Removal of Nitric Oxide through Visible Light

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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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†

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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

9


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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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References

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416 417 418

Figure Captions

419 420

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

421

422 423

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

424

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

425

PTCDA.

426

19


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427 428

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

429

430 431

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

432

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

433

434 435

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

436

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

437

20


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ACS Catalysis

438 439

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

440

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

441

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

442

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

443

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

444

filter was used as the visible light source.

445

446 447

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

448

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

449

and Pd-g-C3N4 (f).

450

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451 452

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

453

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

454

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

455

condition.

456

457 458

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

459

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

460

photocatalysis conditions are the same as in Figure 5.

461

22


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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.

464

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

465

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

466

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

467

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

468

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

469

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

470

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

471

472 473

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


ACS Catalysis

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474

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

475

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

476

literatures (see Figure 9).

477 478

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

479

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

480

481 482

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

483

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

484

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


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485

ACS Catalysis

g-C3N4 was replaced by melamine.

486

487 488

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

489 490 491 492 493 494 495 496 497 498 499 500 501 502 25


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503 504 505

TOC Art Figure

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Removal of Nitric Oxide through Visible Light

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