An Advanced SemimetalâOrganic Bi Spheresâg-C3N4 Nanohybrid

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An Advanced Semimetal-organic Bi Spheres/g-C3N4 Nanohybrid with SPREnhanced Visible-light Photocatalytic Performance for NO Purification Fan Dong, Zaiwang Zhao, Yanjuan Sun, Yuxin Zhang, Shuai Yan, and Zhongbiao Wu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 16 Sep 2015 Downloaded from http://pubs.acs.org on September 21, 2015

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Environmental Science & Technology

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An Advanced Semimetal-Organic Bi Spheres/g-C3N4

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Nanohybrid with SPR-Enhanced Visible-Light Photocatalytic

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Performance for NO Purification

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Fan Dong†,*, Zaiwang Zhao†, Yanjuan Sun†, Yuxin Zhang£, Shuai Yan‡,* and

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Zhongbiao Wu§

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†

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of Environment and Resources, Chongqing Technology and Business University,

Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College

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400067 Chongqing, People’s Republic of China.

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‡

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100029, People’s Republic of China.

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£

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400044, PR China

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§

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China.

Institute of Microelectronics, Chinese Academy of Sciences, Chaoyang, Beijing

College of Materials Science and Engineering, Chongqing University, Chongqing

Department of Environmental Engineering, Zhejiang University, Hangzhou 310027,

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ACS Paragon Plus Environment


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ABSTRACT: To achieve efficient photocatalytic air purification, an advanced

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semimetal-organic Bi spheres/g-C3N4 nanohybrid was constructed through in situ

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growth of Bi nanospheres on g-C3N4 nanosheets. This Bi/g-C3N4 exhibited an

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exceptionally high and stable visible-light photocatalytic performance for NO

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removal due to the surface plasmon resonance (SPR) endowed by Bi metal. The SPR

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property of Bi could conspicuously enhance the visible-light-harvesting and the

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charge separation. The electromagnetic field distribution of Bi sphere involving SPR

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effect was simulated, which reaches its maximum in close proximity to the Bi particle

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surface. When the Bi metal content was controlled at 25%, the corresponding

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Bi/g-C3N4 displayed outstanding photocatalytic capability and transcended those of

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other visible-light photocatalysts. The Bi/g-C3N4 exhibited a high structural stability

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under repeated photocatalytic runs. A new visible light induced SPR-based

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photocatalysis mechanism with Bi/g-C3N4 was proposed based on DMPO-ESR

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spin-trapping. The photo-induced electrons could transfer from g-C3N4 to the Bi metal

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as revealed with time-resolved fluorescence spectra. The function of Bi semimetal as

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a plasmonic cocatalyst for boosting visible light photocatalysis was similar to that of

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noble metals, which demonstrated a great potential of utilizing economically feasible

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Bi element as a substitute for noble metals to advance photocatalysis efficiency.

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1. INTRODUCTION

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Nitrogen oxide (NOx), mostly generated from combustion of fossil fuels and

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vehicle exhaust, are deemed to typical contaminant in air, since they are responsible

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for atmospheric pollutions such as acid rain, haze, and photochemical smog.1 For

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NOx from industrial emissions, traditional techniques such as adsorption, SCR, and

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thermal catalysis strategies could function well, but they are not economically feasible

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for NOx elimination at parts per billion (ppb) level in both indoor and outdoor air.2

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Photocatalysis, as a green and effective technology, is most promising in air

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purification under mild condititions.3-6

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Of the various photocatalytic systems, metal-semiconductor hybrids have been

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regarded as good candidates for photocatalytic degradation of contaminants.7-10 As is

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known, the metal-semiconductor heterojunction could serve as a molecular diode and

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facilitate the interfacial charge transfer.7,8 In particular, TiO2-based photocatalysis is

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especially attractive because TiO2 is widely available, economically feasible, and

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chemically stable.9,10 Nevertheless, the rapid charge recombination and limited visible

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light absorption in the TiO2 results in low quantum efficiency. Hence, it is urgent and

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indispensable to develop more efficient photocatalysts for high-performance

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applications.

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Over the past decades, several novel semiconductor photocatalysts have been

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developed with the purpose of overcoming the obstacles of TiO2, such as

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g-C3N4-based,11-17 W-based,18 and Bi-based photocatalysts,19 and etc. Especially,

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g-C3N4-based photocatalysts have been widely explored in photocatalysis-related 3



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applications. This is mainly attributed to the unique two-dimensional architecture,

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tunable electronic structure, environmentally benign, and excellent chemical

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stability.11,20-22 It is well known that the critical limitation for achieving high

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efficiency is the recombination of photo-excited electron–hole.23 Thus, accelerating

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the electron–hole pairs separation is essential for advancing the photocatalytic

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capability.11-15,18

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It is documented that semiconductors loaded with noble metals, such as Au, Ag, Pt

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and Pd, can effectively improve their photocatalytic activity.7,24,25 The incorporation

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of metal nanoparticles in semiconductors can build a Schottky barrier at the interface,

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which are beneficial for charge transfer between the semiconductor and metal, and

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substantially accelerates the electron–hole separation.10,11,24 Considering the

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over-expenditure of these precious metals, some inexpensive metals with similar

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properties have been demonstrated as decent candidates. Due to the unique

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advantages of low price, easy availability, and transport properties, the semimetal

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bismuth (Bi) could potentially serve as an ideal substitute for noble metals.26-30 Lately,

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Bi was discovered to exhibit decent photocatalytic activity.30-32 We have revealed the

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direct plasmonic photocatalysis of Bi element mediated by surface plasmon resonance

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(SPR).32 Besides, Bi metal could also behave as an excellent co-catalysts to accelerate

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the charge carrier separation of the Bi coupled Bi2O3,26 BiOCl,27, 28 and (BiO)2CO330

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nanocomposites, respectively. All these semiconductors coupled with Bi element

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exhibited highly promoted photocatalytic performance. However, the SPR effect of Bi

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has not been demonstrated theoretically, which is crucial for the understanding the 4


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role of Bi as cocatalyst. Besides, the synthesis, photocatalysis mechanism and

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application of Bi/C3N4 have never been reported.

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In this work, elemental bismuth nanospheres (BiNSs) were introduced onto the

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surface of g-C3N4 nanosheets through an in situ deposition method. The thin g-C3N4

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nanosheets were used as a substrate for the green synthesis of BiNSs loaded g-C3N4

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(Bi/g-C3N4). This novel nanojunction exhibited highly enhanced photocatalytic

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activity and stability for NO removal under visible light irradiation. This enhanced

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photocatalytic performance of Bi/g-C3N4 can be attributed to the co-contributions of

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the improved visible light absorption, the hampered charge recombination and the

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accelerated charge carriers transfer because of the SPR effects of Bi metal. The local

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electromagnetic field representing SPR effects of Bi metal was simulated with a finite

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integration technique (FIT) code, which could provide a direct theoretical evidence

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for understanding the SPR-based photocatalysis mechanism involved with semimetal

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Bi.

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2. EXPERIMENTAL AND THEORETICAL SECTION

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2.1. Preparation of g-C3N4 Nanosheets.

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All the reagents employed in this study were of analytical grade (Sigma-Aldrich) and

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used without further purification. The g-C3N4 was synthesized by polymerization of

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urea under high temperature. In detail, 10 g of urea was added to 20 ml of distilled

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water in 50ml alumina crucible. The crucible was then placed in an oven (70°C) for

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recrystallization under air. Afterwards, the crucible was transferred to the muffle

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furnace. The temperature was raised to 550 °C at a rate of 15 °C /min and was 5



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maintained at this temperature for 2 h. After the reaction, the alumina crucible was

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cooled to room temperature. The fabricated g-C3N4 were collected for further use.

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2.2. Preparation of Bi/g-C3N4 nanohybrid and reference samples.

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In a typical synthesis, 0.364 g of bismuth nitrate pentahydrate was first dissolved in

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10 ml nitric acid solution (1M). The mixture was then transferred into an autoclave

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Teflon vessel of 100 ml with vigorous stirring. After dissolution, ethylene glycol of 55

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ml was added and stirred for 10 min. Afterwards, 0.6 g of Polyvinylpyrrolidone (PVP,

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MW: 24000) was then added into the solution and stirred for 30 min. After all solids

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were dissolved absolutely, different masses (1.00, 0.60, 0.30, 0.15 and 0.075g) of

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as-prepared g-C3N4 nanosheets was added into the above solution and dispersed via

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ultrasonic treatment for 30 min. The aqueous suspension was then hydrothermally

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treated under 160 °C for 12 h. After the reaction, the solid product was collected by

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filtration, washed with distilled water for two times and ethanol for two times, and

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dried at 60 oC for 12 h to obtain the final Bi/g-C3N4 products. This mass ratio of Bi to

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g-C3N4 were controlled at 15, 25, 50, 100, 200% and the corresponding coupling

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Bi/g-C3N4 photocatalysts were labeled as Bi-CN-15, Bi-CN-25, Bi-CN-50,

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Bi-CN-100 and Bi-CN-200, respectively. For comparison, the pristine g-C3N4

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nanosheets was labeled as CN as references. The preparation of reference samples

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was placed in supporting information.

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2.3. Characterization methods.

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The X-ray diffraction (XRD) patterns were recorded using an X-ray diffractometer

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equipped with intense Cu Kα radiation (Model D/max RA, Rigaku Co., Japan). The 6


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morphological structure was analyzed using scanning electron microscope (SEM,

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JEOL model JSM-6490, Japan) and transmission electron microscope (TEM,

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JEM-2010, Japan). X-ray photoelectron spectroscopy (XPS) was carried out to

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investigate the chemical compositions with Al Kα X-ray radiation source (Thermo

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ESCALAB 250, USA). The Brunauer–Emmett–Teller (BET) specific surface area

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were determined using a nitrogen adsorption apparatus (ASAP 2020, USA) with all

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samples degassed at 100 °C for 12 h prior to measurements. The UV-vis diffuse

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reflection spectra (UV-vis DRS) were obtained for the dry-pressed disk samples by

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using a Scan UV-vis spectrophotometer (UV-2450, Shimadzu, Japan) with 100%

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BaSO4 as reference. Photoluminescence (PL, F-7000, HITACHI, Japan) was

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employed to probe the charge separation property. Time-resolved fluorescence decay

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spectra were taken with a fluorescence spectrophotometer (Edinburgh Instruments,

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FLSP-920). Electron spin resonance (ESR) signals of radicals spin-trapped by

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5,5-dimethyl-1-pyrroline N-oxide (DMPO) were recorded on a JES FA200

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spectrometer. Samples for ESR measurement were prepared by mixing the samples in

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a 40 mM DMPO solution tank (aqueous dispersion for DMPO-•OH and methanol

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dispersion for DMPO-•O2-) and irradiated with visible light. The surface photovoltage

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(SPV) measurements were conducted with a home-built apparatus equipped with a

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lock-in amplifier (SR830) synchronized with a light chopper (SR540).

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2.4. Application of Bi/g-C3N4 nanohybrid in photocatalytic air purification.

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The photocatalytic activity of the as-synthesized samples was evaluated by removing

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NO at ppb level in a continuous flow reactor (Scheme S1).29 The detailed description 7



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of the reactor system and detection of NO concentration can be found in the

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supporting information.

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2.5. Trapping experiments for visible light induced active species.

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Potassium iodide (KI) is utilized as an hole scavenger, and tertbutylalcohol (TBA) is

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an effective •OH scavenger. Potassium dichromate (K2Cr2O7) was selected as a

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photo-induced electron scavenger. Photocatalyst (0.20 g) with different trapping

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agents was added into 15 mL of H2O and ultrasonic disperse for 30 min. The aqueous

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suspensions were then equally coated onto two glass dishes. Afterwards, the coated

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dishes were dried at 60 ◦C in an oven. Ultimately, this two dried dishes were utilized

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for further photocatalytic NO removal tests.

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2.6. Simulation of electromagnetic field distribution.

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The simulation of the field distribution at and around the Bi nano particle is carried

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out with a rigorous Maxwell's solver based on the finite integration techniques

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(FIT).33 A time-harmonic inverse iterative method (THIIM) is utilized to overcome

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the instability problem caused by the negative permittivity of the metal.34 The

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implementation of the method is parallelized to deal with the large amount of degrees

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of freedom needed for the simulation of multiscale structures involving plasmonic

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effects.35 The computational domain is a sphere with the Bi particle placed in the

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center and the surrounding is the free space. Perfect matched layer is applied to all the

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sides to truncate the computational region. The refractive index of Bi in the UV to

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near-infrared spectrum is obtained by a fit to the drude model of the empirical data.36

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3. RESULTS AND DISCUSSION 8


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3.1. Phase and chemical composition.

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XRD pattern of the pristine g-C3N4 (Figure S1) shows two typical diffraction peaks

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centered at 27.1° and 13.1°of graphitic C3N4 (JCPDS 87-1526).11 The characteristic

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peaks of Bi (PDF Card 44-1246) confirms the existence of rhombohedral phase Bi

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metal in Bi/g-C3N4 composite samples (Figure S2).28 For Bi/g-C3N4 nanohybrid, the

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diffraction peak at 27.4 for g-C3N4 was not detected because of the shadowing effect

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the strong (012) peak of Bi phase.

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The XPS measurements of Bi-CN-25 further demonstrate the chemical composition

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of the well-coupled Bi/g-C3N4 composites (Figure S3). The Figure S3a depicts the

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presence of Bi, O, C and N. The four characteristic peaks of C1s (Figure S3b), as well

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as three peaks of N1s (Figure S3c) jointly demonstrate the existence of g-C3N4.12,37

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The Bi4f XPS spectra before and after etching reflect considerable differences (Figure

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S3d). The characteristic peaks centered at 156.8 and 162.1 eV are typically ascribed to

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the Bi–Bi bonds of elemental Bi, while peaks around 158.9 and 164.2 eV are the

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characteristic peaks of Bi ions in bismuth oxides.38 The Bi–Bi peak is significantly

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intensified after surface etching, which suggests that a thin bismuth oxide layer is only

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formed on the surface of the elemental Bi. The formation of Bi-O layer could prevent

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the Bi metal from further oxidation. The Bi-O bond can still be detected as the new

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surface of amorphous bismuth oxide of Bi spheres in the sub-layers will be exposed

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after etching. In addition, two peaks of O1s centered at 529.6 and 530.8 eV (Figure

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S3e) was detected, which was originated from Bi-O with lower binding energy and

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the adsorbed species (H2O or O2) on the surface with higher energy.39 9



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Based on the peak ratios (Figure S3a), the molar ratio of Bi to O is about 1:2.5 prior

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to surface etching, which can be ascribed to the oxygen rich surface of the Bi metal.

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After etching of the surface with a depth of 30 nm, the molar ratio between Bi and O

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increases to 3:1, demonstrating that the oxide layer is exclusively generated on the

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surface. EDX elemental mapping of C, N, Bi, and O elements further indicates the

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coexitence of Bi and g-C3N4 in Bi-CN-25 composites (Figure S4). It can be seen that

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the Bi metal is unformly distributed on g-C3N4.

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3.2. Mophological structure.

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Figure S5 shows the SEM images of Bi/g-C3N4 samples. The as-synthesized

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Bi/g-C3N4 consisted of a large quantity of solid nanospheres with diameters 150-200

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nm. These nanospheres were enwrapped with g-C3N4 layers. When increasing Bi

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content coupling with g-C3N4, more and more Bi nanopheres can be formed (Figure

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S5g and S5h).

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Figure 1 shows the TEM images of the pure g-C3N4 and Bi-CN-25 samples. As can

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be observed, the g-C3N4 (Figure 1a) was consisted of thin nanosheets, which are

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beneficial for the growth of Bi nanoparticles on the surface. The uniform dispersion of

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Bi nanospheres with diameters of 150-200 nm are well coupled with the g-C3N4

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nanosheets (Figure 1b). Figure 1c and 1d clearly reveal that an intimate

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metal-semiconductor interface is formed within the Bi/g-C3N4 nanohybrid, which is

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favourable for the charge carriers transfer between the Bi nanospheres and g-C3N4

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nanosheets. Also, as the thickness of the amorphous bismuth oxide is less than 2 nm,

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the photo-induced electrons could pass through the thin layer of amorphous bismuth 10


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oxide via the well-known quantum tunneling. The BET-BJH analysis results (Figure

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S6) suggest the mesoporous architecture and high SBET (Table S1) of the fabricated

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samples, which are beneficial for absorption and the reaction production transfer. (b)

(a)

100 nm

100 nm

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(c)

(d)

50 nm

5 nm

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Figure 1. TEM and HRTEM images of the Bi-CN-25 photocatalysts. 3.3. Visible light photocatalytic air purification. (a)

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(b)

Figure 2. Photocatalytic activities (a) of the samples and reaction constants (b) of 11



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Bi-CN-25 samples for NO degradation in air under visible light illumination (NO concentration: 600 ppb).

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We first surveyed visible-light photocatalytic performance of Bi/g-C3N4 nanohybrid

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for ppb-level NO removal in a continuous reactor to demonstrate the capability in air

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purification (Scheme S1). NO is stable and cannot be photo-oxidized under

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light-illumination without photocatalysts.41 Our previous work has demonstrated that

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pristine Bi nanoparticles showed negligible photocatalytic activity, which implied that

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the amorphous bismuth oxide on the Bi surface is inactive and not responsible for

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photocatalytic removal of NO under visible light irradiation.36 Significantly, the NO

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removal ratio of Bi-CN-15, Bi-CN-25 and Bi-CN-50 samples are increased to 48.5,

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59.7 and 47.6% respectively (Figure 2a), much higher than that of the individual

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g-C3N4 (42.0%). Remarkably, the superior photocatalytic NO removal ratio of

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Bi-CN-25 (59.7%), even outperforms that of other types of decent visible light

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photocatalysts, such as BiOBr (21.3%),42 BiOI (14.9%),43 C-doped TiO2 (21.8%),21

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N-doped TiO2 (36.5%),21 and (BiO)2CO3 (43.5%).44 The CN-Hydro sample shows a

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comparable photocatalytic NO removal ratio (40.1%) in comparison with that of the

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pristine g-C3N4 (42.0%). This result implies that the hydrothermal process does not

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apparently influence the photocatalytic performance. The photocatalytic activity of

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the Au-P25-25, Ag-P25-25 samples (Figure S7) was also checked under the same

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conditions and the NO removal was 33.2% for Au-P25-25 (low efficiency) and 57.6%

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for Ag-P25-25 (but not stable), respectively.

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Bi metal could extremely enhance the photocatalytic performance of the substrate

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photocatalysts as a non-noble metal cocatalyst. Considering the advantages of 12


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low-price and easy availability, Bi metal can act as an excellent potential candidate in

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substituting noble metals (e.g. Au and Ag) to enhance visible light photocatalysis of

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other semiconductors. While, further increasing the Bi contents deposited on g-C3N4

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will hamper its photocatalytic activity, e.g. Bi-CN-100 (31.4%) and Bi-CN-200

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(18.2%). This is understandable because excess Bi could cover most of the surface of

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g-C3N4 and prevent the visible light from irradiation on g-C3N4, hence resulting in a

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decreased photocatalytic performance, similar to other metal-semiconductor

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nanocomposites.27,45,46 The corresponding reaction rate constants determined with

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first-order model (Figure 2b) were calculated to be 1.25 min-1 for Bi-CN-25, which is

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1.67 times than that of pristine g-C3N4 (0.75 min-1).

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3.4. Optical absorption and charge separation. Bi-CN-100

Bi-CN-25

Bi-CN-15

Bi-CN-50

Bi-CN-200

CN

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Figure 3. UV-Vis DRS and the color (inset) of all the fabricated samples.

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The UV-vis DRS of the pristine g-C3N4 nanosheets and Bi/g-C3N4 nanohybrids are

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shown in Figure 3. Apparently, The DRS results demonstrate that the light-harvesting

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are conspicuously enhanced, ranging from the UV light (200 nm) to near-infrared

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light (800 nm) after Bi depositing. This phenomenon could be assigned to a 13



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charge-transfer transition between the Bi spheres and the g-C3N4.24, which is also in

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agreement with the color variation from pale yellow to dark grey (Inset in Figure 3).

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A distinct surface plasmon resonance (SPR) peak centered at 500 nm can be observed

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for pure Bi. Furthermore, compared with the individual g-C3N4 nanosheets, the

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Bi/g-C3N4 nanohybrids exhibits a wide additional absorption peak around 500 nm,

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which is the typical characteristic SPR peak of Bi metal.28,29 This strengthened light

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absorption is crucial for photo-exciting more electrons and holes to enhance the

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photocatalysis efficiency of the Bi/g-C3N4 nanohybrids. In addition, the hydrothermal

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treatment of g-C3N4 resulted in blue-shifted absorption edge (Figure S8 and 3), which

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indicate that the size of g-C3N4 was reduced during hydrothermal processing.

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The PL emission can an effective tool to investigate the separation efficiency of the

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photo-excited electron-hole pairs for a semiconductor.29 As shown in Figure S9a, the

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PL spectra of pure g-C3N4 exhibits an emission peak centered at ca. 468 nm (～2.65

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eV) under excitation at 330 nm, which is mainly assigned to the emission of the band

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gap transition.12 Notably, the emission peak intensity is significantly quenched as Bi

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contents increased, which can be ascribed to the electron enrichment of metal

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nanoparticles, similar to the Mott−Schottky effect.47,48 Because of the diversity of the

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work function, the electron flows from the conduction band of g-C3N4 to the Bi side,

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constructing a potential barrier. No backward flow is permitted once this energy

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barrier is formed. Under visible-light illumination, the photo-generated electrons on

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the conduction band of g-C3N4 will be injected to the Bi metal, which serve as

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electron sinks that could accelerate the photo-generated electrons and holes 14


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separation.27,28,40 This process is thermodynamically favorable as the Fermi level of Bi

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metal is much more positive than that of conduction band position of g-C3N4. Besides,

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the PL peak of the pristine g-C3N4 (CN), hydrothermal g-C3N4 (CN-Hydro) samples

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(Figure S9b) was measured. Compared with the peak of the pristine g-C3N4, the

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hypsochromic shift of PL for the Bi/g-C3N4 nanohybrid was ascribed to the quantum

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sizes effects because the hydrothermal treatment could split the g-C3N4 layers into

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smaller one, consistent with UV-vis DRS result (Figure S8).49

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The ns-level time-resolved fluorescence decay spectra were further utilized to

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investigate the charge transfer dynamics of the samples (Figure S10). The curves can

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be fitted well based on a biexponential decay function (Table S2). The short lifetime

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(τ1) and long lifetime (τ2) is 2.3018 ns and 6.6667 ns for g-C3N4, respectively. While,

298

after the introduction of Bi, the two kind lifetime τ1 and τ2 are decreased to 2.2226 ns

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and 5.9318 ns, which demonstrating that the Bi element could store and shuttle the

300

electrons from photoexcited g-C3N4 to Bi.50,51 In addition, The apparent electron

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transfer (ET) rate (kET) in Bi-CN-25 can be obtained according to the following

302

equation:51

303

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The calculated value is 1.55 × 107 s-1, revealing that the interface formed between Bi

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and C3N4 is favorable for effective electron transfer quenching of the excited state of

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g-C3N4.

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The transfer properties of photo-induced charge at the interface of Bi-CN-25

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sample were further demonstrated by the surface photovoltage (SPV) measurement 15



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(Figure S11).52,53 For g-C3N4 alone, the SPV response is almost negligible, while the

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peak intensity of Bi-CN-25 is extremely enhanced after Bi metal is loaded on the

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surface of the g-C3N4. As is generally known that SPV could reflect the electron

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transition-related separation processes. The stronger the peak intensity, the higher the

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separation efficiency of the charges.54,55 The extraordinarily accelerated charge

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separation in the Bi/g-C3N4 hybrid system can be ascribed to the internal electric field

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of Bi and the electron transfer from g-C3N4 to Bi metal. This is the key to the highly

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enhanced photocatalytic activity of Bi/g-C3N4 hybrid.

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3.5. Active species trapping, electromagnetic field simulation, and photocatalysis

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mechanism. (a)

319 320 321 322

(b)

Figure 4. Active species trapping of Bi-CN-25 samples (a) and DMPO ESR spin-trapping of Bi, CN, and Bi-CN-25 for·O2- radicals (b) under visible light illumination (λ> 420 nm).

323

To determine the involvement of active radical species during photocatalysis,

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trapping experiments (Figure 4a) were performed for detection of the hydroxyl radical

325

(•OH), hole (h+), electron (e-) in the photocatalytic process, taking Bi-CN-25 sample

326

as an example. The scavengers used in this research are tert-butyl alcohol (TBA, 1%)

327

for •OH, potassium dichromate (K2Cr2O7, 1%) for e- and potassium iodide (KI, 1%) 16


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for h+. The degradation behavior of NO is slightly decreased upon the addition of

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TBA, validating that •OH radicals are not the main active species for NO removal.

330

Notably, when the scavenger K2Cr2O7 is added, the photocatalytic capability is

331

remarkably inhibited. This phenomenon implies that photo-excited electrons are the

332

crucial active species responsible for NO removal. Meanwhile, When KI is introduced

333

to the reaction system, the photocatalytic removal of NO is also hampered, which

334

suggested that h+ is also an important active radical in this Bi/g-C3N4 heterojunction.

335

Moreover, the DMPO-ESR analysis (Figure 4b and Figure S12) revealed that after

336

incorporation of Bi nanoparticles on g-C3N4, more •O2- radicals were generated

337

benefiting from the enhanced charge separation, while the production of •OH is

338

inhibited. This result is well consistent with the trapping results in Figure 4. The

339

crucial •O2− radicals are generated by reducing the O2 through the separated electrons.

340 341 342 343 344 345 346

Figure 5. SPR-induced local electromagnetic field in the Bi nanoparticles. The Bi particle is illuminated by plane waves with a wavelength of 420 nm which incident from z-direction. A three dimensional view and two dimensional cross sections perpendicular to the x, y and z axis are shown. The scale bar shows the relative increase in field enhancement T=|E/Einc|2. Although the SPR of Bi element has been described experimentally, the theoretical 17



347

evidence has never been provided.32 Here, the local electromagnetic field arising from

348

SPR effects of Bi metal was first simulated with a rigorous Maxwell's solver based on

349

the finite integration techniques as shown in Figure 5.34, 35 Significant enhancement of

350

electromagnetic field under visible light irradiation can be observed. The

351

electromagnetic field reaches its maximum value in close proximity to the Bi particle

352

surface because of the SPR effect.

353 354 355

Figure 6. The visible-light induced charge separation and proposed photocatalysis mechanism of Bi/g-C3N4 for NOx purification.

356

Based on the experimental and simulation results, the schematic elaboration of

357

photocatalysis mechanism upon Bi/g-C3N4 for NO treatment under visible-light

358

irradiation was presented in Figure 6. Firstly, the incorporation of Bi improved the

359

light-harvesting ability of g-C3N4 ranging from UV to near-infrared light, which

360

favors the production of more active electrons and holes on g-C3N4 surface (Eqs. 1).

361

Secondly, a built-in electric field (Figure 5) induced by the SPR effect of Bi metal will

362

accelerate the photo-excited electrons and holes separation (Figure S8, S9, S10).

363

Thirdly, the nearby Bi nanoparticles could perform as electron traps to facilitate the

364

separation of photo-induced electron-hole pairs (Figure 6).27, 28, 40 The Fermi level

365

(vs.NHE) of Bi can be estimated to be about -0.17 eV.29 The conduction band (CB) 18


Page 18 of 28

Page 19 of 28


366

position of g-C3N4 (−1.12 eV) is much more negative than that of the Fermi level of

367

Bi (Figure 6), which are beneficial for advancing the electron transfer from g-C3N4 to

368

Bi (Eqs. 2). Obviously, the electrons could transfer from conduction band of g-C3N4

369

to Bi metal thermodynamically. Afterwards, the separated electrons will react with the

370

O2 and reduce it to •O2− (Eqs. 3), because the redox potential of O2/•O2− (−0.33 eV) is

371

more positive than that of the CB of g-C3N4 (−1.12 eV). 29,56 While, the photo-driven

372

e− could also reduce O2 to H2O2 on the basis of the redox potential of O2/H2O2 (0.695

373

eV), and afterwards the formed H2O2 would be further transformed into •OH by

374

trapping an electron as shown in Eqs. (3-5) ).57

375

As is well known, the first step reaction is the crucial to determine the reaction rate

376

and the production ratio in multi-steps continuous reactions (O2→•O2−→H2O2→•OH)

377

as presented in Eqs. (3-5). Thus, more •O2− radicals are produced in this

378

photocatalytic system, accompanied with less •OH generation. In addition, the

379

generated h+ on the valence band (VB) of g-C3N4 also plays an important role in direct

380

photo-oxidation of NO (Figure 4a) to the final products (HNO2/HNO3) owing to the

381

strong oxidation power of h+ as shown in Eq. (6). Nevertheless, the h+ on VB of

382

g-C3N4 cannot oxidize the OH− into •OH radicals (Eq. 8), because the potential of the

383

holes at the VB of g-C3N4 (1.57 eV) is more negative than the redox potential of

384

OH−/•OH (1.99 eV).56 Given that •O2− radicals are major reactive oxidation species

385

(Figure 4), they could subsequently oxidize NO to for the final products of HNO2 and

386

HNO3 (Eq. 7).

387

g-C3N4+hv→e−(g-C3N4)+h+(g-C3N4) 19


(1)


388

e− (g-C3N4) → e− (Bi)

(2)

389

e− (Bi)+O2→•O2−

(3)

390

•O2−+2H++ e−→H2O2

(4)

391

H2O2+ e−→•OH+ OH−

(5)

392

NOx +h+(g-C3N4) →NO3−

(6)

393

NOx + •O2−→NO3−

(7)

394

h+( g-C3N4)+OH−× →•OH

(8)

395

Page 20 of 28

3.6. Photochemical and structural stability of photocatalyst. (a)

(b)

396 397 398 399

Figure 7. The stability tests of CN-Hydro and Bi-CN-25 samples for NO removal under visible light illumination (a), and the XRD pattern of the Bi-CN-25 samples before and after stability tests (b).

400

The photochemical and structural stability of catalyst is important for practical

401

applications.58,59 The stability of CN-Hydro and Bi-CN-25 was tested by carrying out

402

the photocatalytic reaction for multiple runs. The results are presented in Figure 7a.

403

The strengthened photocatalytic activity as well as improved stability of Bi-CN-25

404

can be observed in comparison with that of the CN-Hydro sample. This fact

405

demonstrates that incorporation of Bi metal onto the g-C3N4 nanosheets is an effective

406

and potential strategy to significantly enhance the photocatalytic performance of 20


Page 21 of 28


407

g-C3N4 for NO removal. Moreover, the almost unchanged XRD spectra of Bi-CN-25

408

before and after stability test (Figure 7b) further indicates the phase stability of the Bi

409

coupled g-C3N4 photocatalysts.

410

The TEM and HRTEM images (Figure S13) reflects that the morphological

411

structure does not change after repeated irradiations. According to the FT-IR (Figure

412

S14a) and PL (Figure S14b) characterization analysis of Bi-CN-25 samples after

413

several times stability tests, little changes can be detected. These results clearly imply

414

that the first semimetal-organic Bi spheres/g-C3N4 heterojunction photocatalyst have

415

excellent photochemical and structural stability, and thus shows great optential in

416

future applications, such as environmental remediation and solar energy conversion.

417

In summary, we have fabricated a novel semimetal-organic Bi spheres/g-C3N4

418

nanohybrid photocatalyst, which consisted of g-C3N4 sheets and well-coupled

419

semimetal Bi nanospheres. This advanced nanohybrid exhibited highly enhanced

420

visible light photocatalytic activity and stability for NO purification in comparison

421

with pristine g-C3N4. The strengthened photocatalytic performance can be attributed

422

to the co-contributions of the notably improved light-harvesting owing to the SPR

423

effects of Bi nanospheres and the increased separation efficiency of electron-hole

424

pairs because of the electron trapping effect of Bi metal in the hybrid system. A

425

SPR-based visible light photocatalysis mechanism was proposed on the basis of

426

electromagnetic field simulation with rigorous numerical solver of Maxwell’s

427

equations and active species trapping. The theoretical simulation has revealed the

428

maximum intensity of electromagnetic field near the Bi particle surface owing to the 21



Page 22 of 28

429

SPR effect. This work could not only provide new insights into of the understanding

430

of

431

semimetal-semiconductor hybrid, but also pave a new way for optimizing the

432

photocatalytic performance with non-noble metals as plasmonic cocatalyst to achieve

433

the enhanced capability.

noble

metal-like

catalytic

behavior

of

Bi

element

in

Bi-based

434 435

ASSOCIATED CONTENT

436

Supporting Information.

437

Preparation of reference samples. Description of the reactor system and detection of

438

NO concentration. Diagrams of equipment for NO removal. XRD pattern of g-C3N4.

439

XRD of Bi/g-C3N4. XPS spectra of the Bi-CN-25 sample before etching. EDX

440

mapping of Bi-CN-25 sample. SEM images of the samples. N2 adsorption-desorption

441

isotherms and the corresponding pore-size distribution curvesof the samples. UV-Vis

442

DRS of g-C3N4 and the CN-Hydro samples. PL spectra of g-C3N4, CN-Hydro and

443

coupled Bi/g-C3N4. The time-resolved fluorescence spectra of Bi, CN, and Bi-CN-25.

444

The SPV spectra of Bi and Bi-CN-25. The DMPO spin-trapping ESR spectra of Bi,

445

CN, and Bi-CN-25. The TEM, HRTEM, FT-IR and PL results of Bi-CN-25 after

446

stability test. These materials are available free of charge via the Internet at

447

http://pubs.acs.org.

448

Corresponding Author

449

∗ To whom correspondence should be addressed. Phone: +86 23 62769785 605. Fax:

450

+86 23 62769785 605. E-mail: [email protected] (Fan Dong), [email protected]

451

(Shuai Yan)

452

ACKNOWLEDGMENTS

453

This research is financially supported by the National Natural Science Foundation of 22


Page 23 of 28


454

China (51478070, 21501016, 51108487).

455

Author Contributions

456

The manuscript was written through contributions of all authors. All authors have

457

given approval to the final version of the manuscript.

458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480

482

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Graphical Abstract


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An Advanced SemimetalâOrganic Bi Spheresâg-C3N4 Nanohybrid

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