert-butyl alcohol (TBA) for ·OH,36 p-benzoquinone (PBQ) for ·O2−

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H2O2 38 in the photocatalysis. As shown in Figures 8a and 8b, the addition of TBA did not change the

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NO removal rate over either g-C3N4 or PI-g-C3N4, suggesting no significant contribution from ·OH

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to the photocatalytic removal of NO. Addition of PBQ clearly depressed the NO removal on g-C3N4

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(Figure 8a), but not PI-g-C3N4 (Figure 8b), implying different roles of ·O2− in the two systems. Also

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clearly seen from Figure 8 is that the addition of CAT did not affect the NO removal rate over either

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g-C3N4 or PI-g-C3N4. On the other hand, the presence of CAT dramatically increased the NO2

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concentration in PI-g-C3N4 system (Figure 8d), but brought little effect on NO2 generation over

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g-C3N4 (Figure 8c). These results indicate that H2O2 facilitates the further conversion (oxidation) of

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NO2 to NO3- in the PI-g-C3N4 system. Based on these comparative observations, we conclude that

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both the photogenerated hole and H2O2 are indispensable, playing the synergic roles, in the NO

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removal over PI-g-C3N4, enabling ultimate conversion to NO3- ion, whereas for the NO removal over

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g-C3N4 the synergic roles are played by the hole and ·O2−, resulting in conversion of NO to NO2

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(consistent with the previous observation on g-C3N4 24).

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and catalase (CAT) for

From the comparative investigations above, the photocatalytic removal of NO over PI-g-C3N4

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may involve the following reactions, Eq. (1) to (4),

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PI-g-C3N4 + visible light → h+ + e-

(1) 12

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2h+ + NO + H2O → NO2 + 2H+

(2)

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e- + O2 + 2H+→ H2O2

(3)

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2NO2 + H2O2→ 2NO3- + 2H+

(4)

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Efficiency of visible light photocatalysis depends on the visible light absorption of the catalyst.

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As shown in Figure 4a, PI-g-C3N4 (compared to g-C3N4) has increased absorption in the visible

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region, particularly above 500 nm, which is solely due to the absorption by PTCDI part. This

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increased visible absorption caused significant enhancement in photocatalytic removal of NO as

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shown in Figure 5. Such enhancement could be explained in two possible ways, one is localized on

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the PTCDI part, and the other is a cooperative process involving both g-C3N4 and PTCDI parts

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(Scheme 2). For the first case, visible excitation of the PTCDI (bandgap 2.5 eV, see the caption of

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Figure 9) produces electrons and holes, which then initiate the surface redox reactions as observed

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for other semiconductor photocatalysts. However, the efficiency of this catalysis is limited by the

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fast charge recombination process, which is actually evidenced by the strong fluorescence of PTCDI

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measured from the PI-g-C3N4 sample (Figure 4b). PTCDI alone is unlikely to afford the high

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efficiency of visible photocatalysis. Thereby, the cooperative process (Scheme 2) becomes the

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reasonable interpretation for the observed enhancement in visible photocatalysis.

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As shown in Scheme 2 (The band potential were acquired from the Mott-Schottky plots, Figure

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9), both the g-C3N4 and PTCDI parts are excited under visible light (λ > 420 nm), followed by the

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electron transfer between g-C3N4 and PTCDI, which can be classified into two models. The first is

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the common donor-acceptor charge separation as illustrated in Scheme 2a, in which electron transfer

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from the CB of g-C3N4 to the CB of PTCDI, leaving a hole in the VB of g-C3N4. If the charge

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migration occurs via this model, the enhancement of photocatalytic NO removal should also be

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observed under the UV-light illumination (λ < 400 nm). The absorption coefficient of PTCDI in the 13

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UV region is about 15 times lower than the maximal in the region of 500-550 nm (also note: for

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PTCDI the absorption coefficient does not change with different side substitutions39). From the

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UV-vis absorption spectrum measured for the PI-g-C3N4 sample in this study (Figure 4a), the

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absorption of PTCDI part in the UV region should be 15 times lower than the maximal peak in the

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region of 500-550 nm (where g-C3N4 has no absorption). By careful examining the absorption data,

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we can conclude that the absorption of PI-g-C3N4 in the UV region is truly dominant by the

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absorption by g-C3N4 (> 99% in absorption coefficient). That's to say, under UV irradiation (λ < 400

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nm) only the g-C3N4 part is excited. After the excitation, photogenerated electrons will transfer from

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the CB of g-C3N4 to the CB of PTCDI, producing electron and hole located at the PTCDI and

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g-C3N4, respectively, the same charge separation as illustrated in Scheme 2a. However, as shown in

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Figure 10, under the UV irradiation the photocatalytic removal of NO on PI-g-C3N4 was even slower

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than that on g-C3N4. This observation suggests that the common charge separation model shown in

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Scheme 2a is not likely the case in the visible photocatalysis of PI-g-C3N4. The second model is the

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Z-scheme charge migration (Scheme 2b), for which the visible excitation initiates the electron

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transfer from the CB of PTCDI to the VB of g-C3N4. This results in a different way of charge

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separation with an electron located in the CB of g-C3N4 and a hole in the VB of PTCDI.40 The lower

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level of VB of PTCDI (by 0.48 eV compared to that of g-C3N4) provides stronger oxidizing power

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for the hole, thus enabling direct oxidation of NO to NO2 (Eq. 2), which is consistent with the results

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shown in Figure 5 as discussed above. Meanwhile the electron located in the CB of g-C3N4

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possesses higher reducing power (by 0.68 eV compared to that of PTCDI), thereby enabling direct

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reduction of O2 to H2O2 (Eq. 3), which is also consistent with the results presented in Figure 7 and 8.

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Because the NO2, H2O2 and NO3- species are formed at different sites in PI-g-C3N4 system, the

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deactivation of catalysis caused by the occupation of active sites could be alleviated greatly (Scheme 14

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3). The heterojunction charge separation as illustrated by the Z-scheme reduces the probability of

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charge recombination that is often encountered in the single-component photocatalyst, thus

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producing increased density of holes and electrons, which can act as charge carriers when the

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catalyst material is employed in a circuit. This is evidenced by the results shown in Figure 11,

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wherein the photocurrent generated over PI-g-C3N4 (4.6 µA cm-2) was about 15 times higher than

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that over g-C3N4 (0.3 µA cm-2).

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Conclusions

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In summary, an all-solid-state Z-Scheme heterojunction (PI-g-C3N4) has been successfully

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constructed. As tested for photocatalytic removal of NO under visible light, significant enhancement

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in catalytic activity was observed for PI-g-C3N4 in comparison to the pristine g-C3N4. The Z-Scheme

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heterojunction creates charge separation with the electron populated to the higher CB and hole to the

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lower VB, thus enhancing the redox reaction power of the charge carriers. The strong hole can

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directly oxidize NO to NO2, while the strong electron can directly reduce O2 to H2O2. Since NO2 and

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H2O2 can react at a different location (via diffusion), the NO3- ion produced would cause minimal

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deactivation to the catalyst. This study provides new insight into the design of effective

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photocatalysts that can be operated under visible light, facilitating the utilization of solar energy.

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Acknowledgment

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Financial support by the National Nature Science Foundation of China (Grant No. 21473248), the

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CAS/SAFEA International Partnership Program for Creative Research Teams, the CAS “Western

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Light” program (2015-XBQN-B-06), and NSF (CBET 1502433) is gratefully appreciated.

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Supporting Information

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The preparation of Pd-g-C3N4; the photoactivities for NO removal of other samples which have

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different content ratio of PI to g-C3N4. This material is available free of charge via the Internet at 15

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http://pubs.acs.org.

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Figure Captions

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Scheme 1. Synthetic route of PI-g-C3N4.

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Figure 1. (a) g-C3N4 (yellow) and PI-g-C3N4 (pink) solid showing different colors. (b) XRD patterns

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of PI-g-C3N4, g-C3N4 and PTCDA. (c) Small-angle XRD patterns of PI-g-C3N4, g-C3N4 and

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PTCDA.

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Figure 2. TEM image of g-C3N4 (a) and PI-g-C3N4 (b).

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Figure 3. (a) XPS spectra of g-C3N4 and PI-g-C3N4; (b, c) high-resolution XPS spectra of O 1s (b)

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and N 1s (c) of g-C3N4 and PI-g-C3N4.

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434 435

Figure 4. (a) UV-vis absorption spectra of g-C3N4, PI-g-C3N4 and PTCDA. (b) Photoluminescence

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(PL) spectra of g-C3N4, PI-g-C3N4 and PTCDA (excited at 500 nm).

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Figure 5. (a) The relative change of NO concentration (C/C0) as a function of irradiation time tested

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over g-C3N4, PI-g-C3N4, and PTCDA. (b) NO2 concentration changing with irradiation time tested

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over g-C3N4 and PI-g-C3N4. (c) The FTIR spectra of PI-g-C3N4 before and after used in

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photocatalytic removal of NO. In all the experiments, initial concentration of NO was 600 ppb, the

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amount of g-C3N4, PI-g-C3N4, and PTCDA used was 50 mg, a 300 W Xe lamp with a 420 nm cutoff

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filter was used as the visible light source.

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Figure 6. Repeated testing of the photocatalytic NO removal over PI-g-C3N4 (a), g-C3N4 (c) and

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Pd-g-C3N4 (e); NO2 concentration changing in the repeated testing over PI-g-C3N4 (b), g-C3N4 (d)

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and Pd-g-C3N4 (f).

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Figure 7. (a) DMPO spin-trapping ESR spectra recorded for ·O2- in the g-C3N4 and PI-g-C3N4

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systems (under λ > 420 nm irradiation), where the concentration of DMPO was 25 mmol L-1. (b)

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Comparison of H2O2 generation in the g-C3N4 and PI-g-C3N4 systems under the same photocatalytic

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condition.

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Figure 8. The influence of different scavengers (KI for h+, K2Cr2O7 for e-, TBA for ·OH, PBQ

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for ·O2-, CAT for H2O2) on the photocatalytic removal of NO using g-C3N4 and PI-g-C3N4. The

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photocatalysis conditions are the same as in Figure 5.

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462 463

Figure 9. Mott-Schottky plots for g-C3N4 and PI-g-C3N4 at frequency of 1 k Hz obtained in darkness.

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Both g-C3N4 and PI-g-C3N4 samples display n-type semiconductor characteristic. The flat-band

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potential (equal to CB band in n-type semiconductor) was measured at -1.37 eV vs. NHE for pure

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g-C3N4, and -1.32 eV vs. NHE for the g-C3N4 part in PI-g-C3N4. With the known bandgap of 2.7 eV

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for g-C3N4,20 the VB potential of g-C3N4 can be calculated to be 1.38 eV vs. NHE in the PI-g-C3N4

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composite. The second linear region in the Mott-Schottky plot of PI-g-C3N4 is attributed to the CB

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band of PTCDI part, corresponding to a potential of -0.64 eV vs. NHE. With the known bandgap of

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2.5 eV for PTCDI,24 the VB potential of PTCDI can be calculated to be 1.86 V vs. NHE.

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472 473

Scheme 2. Two models of charge separation proposed for PI-g-C3N4 under visible irradiation: (a) 23

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conventional donor-acceptor charge transfer, (b) Z-scheme electron transfer. Energy levels (vs. NHE)

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are obtained from the flat potential (CB) measured in this study and the band gap values reported in

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literatures (see Figure 9).

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Figure 10. The photocatalytic removal of NO under UV-light irradiation (λ < 400 nm). For UV

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irradiation (λ< 400 nm), more than 99% of the irradiation will be absorbed by the g-C3N4 part.

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481 482

Figure 11. Photocurrent-time curves of g-C3N4 and PI-g-C3N4 electrode in 0.1 M KCl aqueous

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solution under visible light irradiation (λ >420 nm). For comparison, photocurrent-time curve of PI

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was also detected under the same condition. PI was prepared in the same manner as PI-g-C3N4, but 24

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g-C3N4 was replaced by melamine.

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487 488

Scheme 3. NO photocatalytic mechanism of PI-g-C3N4 under visible light irradiation.

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TOC Art Figure

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