无机盐辅助制造石墨碳氮化物增强罗丹明B的光催化降解

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Materials Letters

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Inorganic salt-assisted fabrication of graphitic carbon nitride with enhanced photocatalytic degradation of Rhodamine B

Lei Luo a ,Anfeng Zhang a ,Michael J.Janik b ,Keyan Li a ,Chunshan Song a ,b ,?,Xinwen Guo a ,

?

a

State Key Laboratory of Fine Chemicals,PSU-DUT Joint Center for Energy Research,School of Chemical Engineering,Dalian University of Technology,Dalian 116024,P.R.China b

EMS Energy Institute,PSU-DUT Joint Center for Energy Research and Department of Energy &Mineral Engineering,Pennsylvania State University,University Park,Pennsylvania 16802,United States

A R T I C L E I N F O

Keywords:

Porous materials Semiconductors Carbon materials

A B S T R A C T

With the assistance of inorganic salt,graphitic carbon nitride nanosheets were prepared with increased mesoporous surface area for promoting the photocatalytic performance compared with the one without.Series of photocatalysts with di ?erent mass ratio of NH 4NO 3/melamine were prepared and characterized by XRD,FT-IR,XPS,TEM,SEM,N 2physical adsorption,UV –vis and PL spectrometries.Photocatalytic degradation of RhB under visible light irradiation (λ>420nm)was applied to evaluate catalytic properties.The catalytic activity maximizes at an intermediate NH 4NO 3/melamine ratio at 0.15,giving a maximum performance,RhB totally being degraded within 30min and kinetic constant reaching 0.167min ?1that is 4.5times as high as that on BCN.The NH 4NO 3assisted procedure is a facile,repeatable,environmental friendly,and e ?cient method for preparing g-C 3N 4nanosheets with high photocatalytic performance.

1.Introduction

Graphitic carbon nitride (g-C 3N 4)is a promising metal-free visible light photocatalyst [1–3].However,the photocatalytic activity of the pristine g-C 3N 4is low due to its limited speci ?c surface area,fast rate of charge recombination,and low electronic conductivity [4–6].Hard-templating method is a controllable,?exible and precise strategy for introducing a nanostructure,but the usage of hazardous ?uoride-contained reagents (HF,NH 4HF 2)to remove the template as well as multiple step procedures with long operation periods limit its practical application.In contrast,top-down approaches such as liquid exfolia-tion [7–10]or thermal exfoliation [11–13]are utilized successfully to break the stacking system of bulk g-C 3N 4to nanosheets and increase its photocatalytic activity.Though exfoliated nanosheets exhibit promoted photocatalytic activity with enhanced photoabsorption and photore-sponse [8],their yield is still lower than 40%[5].It would be advantageous to prepare g-C 3N 4from one single step,with shorter preparation time,and with enhanced photocatalytic activity.

In this work,g-C 3N 4was synthesized by the thermal condensation of melamine,with the assistance of NH 4NO 3,to increase the speci ?c mesoporous surface area,enhance the charge separation e ?ciency,and promote the photocatalytic performance.Photocatalytic degradation of RhB under visible light irradiation was performed to evaluate the g-

C 3N 4photocatalytic activity.2.Experimental

Graphitic carbon nitride was synthesized from the thermal con-densation of melamine with or without the assistance of NH 4NO 3.In a typical synthesis procedure,3.0g melamine precursor was mixed with a speci ?ed amount of NH 4NO 3then ground to a ?ne powder.The powder was placed into a 30mL covered crucible and heated in static air to 550°C at a heating rate of 10°C/min.After cooling,the obtained yellow product was ground for further characterization or evaluation.Samples are denoted as CNx where x/100represents the mass ratio of NH 4NO 3/melamine.For sample CN15,the mass ratio of NH 4NO 3/Melamine is 0.15.Bulk carbon nitride (BCN)was prepared without NH 4NO 3.The relevant characterization and evaluation can be found in Electronic Supplementary Information .3.Results and discussion

The crystal structure of the as-prepared samples BCN and CN15were characterized with XRD,FT-IR,and XPS spectra,which together demonstrated the characteristic structure of g-C 3N 4(shown in Fig.S1).The morphologies of the as-prepared catalysts were investigated with

https://www.360docs.net/doc/a31313381.html,/10.1016/j.matlet.2016.11.043

Received 20September 2016;Received in revised form 8November 2016;Accepted 9November 2016?

Corresponding authors.

E-mail address:guoxw@https://www.360docs.net/doc/a31313381.html, (X.Guo).

Materials Letters 188 (2017) 130–133

Available online 15 November 2016

0167-577X/ ? 2016 Elsevier B.V. All rights reserved.

TEM,SEM and nitrogen physical adsorption.Fig.1shows SEM and TEM images of the as-prepared sample BCN and CN15.The BCN sample is dense with a large particle size,whereas CN15appears as a loose and soft material.The TEM images of CN15show ultrathin nanosheets.By introducing NH 4NO 3,the layers of bulk g-C 3N 4are separated,modifying the nanostructure.Nitrogen physical adsorption further supports the result obtained from SEM and TEM and adds quantitative analysis on the pore and textural properties.The adsorp-tion-desorption isotherms are shown in Fig.S2,with textural proper-ties presented in Table 1.By introducing NH 4NO 3,the S BET of CN15is much higher than that of pristine BCN,suggesting that the gas products (N 2,NO,NO 2,etc.)during the pyrolysis of NH 4NO 3can play a templating role in thermal condensation of melamine.

The optical properties of the CNx samples were characterized with UV –vis and PL spectra and displayed in Fig.2.As shown in Fig.2a,samples BCN and CN15displayed similar photoabsorpion from ultra-violet to visible light,and their band gaps are both at 2.75eV.The photoluminescence spectrum emission arises from the recombination of photogenerated electron-hole pairs,and is therefore useful to probe the transfer and recombination processes of these photogenerated carriers [14,15].As shown in Fig.2b,the strong photoluminescence of sample BCN is greatly reduced in the CN15sample,suggesting that the

energy-wasting carrier recombination will be suppressed.The im-proved charge separation e ?ciency can be attributed to the pore structure and increased surface area that facilitates the transfer and separation of photogenerated charge carriers,prolongs their lifetime,and thus lowers the PL intensity [16].The enhanced electron-hole separation e ?ciency may lead to improved photocatalytic quantum e ?ciency and activity.

The photocatalytic degradation of RhB under visible light irradia-tion was applied as a probe reaction to evaluate the photocatalytic activity of the CNx samples.The amount of RhB adsorbed to the g-C 3N 4before light irradiation was less than 20%for all CNx samples.As shown in Fig.3a,RhB is degraded in the presence of each photo-catalyst,however,the photocatalytic activity of all CNx samples exceeds BCN.The kinetic constants over the di ?erent photocatalysts are presented in Fig.3b and c.BCN presents the lowest rate constant of k =0.037min ?1,and k increased signi ?cantly as the mass ratio of NH 4NO 3increased.The kinetic constant reaches a maximum for CN15at k =0.167min ?1.Further increasing the mass ratio of NH 4NO 3led to a decrease of the kinetic constant.The enhanced photocatalytic activity arises from both an increase in surface area and increased electron-hole separation e ?ciency,as evidenced by the PL spectra.The increased speci ?c surface area of the nanosheets can not only increase the active site,but also shorten the distance photogen-erated electron-hole pairs migrate to reach the organic pollutant at the surface with the strong oxidative hole.For sample CN15,the reusability was also investigated by ?ve consecutive reactivity studies (Fig.3d).As the irradiation time was prolonged,RhB degraded without evident deactivation,which indicates stability of the CN15photocatalyst.4.Conclusions

Graphitic carbon nitride was prepared from the pyrolysis of melamine with the assistance of NH 4NO 3.This procedure increased the mesoporous surface area,enhanced the absorption of visible light,improved the electron-hole separation e ?ciency,and promoted the photocatalytic https://www.360docs.net/doc/a31313381.html,x samples demonstrated more rapid RhB photodegradation than the BCN sample prepared without NH 4NO 3assistance.The mass ratio of NH 4NO 3/melamine of 0.15

500 nm 4μm 500 nm

200 nm 500 nm 200 nm

(d)(f)

(e)Fig.1.SEM (a)and TEM (d)images of BCN,SEM images of CN15(b and c),TEM images of sample CN15(e and f).

Table 1

Properties of BCN and CN15.Sample

Molar ratio of C/N a S BET (m 2/g)b V pore (cm 3/g)c Band Energy (eV)d

BCN 0.67015.10.03 2.75CN5/

12.90.04 2.74CN150.67324.40.05 2.75CN25/24.30.06 2.75CN50/23.60.07 2.73CN100/24.50.06 2.75CN200

/

14.3

0.03

2.75

a The average value of two individual measurements by element analysis,

b Calculated by BET method.

c Calculate

d at P/P 0=0.95.d

Measured by UV –vis spectra.

shows the highest catalytic activity with a kinetic constant of 0.167min ?1that is 4.5times as high as that on BCN.The synthesis strategy involving NH 4NO 3-assisted preparation of g-C 3N 4is a facile,repeatable,environmentally friendly and e ?cient way to prepare g-C 3N 4with a high photocatalytic performance.

Acknowledgements

This work was supported by State Key Program of National Natural Science Foundation of China (Grant no.21236008,21306018),and Fundamental Research Funds for the Central Universities (Grant no.DUT16LK12).

Appendix A.Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.matlet.2016.11.043.

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