Chlorine-functionalized carbon dots for highly efficient photodegradation of pollutants under visible-light irradiation

Accepted Manuscript Title: Chlorine-functionalized carbon dots for highly efficient photodegradation of pollutants under visible-light irradiation Author: Shengliang Hu Yanli Ding Qing Chang Jinlong Yang Kui Lin PII: DOI: Reference:

S0169-4332(15)01692-X http://dx.doi.org/doi:10.1016/j.apsusc.2015.07.125 APSUSC 30852

To appear in:

APSUSC

Received date: Revised date: Accepted date:

14-6-2015 17-7-2015 19-7-2015

Please cite this article as: S. Hu, Y. Ding, Q. Chang, J. Yang, K. Lin, Chlorine-functionalized carbon dots for highly efficient photodegradation of pollutants under visible-light irradiation, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.07.125 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Chlorine-functionalized carbon dots for highly efficient photodegradation of pollutants under visible-light irradiation Shengliang Hua,∗, Yanli Dinga, Qing Changa,*, Jinlong Yangb, Kui Linc,* a

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School of Material Science and Engineering, North University of China, Taiyuan 030051,

P. R. China b

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  State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University,

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Beijing 100084, P. R. China.

Analytical Instrumentation Center, Tianjin University, Tianjin 300072, P. R. China.

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Abstract

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Chlorine-functionalized carbon dots (Cl-CDs) were prepared by the substitution reaction between Cl radicals into thionyl chloride molecules and carbon dots with containing

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OH/COOH groups at their surface (O-CDs). The obtained Cl-CDs with a size of 2-5 nm

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contain 2-3% Cl atoms and emit blue light. Compared with amine-functionalzed carbon dots (N-CDs) and O-CDs, Cl-CDs exhibit much higher photocatalytic activity under visible-light irradiation. The thermally and chemically stable phthalocyanine can be even degraded quickly through Cl-CDs. This work suggests that surface engineering of carbon dots with heteroatoms can be used to enhance their photochemical properties. Keywords

Carbon materials; Nanoparticles; Surfaces; Photocatalytic activity                                                               ∗

Corresponding author. E-mail address: [email protected] (S. Hu); [email protected] (Q. Chang);

[email protected] (K. Lin)

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1. Introduction Carbon dots (CDs) exhibit unique and tunable photoluminescence behaviors, exceptional physicochemical properties without incurring the burden of intrinsic toxicity

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or elemental scarcity [1]. On the other hand, they can be synthesized inexpensively and easily by using various carbon sources, such as soot, fruit, grass and organic molecules

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[1-3]. Therefore, CDs are inspiring intensive research on their potential applications in

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many fields [4]. With industrialization and population growth, the environmental contamination caused by organic pollutant is becoming an overwhelming problem all over

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the world. To remove them from various waters and wastewaters, photocatalysis has

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emerged as one of the most promising technologies because it represents an easy way to utilize the energy of either natural sunlight or artificial indoor illumination, and is thus

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abundantly available everywhere in the world [5]. Although the recent studies have

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indicated that CDs can be employed to harvest visible light and convert it to shorter wavelength light through up-conversion, they only act as one component of the composites to enhance photocatalytic activity of the other component such as TiO2, Cu2O, Ag3PO4, Au [4,6]. Thus, the green and low cost CDs do not play a key role in the use of sunlight to degrade organic pollutants.

According to previous researches, engineering CDs with heteroatoms or surface groups may control the photoinduced charge behaviors [2,7,8]. In the present work, we design chlorine atoms (Cl) on the surface of CDs as ideal active sites for photocatalytic reaction to proceed. The Cl-functionalized CDs (Cl-CDs) show highly efficient photocatalytic performance. For example, they not only can degrade the common dyes 2   

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(such as methylene blue, rhodamine-B) [2], but also can oxidize phthalocyanine (Pc) into CO2, water by using molecular O2 as oxidant under visible-light irradiation. Pc has a two-dimensional 18-п-electron aromatic system. Specially, it is a thermally, chemically

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and photo-chemically stable compound [9]. This leads to high difficulty in degrading Pc molecules through general photocatalysts, e.g. TiO2.

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2. Experimental section

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Cl-CDs were obtained by two steps. CDs that contained OH and/or COOH O-contained groups at their surface (named O-CDs) were first synthesized by thermal

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decomposition of organic molecules such as citrate, ethylene glycol, which had been

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reported widely [7]. Then surface chlorination of O-CDs was accomplished through the substitution reaction between Cl into thionyl chloride (SOCl2) molecules and OH/COOH

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3. Results and discussion

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radicals. Detailed experimental procedures were given in the Supplementary section.

Fig. 1(a) shows a transmission electron microscopy (TEM) image of Cl-CDs. The uniformly dispersed nanoparticles are observed and their sizes mainly range from 2 to 5 nm. X-ray photoelectron spectroscopy (XPS) was used to analyze element compositions of Cl-CDs (Fig. S1). The measured result indicates that Cl-CDs contain 2-3% of Cl atoms and 10-12% of O atoms besides carbon atoms. High-resolution XPS scans for Cl 2p peaks of Cl-CDs is shown in Fig. 1(b). The bonding energy peak of Cl 2p is centered on about 200 eV, suggesting the covalent bonding of the chlorine atoms to the carbon skeleton [10]. This result is confirmed by the infrared (IR) spectra (Fig. S2). Zinc Pc (ZnPc) is used for photodegradation studies to show photocatalytic activity 3   

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of Cl-CDs. Due to water-insolubility of ZnPc, Cl-CDs are dispersed in the mixed solution of water and N,N-dimethylmethanamide (DMF). Fig. 1(c) shows photoluminescence (PL) spectra of Cl-CDs and the blend of Cl-CDs and ZnPc (Cl-CDs+ZnPc) in the mixed

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solution of DMF and H2O, respectively. It can be seen that the addition of ZnPc results in PL reduction. However, the corresponding UV-vis absorption intensity increases in

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contrast with Cl-CDs or ZnPc alone (Fig. 2a). After visible-light irradiation from a 300 W

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xenon lamp, the sample absorption significantly decreases in the 600-800 nm region. The temporal evolution of ZnPc concentration during photocatalytic degradation is shown in

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Fig. 2(b). ZnPc was degraded quickly with complete removal occurring after only 90 s of

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reaction. To compare, ZnPc were displaced by copper Pc (CuPc) to investigate photocatalytic activity of Cl-CDs at the same conditions. As shown in Fig. 2b, CuPc was

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also degraded quickly similar to ZnPc through Cl-CDs, indicating their powerful

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capability in photodegradation of dyes.

In order to further prove highly efficient photodegradation performance of Cl-CDs, the blend solution of Cl-CDs and ZnPc was placed in the experimental room (low-light environments) without xenon lamp irradiation. As shown in Fig. 3(a), the sample color is transformed into yellow from green after 10 days. In contrast, ZnPc solution does not change color at the same past time (Fig. 3b). The mixed solution of Cl-CDs and ZnPc can emit blue and red light through different wavelength excitation. The blue light emission obtained at 400 nm excitation is attributed to Cl-CDs (Fig. S3) while the red light emission obtained at 600 nm excitation originates from ZnPc molecules (Fig. S4). Fig 3(c) shows the PL intensity changes of Cl-CDs and the mixture with the standing 4   

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time. The PL intensity of Cl-CDs decreases with the standing time while that of the mixture increases. After standing for 10 days, the blue light emission of both samples becomes stable. Similarly, Fig. 3(d) gives the PL intensity changes of ZnPc and the

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mixture with the standing time. The little effect of the red light emission from ZnPc solution on the standing time is observed. However, the red light emission of the mixed

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solution decreases significantly in several days. This implies that ZnPc molecules have

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been degraded by Cl-CDs under the weak visible-light irradiation.

By contrast, we performed the similar photodegradation experiments through

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blending amine-functionalized CDs (N-CDs) and O-CDs with ZnPc in mixed solution of

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DMF and H2O, respectively (Fig. 4). The degraded rate of N-CDs and O-CDs to ZnPc is much slower than that of Cl-CDs. Therefore, highly photocatalytic activity of Cl-CDs

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could be attributed to the chlorine-contained groups at the surface.

Scheme 1 Photocatalytic mechanism of Cl-CDs

For CDs with a very small size, most of carbon atoms are exposed on the surface, thus their space charge distribution will depend on surface state. In the space charge region, the electric potential (V) of a point (x, y, z) can be described as,[11] ∂ 2V ( x, y, z ) ∂ 2V ( x, y, z ) ∂ 2V ( x, y, z ) ρ + + =2 2 2 ∂x ∂y ∂z ε rε 0

(1)

where εr, ε0 and ρ are the relative dielectric constant, the vacuum permittivity and the 5   

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space charge density, respectively. For simplicity, the space charge distribution was considered as one dimension. Then equation (1) can be written as d 2V ( x) ρ =2 dx ε rε 0

(2)

ρ x2 2ε r ε 0

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V (x) = -

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To solve equation (2), V can be obtained as,

(3)

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It can be seen that V is a function of the x coordinates and is affected by ρ that is connected with the kinds and quantities of surficial heteroatoms. Therefore, the difference

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of V is created in a nanoparticle when Cl atoms are introduced at the surface of O-CDs.

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The changes of V with Cl and O-contained positions could facilitate electron-hole pair separation and induce faster carrier migration.  Thus, the more excited-electron and hole

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are produced. As shown in Scheme 1, they will attack O2 and OH- (and/or H2O) to

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generate more active hydroxyl radicals (·OH). Since the ·OH radicals play a key role in oxidizing dyes into CO2 and water shown in equation (4), Cl-CDs exhibit higher efficiency than O-CDs for photodegradation of Pc. O2 /H 2 O Pc + • OH → PcOH • ⎯⎯⎯→ CO 2 + H 2 O                           (4)

On the other hand, the ·OH quantities produced from Cl-CDs and O-CDs were estimated by employing terephthalic acid (TA) as ·OH probe [12,13]. The experimental results support our above proposal that Cl-CDs can generate more ·OH radicals than O-CDs (Fig. S5 in Supplementary File). Therefore, this work has opened a new way to tune the photochemical properties of CDs through heteroatoms although further works may be required. 6   

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4. Conclusion

Cl-CDs were synthesized through the substitution reaction between Cl radicals into

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thionyl chloride molecules and OH/COOH groups at the surface of O-CDs. The XPS results show that Cl atomic content in Cl-CDs with the size of 2-5 nm is 2-3%. However,

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such Cl-CDs exhibit much higher photodegradation of Pc molecules than O-CDs and

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N-CDs. It is believed that the Cl-contained groups at the surface of CDs play a key role in driving the photochemical reaction.

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Acknowledgements

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We thank for financial support from the National Natural Science Foundation of China (Nos. 51272301, 51172214, 51172120), China Postdoctoral Science Foundation

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funded project (Nos. 2012M510788, 2013T60269), Shanxi Province Science Foundation

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for Youths (2014021008), 131 Talent Plan of Higher Learning Institutions of Shanxi. Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at http://dx.doi.org

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[3] Liu S, Tian J, Wang L, Zhang Y, Qin X, Luo Y, Asiri A., Al-Youbi A., Sun X. Hydrothermal treatment of grass: A low-cost, green route to nitrogen-doped, carbon-rich, photoluminescent polymer nanodots as an effective fluorescent sensing platform for

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label-free detection of Cu(II) ions. Adv Mater 24 (2012) 2037-2041 [4] Lim SY, Shen W, Gao Z. Carbon quantum dots and their applications. Chem Soc Rev

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[5] Chen C, Ma W, Zhao J. Semiconductor-mediated photodegradation of pollutants under visible-light irradiation. Chem Soc Rew 39 (2010) 4206-4219

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[6] Cao L, Sah S, Anilkumar P, Bunker CE, Xu J, Fernando K, Wang P., Guliants E.,

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Tackett K., Sun Y-P. Carbon nanoparticles as visible-light photocatalysts for efficient CO2 conversion and beyond. J Am Chem Soc 133 (2011) 4754-4757

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[7] Tian R, Hu S, Wu L, Chang Q, Yang J, Liu J. Tailoring surface groups of carbon

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quantum dots to improve photoluminescence behaviors. Appl Surf Sci 301 (2014) 156-160 [8] Tetsuka H, Asahi R, Nagoya A, Okamoto K, Tajima I, Ohta R. Optically tunable amino-functionalized grapheme quantum dots. Adv Mater 24 (2012) 5333-5338 [9]

Bottari

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phthalocyanine-carbon nanostructure systems: Synthesis, photoinduced electron transfer, and application to molecular photovoltaics. Chem Rev 110 (2010) 6768-6816 [10] Tan YZ, Yang B, Parvez K, Narita A, Osella S, Beljonne D, Feng X., Mullen K. Atomically precise edge chlorination of nanographenes and its application in grapheme nanoribbons . Nat Commun 4 (2013) 2646 [11] Z Zhang, JT Yates Jr. Band bending in semiconductors: chemical and physical 8   

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consequences at surface and interfaces. Chem Rev 112(2012)5520-5551 [12] T Hirakawa, Y Nosaka. Properties of O2·- and OH· formed in TiO2 aqueous suspensions by photocatalytic reaction and the influence of H2O2 and some ions.

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Langmuir 18(2002)3247-3254 [13] TJ Mason, JP Lorimer, D. M. Bates, Y Zhao. Dosimetry in sonochemistry: the use of terephythalate

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monitor.

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Sonochem

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aqueous

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1(1994)S91-S95

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Figure captions Fig. 1 (a) TEM image of Cl-CDs; (b) Cl 2p XPS of Cl-CDs; (c) PL emission of Cl-CDs

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and the mixed solution of Cl-CDs and ZnPc excited at 400 nm. Fig. 2 (a) UV-vis absorption spectra: Cl-CDs, ZnPc, Cl-CDs+ZnPc and the sample after

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visible-light irradiation of Cl-CDs+ZnPc; (b) A plot of the extent of photodegradation of

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ZnPc and CuPc vs. time for Cl-CDs.

Fig. 3 (a) Photos of the mixed solution of Cl-CDs and ZnPc stood for 0 and 10 days,

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respectively; (b) Photos of ZnPc solution stood for 0 and 10 days, respectively. (c) PL

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intensity change of the samples (containing the same concentration of Cl-CDs) excited at 400 nm with the standing time; (d) The PL intensity changes of the samples (containing

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the same concentration of ZnPc) excited at 600 nm with the standing time.

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Fig. 4 Photocatalytic degradation of ZnPc under visible light irradiation (λ > 420 nm),

using Cl-CDs, O-CDs and N-CDs, respectively Figures

Figure 1

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Figure 2

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Figure 3

Figure 4

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Graphical Abstract 

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Chlorine‐functionalized  carbon  dots  (Cl‐CDs)  were  synthesized  through  the  substitution 

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reaction. Cl‐CDs show highly photocatalytic activity under visible‐light irradiation, and can 

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quickly  degrade  phthalocyanine  with  the  thermal  and  chemical  stability.  This  work  suggests  that  surface  engineering  of  carbon  dots  with  heteroatoms  can  be  used  to 

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enhance their photochemical properties. 

Research Highlights   

• Chlorine‐functionalized carbon dots (Cl‐CDs) were synthesized by substitution reaction.  • Cl‐CDs show highly photocatalytic activity under visible‐light irradiation.  • The thermally and chemically stable phthalocyanine is even photodegraded by Cl‐CDs.     

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Supplementary File

Shengliang Hua,∗, Yanli Dinga, Qing Changa,*, Jinlong Yangb, Kui Linc,* a

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School of Material Science and Engineering, North University of China, Taiyuan 030051,

P. R. China b

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  State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University,

Analytical Instrumentation Center, Tianjin University, Tianjin 300072, P. R. China.

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c

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Beijing 100084, P. R. China.

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1. Experimental details

Preparation of O-CDs: O-CDs were obtained by hydrothermal treatment of

the

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aqueous solution of organic molecules (such as citric acid, ethylene glycol) in a

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poly(tetrafluoroethylene) (Teflon)-lined (PTFE) autoclave under the heating at 180-200 ℃,which was reported previously. For example, 25 mL ethylene glycol was put into the

PTFE autoclave and heated at 200 for 5 h. The obtained homogeneous solution contains O-CDs after cooling to room temperature. Our previous characterizations have indicated that such O-CDs mainly contain O-related groups. Preparation of Cl-CDs and N-CDs: The obtained O-CDs were mixed with thionyl                                                               ∗

Corresponding author. E-mail address: [email protected] (S. Hu); [email protected] (Q. Chang);

[email protected] (K. Lin)

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chloride solution in a glass vial. After stirring for 30 min, the mixture were transferred to a PTFE autoclave again and then heated at 180℃ for 5 h. Finally, Cl-CDs were synthesized after cooling the room temperature. For example, 10 mL of O-CDs was mixed with 536

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μL of thionyl chloride, and then Cl-CDs were obtained following the above steps. Similarly, N-CDs were also obtained by replacing thionyl chloride with ammonia solution

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(25 wt% in H2O) on the basis of the preparing process of Cl-CDs. To remove excess

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ammonia and thionyl chloride possibly, the obtained samples was heated at 100 ℃ for 1 h in the atmosphere.

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Photocatalysis Experiments: For the photocatalytic activity comparison, the same

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amount (1 mL) of Cl-CDs, O-CDs and N-CDs was used, and then dispersed in 1 mL of 500 mg/L ZnPc (or CuPc) of DMF solution. The obtained mixture was diluted by adding

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18 mL of DMF solvents. Then the diluted solution was transferred to a quartz cell with an

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inner cell edge length of about 2 cm and a small capped opening at the top. Before illumination, the cell was constantly stirred for 30 min in the dark to ensure the establishment of an adsorption–desorption equilibrium. After that the cell was irradiated with light from a 300 W xenon lamp placed 25 cm away. The light intensity reaching the cell was about 550 mW/cm2. Visible and IR light were obtained by using cutoff filters to remove light of λ < 420 nm. UV-vis absorption spectra of the samples were taken before and after irradiation by removing the cap to withdraw the solution. Characterization: Fluorescence spectra and absorption were performed on a Hitachi

F4500 fluorescence spectrophotometer and a Shimadzu UV-2550 UV-vis spectrometer, respectively. Diluted supernatants containing O-CDs, N-CDs and Cl-CDs were dropped 14   

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onto copper grids covered with amorphous carbon film to prepare specimens for transmission electron microscopy (TEM) observation, which was performed in a FEI Tecnai G2 F20 microscope with a field-emission gun operating at 200 kV. XPS data of all

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samples was collected by a Kratos AXIS 165 mutitechnique electron spectrometer having an Al Ka X-ray source for determining the composition and chemical bonding

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configurations. The infrared (IR) spectra were obtained on a Thermo Nicolet 360 FT-IR

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spectrophotometer.

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2. Supporting results

Figure S1 XPS survey of Cl-CDs

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Figure S2 IR spectra of Cl-CDs and the mixture of O-CDs and SOCl2

Fig. S3 PL spectra of Cl-CDs excited by the different wavelength excitation

Fig. S4 PL spectra of ZnPc and the mixed solution of Cl-CDs and ZnPc excited at 600 nm.

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Fig. S5 The fluorescence intensity changes for the supernatant liquid of the irradiated

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Cl-CDs and O-CDs suspension containing TA with the irradiation time. It is known that ·OH reacts with TA to generate 2-hydroxyterephthalic acid (TAOH), which emits a

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unique fluorescent signal with its peak centered at around 426 nm (see Ref. 12 and 13).

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The more ·OH, the more TAOH are produced and then results in the stronger fluorescence emission. Thus, the photocatalytic activity of CDs in generating ·OH radicals can be

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estimated.

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