Influence of hydrogen peroxide in enhancing photocatalytic activity of carbon nitride under visible light: An insight into reaction intermediates

Accepted Manuscript Title: Influence of hydrogen peroxide in enhancing photocatalytic activity of carbon nitride under visible light: An insight into reaction intermediates Authors: Dipendu Saha, Mathew M. Desipio, Tyler J. Hoinkis, Erik J. Smeltz, Ryan Thorpe, Dale K. Hensley, Shirley G. Fischer-Drowos, Jihua Chen PII: DOI: Reference:

S2213-3437(18)30409-3 https://doi.org/10.1016/j.jece.2018.07.030 JECE 2525

To appear in: Received date: Revised date: Accepted date:

5-5-2018 13-6-2018 14-7-2018

Please cite this article as: Saha D, Desipio MM, Hoinkis TJ, Smeltz EJ, Thorpe R, Hensley DK, Fischer-Drowos SG, Chen J, Influence of hydrogen peroxide in enhancing photocatalytic activity of carbon nitride under visible light: An insight into reaction intermediates, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.07.030 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.

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Influence of Hydrogen Peroxide in Enhancing Photocatalytic Activity of Carbon Nitride Under Visible Light: An Insight into Reaction Intermediates

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Dipendu Saha*1, Mathew M. Desipio1, Tyler J. Hoinkis1, Erik J. Smeltz1, Ryan Thorpe2, Dale K. Hensley3, Shirley G. Fischer-Drowos4, Jihua Chen3 of Chemical Engineering, Widener University, One University Place, Chester, PA 19013, 2Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA 3 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory. Oak Ridge, TN 37831, USA4 4Department of Chemistry, Widener University, One University Place, Chester, PA 19013

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1 Department

*Corresponding author’s e-mail: [email protected], phone: +1 610 499 4056

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Graphical Abstract

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Synopsis: Carbon nitride, a photocatalyst, decomposes organic pollutants into harmless products in presence of hydrogen peroxide and under visible light irradiation.

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Abstract

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Carbon nitride and hydrogen peroxide degraded methylene blue under visible light. Hydrogen peroxide prevented electron/hole pair recombination of photocatalyst. Hydrogen peroxide also generated additional hydroxyl radials to oxidize paraquat. Intermediates of photocatalysis were identified by LCMS analysis.

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Highlights

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Carbon nitride is a known photocatalyst that can be activated under visible light irradiation. In this research, the combined effect of carbon nitride and hydrogen peroxide in photocatalytic degradation of methylene blue (MB) in water was investigated under visible light. Carbon nitride was synthesized by thermal polymerization of dicyandiamide and characterized by x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Energy dispersive x-ray (EDX), Fourier transform infrared spectroscopy (FTIR) and pore textural properties. The kinetic study of degradation of MB with carbon nitride and in presence of hydrogen peroxide showed complete degradation of MB before 45 minutes of time interval. The kinetic studies were also performed by varying the dose of photocatalyst and initial concentration of MB and it was shown that higher dose of photocatalyst and lower concentration of MB resulted in higher rate of degradation. The dual role of hydrogen peroxide can be related to the generation of additional highly reactive hydroxyl radicals in presence of photocatalyst and prevention of recombination of holes and electrons of carbon nitride thereby improving its photocatalytic activity. The pH study

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revealed that neutral pH of the solution results in the highest rate of degradation. Analysis of liquid chromatography-mass spectroscopy (LCMS) results helped to reveal the reaction steps of photocatalysis of MB and it was found that the three stages of photodegradation of MB were demethylation, ring shortening and ring opening type of reactions. Keywords: Photocatalyst, carbon nitride, hydrogen peroxide, methylene blue, LCMS

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1. Introduction

In the field of environmental remediation, semiconductor-based photocatalysts have been investigated rigorously for the degradation of different kinds of organic pollutants. The most common photocatalyst that is used today is titanium dioxide or titania (TiO 2). Two common forms of TiO2 are rutile and anatase and they have slightly different bandgap 1,2

. In order to overcome the UV light

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ultraviolet (UV) regions (wavelength, λ<420 nm)

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energies of 3.0 and 3.2 eV, respectively. All forms of titania are photoactive only in the requirement of traditional photocatalysts, or in other words, lower the bandgap energy, a

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new type of photocatalyst has been developed. It is polymeric and graphitic carbon nitride

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(C3N4). It has relatively lower bandgap energy of 2.7-2.8 eV thereby making it active in the visible light of 450-460 nm wavelength (λ). Its facile and inexpensive synthesis method

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includes simple heating of ‘melon’ types of organic molecules, like dicyandiamide, melamine

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or triazine in air. Although the existence of carbon nitride can be traced to back to3 1834, the modern research on this material as a photocatalyst did not start before 20064 and after that, there was a potential shift in the research interests from inorganic photocatalysts to organic

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and polymeric photocatalysts5,6. Besides harvesting solar energy and splitting water to generate H2, to date, the number of studies on the role of carbon nitride in environmental

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remediation has grown significantly. In aqueous phase decontamination, carbon nitride has been widely used in the decomposition of several types of dyes or reduction of CO2. A

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detailed review on the activity of carbon nitride can be found elsewhere7. Hydrogen peroxide (H2O2) is often used as a ‘green’ oxidant to oxidize and decompose

organic matters irrespective of their type. Hydrogen peroxide, especially in presence of UV light, produces highly reactive hydroxyl radical (OH.) in water that can oxidize and degrade

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the organic matters8,9. Mostly commonly, it is used in Fenton’s reagent, which is a mixture of Fe(II)/Fe(III) and H2O2 and produces a large amount of hydroxyl radical to degrade the organic materials10,11. Despite being very effective for the decomposition of organic materials, the key problems of Fenton’s reagent are very low pH requirement (<3) and the accumulation of large amount iron sludge at the end of the process thereby creating a

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secondary pollution12,13,14 . In order to avoid the secondary pollution owing to the generation of iron sludge, a heterogeneous type of Fenton’s reagent has been designed to deposit iron onto a solid support, like silica, zeolite, carbon nanotube or other clay materials15,16,17,18,19. The other approach to incorporate H2O2 to degrade organic pollutants is to employ it with a suitable photocatalyst. It is has been successfully used with TiO2 to enhance the overall activity of photocatalyst to decompose the organic materials20. However, as mentioned

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earlier, TiO2 has a high bandgap energy and hence require a UV source. Recently, H2O2 is also

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used with carbon nitride to successfully decompose organic molecules in presence of visible

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light irradiation21.

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Organic dye is a common type of water pollutant and a concern for wastewater treatment. It is released from different types of industries, like textiles, food, printing and

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cosmetics and possesses quite serious threat to the environment due its toxicity and lack of

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biodegradability22,23,24 . In this work, methylene blue (MB) has been chosen as a model pollutant to investigate it’s photodegradation by carbon nitride in presence of hydrogen

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peroxide and under visible light irradiation. In the course of the study, the kinetics of reaction, dose of photocatalyst, concentration of pollutant and the pH of reaction mixture

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were varied to understand the photodegradation of the dye. With the help of liquid chromatography-mass spectroscopy (LCMS), different degradation products and the

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possible reaction pathways have also been identified. 2. Experimental Graphitic carbon nitride (g-C3N4) was synthesized by the traditional method reported by other researchers7. Typically, in one batch, 5 g dicyandiamide (Sigma-Aldrich) was put in a porcelain boat and inserted in Linderberg-BlueTM tube furnace. The furnace was heated to

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550 C through the period of 4 hours and cooled down to room temperature in air. The yellow mass obtained was ground in a mortar and pestle and used without any further modification. Carbon nitride was characterized with X-ray photoelectron spectroscopy (XPS),

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fourier transform infrared spectroscopy (FTIR), scanning electron microscopic images (SEM), thermogravimetric analysis (TGA) and pore textural properties including BET specific surface area. XPS data was obtained in Thermo-Fisher K-Alpha instrument with an Al-Kα x-ray anode. The energy of x-ray was 1486.6 eV with the solution of 0.5 eV. The sample was mounted on a carbon tape and the charge neutralization was performed by 2 eV Ar+ ions. Scanning electron microscopic images were performed in Carl Zeiss Merlin SEM microscope

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operating at 1 kV. BET surface area was obtained by analyzing N2 adsorption-desorption plot

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at 77 K, which was measured in Quantachrome’s Autosorb-iQ surface area and porosity

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analyzer. Thermogravimetric analysis (TGA) was obtained in TA instrument’s SDT Q600

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simultaneous TGA-DSC analyzer.

In order to study the kinetics of photocatalytic activity of carbon nitride in

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presence of hydrogen peroxide, 25 mL of 10 mg/L methylene blue (Sigma-Aldrich) solution

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was used along with 0.2 g carbon nitride and 5 mM H2O2 (Sigma-Aldrich). The abovementioned mixture was stirred in a 200 mL round bottom flask for the time interval of 5, 10,

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15, 30 and 45 minutes. A 315 watt ceramic metal halide lamp (Hydroponics) with a UV protective shield (Edmund Optics) was used as a source of visible light. For control purposes,

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pure stock solution of methylene blue and methylene blue/carbon nitride mixture with and without hydrogen peroxide were studied in both darkness and under visible light for the same time interval. One mixture was used for each time interval and upon completion of the

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required time, the flask was taken out and centrifuged several times to remove carbon nitride from the solution. All the operations after the kinetic runs were performed under darkness to avoid any further photoreaction. The residual concentration of methylene blue (MB) was measured in a Thermo-Scientific Genesis UV-Vis spectroscope at the wavelength

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of 668 nm against a known calibration plot. All the runs were performed in duplicate and the average values were reported along with standard deviation. In the course of experiments, the influence of photocatalyst dose and initial dye (MB) concentration were also studied. The photocatalyst dose was varied in the amounts of

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0.1, 0.15, 0.2 and 0.25 g along with 25 mL of 10 mg/L MB and 5 mM H2O2. In order to study the effect of initial dye concentration, the concentrations of MB were varied for 5, 10, 15 and 20 mg/L along with 0.2 g carbon nitride and 5 mM H2O2. In order to understand the role of pH of the reactant mixture, the initial pH was varied in 3, 5, 6.3, 7, 8 , 10 and 12 with the help of suitable buffers. For all the pH runs, 10 mg/L 25 mL MB solution was used along with 0.2

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g photocatalyst and 5 mM H2O2. The reaction time was fixed to 30 mins only.

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Liquid chromatography-Mass spectroscopy (LCMS) analysis was performed in

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Thermo Scientific Q Exactive orbitrap instrument. The column used was Waters Acquity UPLC BEH C18 with dimensions of 1.7um 201m X 50 mm. For each sample, only 20 L sample

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was injected onto the column. The samples after 5, 10, 15, 30 and 45 minutes kinetic run was employed for LCMS analysis. In order to study the residual of the intermediate compounds,

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same fashion.

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two additional samples after 3 hr and 8 hr of reaction were also analyzed under LCMS in the

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3. Results and Discussion 3.1 Materials Characteristics

According to XPS results, The total C, N and O contents of carbon nitride were in

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the range of 40.7, 57.8 and 1.7 at.%, respectively. The detailed peak fitting results for C-1s and N-1s spectra of XPS are given in figure 1(a) and 1(b), and the quantitative contributions

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are provided in Table 1. In C-1s peak fitting results, the largest peak appears at about 288.2 eV of energy level. It is attributed to sp2 hybridized carbon in the form of N-C=N in graphitic carbon nitride25,26,27,28 and constitutes of 36.3 at.%. The other two peaks of C-1s spectra appeared at 284.8 and 286.2 eV, which belong to sp2 C=C bonds of carbon containing contaminations and sp3 hybridized carbon from defects in graphitic carbon nitrideError! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined..

A small

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peak at about 289.5 eV may also be attributed by the carbonate groups (oxygen containing defects in carbon nitride). In N-1s spectra, the largest peak at 398.7 eV belongs to sp2 hybridized nitrogen in the form of C-N=C of carbon nitride29,Error! Bookmark not defined.,Error! Bookmark not defined.,30

and its quantitative contribution is 41.0 at.%. The other two larger peaks

at 399.8 and 401.2 eV belong to sp3 hybridized nitrogen in the form of H-N-(C)3 and amino Bookmark not defined.,

Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,Error!

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nitrogen (C-NHx)Error!

respectively. Besides those peaks, two smaller peaks at around 404 and

406 eV belonged to graphitic and nitro/nitroso type of nitrogen functionalities31 respectively. The FTIR spectra of carbon nitride are shown in figure 2. The broad peak at around 3154 cm-1 belongs to N-H stretching and deformation mode32. Few narrow peaks in the region of 2000-2500 cm-1 belong to the presence of NC or CN bonds33; their

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contribution is very low in the system owing to the continuous C and N conjugated bonds.

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The peak at about 1560 cm-1 is attributed to C=N bond34, whereas the peak at 1408 cm-1 is Bookmark not defined..

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ascribed to sp3 C-C bonds35. The peak at 1274 cm-1 is attributed to C-N heterocyclesError! The sharp peak at 805 cm-1 belongs to the breathing mode of C-N

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heterocycles in triazine units36,37 of carbon nitride.

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Table 1. Detailed quantitative contributions of chemical functionality as obtained from XPS Functionality

Amount (at.%)

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sp2 (N-C=N)

36.3

sp2 (defects)

3.3

sp3

<0.1

CO3-

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sp2 (C-N=C)

41.0

sp3 (H-N-(C)3)

8.0

Amino(C-NHx)

5.6

Graphitic

2.2

Nitro/nitroso (NOx)

1.0

--

1.7

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Atom type

O

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The SEM images in micron and nanoscale of as-synthesized carbon nitride are given in figure 3(a) and (b), respectively. The surface of carbon nitride appears to be very rough and no ordered structure is noticed. Energy dispersive x-ray (EDX) was performed in

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SEM image the results are given in figure 3(c). It was observed that carbon and nitrogen are the largest contributions in carbon nitride followed by a very small amount of oxygen. The mapping for C, N and O atoms are shown in the inset of figure 3(c). Nitrogen adsorptiondesorption at 77 K on carbon nitride is shown in figure 4. The BET specific surface area calculated from this plot is about 0.5 m2/g. Thermogravimetric analysis (TGA) of carbon nitride in N2 and air is shown in figure 5. Only a small loss of mass of about 5 % upto 100 C

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may be attributed to the loss of moisture. It was observed that MB is stable at about 500 C

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and then started to decompose very rapidly. The thermal stability of carbon nitride in N2 may

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be slightly higher than that in air as the derivative of peak of thermal decomposition under

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N2 atmosphere appeared at about 720 C compared to 633 C in air.

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3.2 Photodegradation of Methylene Blue

For all the photocatalytic studies, the concentration of methylene blue was set to

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10 mg/L. The kinetics of photodegradation of MB at different time intervals is given in figure 6. Stock solution, i.e., pure MB without any photocatalyst or H2O2 did not undergo any

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photodegradation under visible light. MB with 5 mM H2O2 at light underwent about 17 % photodegradation under the same conditions and within the time period of 45 mins, which

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is not more than about 8 % higher than the same mixture under darkness. Pure carbon nitride in darkness and light, was successful to degrade methylene blue to about 27.6% and

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46.5 %, respectively, in 45 mins. Carbon nitride, in presence of H2O2 and in darkness, demonstrated slightly lower performance compared to that of pure carbon nitride under visible light illumination. Since, the catalyst cannot be activated in darkness, the degradation of MB is caused by the H2O2 itself; most likely, H2O2 was dissociated by both presence of light and on the catalyst surface to generate reactive hydroxyl radials that reacted with MB. It is obvious from the plot that combined effect of carbon nitride and H2O2 under the illumination

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of visible light resulted in significantly faster decay. It caused almost complete photodegradation of methylene blue in 45 mins. Enhancement of photodegradation by the combined influence of photocatalyst and hydrogen peroxide is similar to that of other TiO2/H2O2 systems20. It is important to note that pure H2O2 was also able to degrade MB in

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a very slow fashion. H2O2 may slowly dissociate to form hydroxyl radical (OH.) that can attack and degrade an organic molecule. This dissociation is faster in presence of UV light, but very slow under visible light. Owing to the activation of photocatalyst in visible light, the rate of photodegradation caused by pure carbon nitride was faster than that by pure H2O2, unlike a traditional TiO2/H2O2 mixture reported in literature, where the rate of degradation of organic molecule is similar in presence of TiO2 or H2O2 under UV light irradiation20. It is

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important to notice that the rate of photodegradation of MB is faster in first 15 minutes

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followed by a slightly slow rate of reaction in the remaining time period. Such difference may

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be attributed to the fact that, initially, MB was the key recipient of the hydroxyl radials (OH.)

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to undergo the oxidative reaction. However, in later period, smaller molecular fragments also receive those radicals for oxidation and hence less number of radicals becomes available

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for MB resulting in lowering the rate of its photodegradation. More details about the possible

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degradation products are given in the next section on LCMS analysis. It needs to be mentioned that at a very low dose of hydrogen peroxide (<0.5 mM),

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the influence of H2O2 in photocatalysis is negligible. At a higher dose (>5 mM), two problems seemed to arise. Firstly, H2O2 was too aggressive to oxidize MB even after the photocatalysis

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experiment was stopped and in the course of photocatalyst separation (centrifuge) before the sample was tested by UV-Vis spectroscopy and hence a false result was manifested.

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Secondly, higher dose of H2O2 also slightly degraded carbon nitride itself as observed by XPS. We found that that the H2O2 concentration around 5 mM minimized those ill-effects, but significantly enhanced the oxidation of MB compared to photocatalyst alone. Furthermore, it is also well known that the elevated dose of H2O2 may lower the photocatalytic activity by scavenging effects of free radicals, however, in our trial runs, we did not observe lowering of photocatalytic oxidation of MB at the highest concentration of H2O2 and therefore, it is very

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unlikely that we had exceeded the limits where H2O2 may lower the photocatalytic degradation. The enhancement of photodegradation of MB in presence of carbon nitride and H2O2 may be attributed to the combined effects of photodissociation of H2O2 in presence of

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light and on the surface of photocatalyst and the complex transport of highly reactive hydroxyl radical (OH.) in the reactant mixture. According to the literature38,20 the following reaction schemes of the photolysis of H2O2 are possible. H2O2+h  2OH. ….. (1) 2H2O2 2H2O + O2 ….. (2)

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…… (4) …….(5)

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h++OH-  OH. e- + O2O2-

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e- + H2O2 OH.+OH- ….(3)

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RH+OH. H2O+R.  further oxidation …(6) (RH = organic molecule) From the reaction schemes, it is clear that the crucial part is the generation of hydroxyl

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radicals from the photolysis of H2O2 that can be used to react with the organic molecules.

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H2O2 can also act as electron scavengers in the course of photocatalysis. The hydroxide ion that was produced from electron scavenging reaction of H2O2 may, in turn, react with holes

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(h+) thereby minimizing the recombination of electrons and holes in the molecular orbital of the photocatalyst32,39. The superoxide ion (O2-) that is produced in the course of the reaction may also be used to oxidize the organic molecule but it is not as strong oxidant as an hydroxyl

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radical20. In addition to photolysis, H2O2 may also dissociate on the surface of photocatalyst

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and the following reactions are plausible according to the reports of Miller et al.40 C+ + H2O2  C+H++HO2. ……(7) (C+ = Oxidized catalyzed surface) C+ H2O2  OH-+ OH.…... (8) (C = Reduced catalyzed surface) C+ + O2-  C+O2 ……(9) . + C + OH 2  C +HO2 …....(10) C + OH.  C++ OH…… (11)

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It is obvious that in these model equations, H2O2 may react with both oxidized and reduced catalyst surface and both the catalyst and H2O2 can undergo simultaneous redox reactions. In the course of these reactions, more hydroxyl radicals and ions are generated that may, in turn, act as electron/holes scavengers and react with organic molecules as well. As shown in the reaction (9)-(11), superoxide, hydroxyl and perhydroxyl radials may also react on

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catalyst surface that can further enhance the photocatalytic reaction in a cumulative fashion. These set of reactions may explain the increase in overall rate of photodegradation of methylene blue in g-C3N4/H2O2 system.

The influence of two reaction parameters, including the dose of photocatalyst and initial methylene blue concentration is shown in figure 7(a) and (b). In figure 7(a), it is observed that rate of photocatalysis (mg/g of photocatalysis) is higher for the larger dose of

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photocatalyst at all time intervals. Larger dose photocatalyst essentially provides larger

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amount of electron-hole pairs under visible light giving rise to larger amount of hydroxyl

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radicals, which, in turn, increases rate of photodegradation of MB. It is also needs to mentioned that, as reported for TiO2/H2O2 system, higher dose of photocatalyst may lower

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the rate of reaction due to the higher turbidity of the reaction mixture that may block the part of incident light onto the system. In our system and with the maximum dose of

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photocatalyst, we did not observe such effect and hence it may be concluded that much

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higher dose of carbon nitride would be required to initiate this effect. Lower initial concentration of dye resulted in elevated degradation (fig 7(b)), which is quite intuitive and

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can be correlated with the limited number active sites of carbon nitride or limited number of hydroxyl radials generated in the course of photocatalysis.

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Influence of solution pH in the photodegradation of MB is shown in figure 8. The

rate of photodegradation caused by carbon nitride is poor in both lower and higher pH and demonstrated best performance within the neutral pH values of 7-8. Different reasons that

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cause the pH dependency of the photocatalytic reactions are photocatalyst’s surface charge, flat band potential, dissociation of the medium and coulombic interactions between the catalyst and dye20. At the lower and higher pH, catalyst surface may be positively or negatively charged due to the surface adsorption of H+ or OH- ions, respectively. Similarly, depending on pKa values, the MB molecule itself may attain positive or negative charge in lower and higher pH values. Therefore, at lower or higher solution pH, coulombic repulsions 11

between the similarly charged photocatalyst and MB molecule prevents intimate contact between two species. As highly reactive hydroxyl radicals are generated only at the contact point of the photocatalyst and water, it cannot reach the MB molecule and hence the overall rate of photocatalytic degradation of MB decreases.

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3.3 LCMS Analysis: Reaction Intermediates

Samples with all the time intervals of kinetic run along with pure methylene blue (MB) and two additional samples at 3 hr and 8 hr of time intervals were fed into the liquid chromatography-mass spectroscopy (LCMS) system. Pure methylene blue is designated as “0 min” in the figure. Total ion chromatogram (TIC) of the all samples is shown in figure 9, where the retention time is plotted against intensity of the signal. Each signal was resolved

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in mass spectroscopy to attain the species with known m/z values. The molecular structures

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of the key species with known m/z values from signal of mass spectroscopy are isolated from

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the daughter ions and inserted on the same figure. Different other literature that reported molecular fragments of MB under photolysis41,42,43 are also consulted in order to identify the

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reaction intermediates of MB in our system. In pure MB solution, the key species is attributed to m/z=284, which is methylene blue itself without the chloride ions. We also detected a

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small amount species with m/z=270 which is nothing but MB molecule with short of one

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methyl group (-CH3) attached to either of the tertiary N-atoms and often termed as Azure B (AB). Presence of AB in pure MB suggests the possible presence this species in the dye as

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impurity, which was obtained from commercial sources. The strong peak due to the presence of un-degraded MB was detected clearly upto 15 mins of sample and then drastically

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decreased in 30 mins. Such a trend clearly supports of the change in residual MB concentration that was detected in UV-Vis spectroscopy and shown in figure 6. Besides Azure B, the other molecule with three-member ring structure that was detected in 5-15 minutes

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of time interval is m/z= 256. It is termed as Azure A (AA) and short of two methyl groups from the terminal N-atom compared to pure MB. It also needs to be mentioned that a heavier species compared MB is also detected in the sample of 5 min interval with m/z= 298. Possibly, this species has the same structure as that of MB except an additional methyl group attached to any of the three rings. Starting from 10 min interval, a single ring structure appeared in the system with m/z=219 and a possible amino-quinone type of structure. From 12

these time intervals, two other prominent species appeared in the system with m/z=219 and 127. Based on the reaction intermediates of the MB under photolysis and as reported by previous researchers, we suggest that those could be the open ring structures with possible sulfonic acid and amine containing groups in the molecule and shown in the figure. In the last sample of 8 hr interval, we still found the partial presence of an unreacted molecule with

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m/z=127.

Since the key intermediate products detected in LCMS are designated as m/z= 298, 284, 270, 256, 219, 127 and 123, we have plotted the change in relative intensity of those molecules with time in figure 10. According to the figure, the intensity of MB (m/z = 284) went to very low after 30 mins of time interval. The three-member ring structures, i.e.,

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demethylation products of MB, AB (m/z = 270) and AA (m/z = 256) almost degraded

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completely within 45 minutes of time interval. The fragments with m/z = 219 and 127 still

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had a slight increasing trend within 45 minutes but diminished within 3 hr time. The fragment with m/z = 127 and 122 continued to increase significantly within the 45 minutes

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of time. Although there is a definite decreasing trend for this species after 8 hr, it did not

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mineralize those fragments.

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diminish completely and most likely, even a longer time interval was required to completely

The possible reaction intermediates of photocatalytic oxidation of MB are shown

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in figure 11. There are four stages by which MB molecule can undergo a complete photocatalytic degradation, (i) demethylation, (ii) ring shortening, (iii) ring opening and (iv)

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final oxidation or mineralization. In the first step of demethylation, four methyl groups gradually break down from the two tertiary nitrogen atoms at the two side chains of the MB molecule. In the course of demethylation, four intermediate compounds that could be

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generated are azure B (AB), azure A (AA), azure C (AC) and thionine, and all the structures have the intact three member aromatic rings in the center35. In our system, although we have detected the presence of AB and AA from the signal of mass spectral data, we did not the presence of AC and thionine. Most likely, either the reaction pathways were somewhat different from that of Cr-Ti catalyst that generated all the intermediates35 or those species were quickly oxidized in the course of reaction without leaving their fingerprints in the 13

analysis. According to literature37, the progressive oxidation of methyl groups were performed by the hydroxyl radical, which gradually converts methyl groups to alcohol, aldehyde, carboxylic acid and ultimately decarboxylates to CO2 by photo-Kolbe type of reactions as given below.

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-N(CH3)2+ OH. -N(CH3)-CH2.+H2O ……………(15)

-N(CH3)-CH2. + OH.  N(CH3)-CH2OH……………(16)

-N(CH3)-CH2OH + OH.  N(CH3)-CH.OH+H2O……………(17) -N(CH3)-CH.OH+ OH.  N(CH3)-CH(OH)2 ……………(18)

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-N(CH3)-CH(OH)2  N(CH3)-CHO+H2O……………(19)

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-N(CH3)-CHO+ OH.  N(CH3)-C.=O+H2O……………(20)

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-N(CH3)-C.=O + OH.  N(CH3)-COOH……………(21)

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-N(CH3)-COOH + h+  N.-(CH3) + H+……………(22)

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The methyl-amine radical attached to phenolic group can undergo further

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degradation for further demethylation or the hydroxylations of aromatic ring itself giving rise to the ring shortening. The amine group also react with hydroxyl radicals to produce

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ammonia, ammonium ion or the nitrates37. -R-NH2 + OH.  R-OH + NH2...............(23) NH2. + H.  NH3 ……………….(24) NH3 + H+  NH4+ ……………….(25)

The only prominent short ring structure that was detected in our system is an one ring amino-quinone type of structure (m/z = 123). Although few other short ring structures, like phenol, aniline or benzenesulfonic acid were suggested for photocatalytic degradation of MB under TiO2/UV system, we could not detect those species in our study with carbon nitride.

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Most likely, other short ring structures were too unstable and broken onto smaller fragments or open ring structures very quickly. The two possible key open ring structures that were detected in our system are amine and amine/sulfonic acid containing species with m/z = 127 and 219, respectively. The

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sulfur containing species with MB may undergo progressive oxidation to ultimately convert to sulfonic acid (-SO3H) or sulfates (SO4-) by the following possible reactions37 -S+= + OH.  -S(=O)- + H+ …………..(26)

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-S(=O)- + OH.  -SO2 + H ……………..(27)

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-SO2 + OH.  -SO3H …………………(28)

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-SO3H + OH.  SO4- + 2H+ ……………..(29)

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We were unable to specifically find the smaller fragments that could be generated from the photolysis of the open ring structures in the signal of mass spectroscopy that could be

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4. Conclusion

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possibly identified from the pool of several daughter ions.

Based on the results, the overall conclusion can be made that the rate of

photodegradation of methylene blue in water by carbon nitride and in presence of visible

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light can be increased significantly by addition of hydrogen peroxide in the system. Hydrogen peroxide acted as a source of additional hydroxyl radicals that increased the oxidation of MB.

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Furthermore, the dissociation products of hydrogen peroxide might have also prevented the recombination of holes and electrons of carbon nitride that in turn increased the overall activity of photocatalyst. LCMS analysis of the samples helped to identify the molecular fragments of photodegradation reactions of MB and possible reaction pathways. Before final mineralization, the possible photodegradation steps were demethylation, ring shortening and ring opening of MB. The overall results suggest that addition of hydrogen peroxide can 15

enhance the photodegradation of an organic molecule in presence of carbon nitride under visible light irradiation. Acknowledgements M.D., T.H., and E.S. acknowledge the finding from School of Engineering of Widener

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University. All the authors acknowledge Dr. PapaNii Osare-Oaki of University of Delaware for helping with LCMS study. SEM (D.K.H and J.C.) experiments were partially conducted under the user proposal (CNMS2016-302) at the Center for Nanophase Materials Sciences,

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ORNL, which is a DOE Office of Science User Facility.

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Figure 1. XPS peak fitting results of carbon nitride for C-1s (a) and N-1s (b)

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Figure 2. FTIR peaks of carbon nitride

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Figure 3. SEM image of carbon nitride in micron scale (a) and nanometer scale (b). Energy dispersive x-ray (EDX) peaks for C, N and O (inset: EDX mapping for C, N and O) (c)

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Figure 4. Nitrogen adsorption-desorption plot of carbon nitride at 77 K

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1.6

90

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80 70

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air N2

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N2: Derivative

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Air: Derivative

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Wt.%

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0 200

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-0.2 600

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Figure 5. Thermogravimetric analysis (TGA) of carbon nitride in air and N2

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Figure 6. Kinetics of photodegradation of methylene blue at different reaction conditions. The initial concentration of methylene blue was set to 10 mg/L.

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Figure 7. Kinetics of photodegradation of methylene blue as a function of photocatalyst dose (a) and initial dye concentration (b). The initial concentration of methylene blue was set to 10 mg/L (reaction time 30 mins).

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Figure 8. Influence of solution pH in the photodegradation of methylene blue (25 mL 10 mg/L MB, 0.2 g C3N4, 20 mM 36% H2O2, reaction time: 30 mins)

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Figure 9. Total ion chromatogram (TIC) of LCMS analysis

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Figure 10. Variation of relative intensity of different intermediate products with time as obtained in LCMS analysis

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Figure 11. Possible reaction pathways of photodegradation of methylene blue with carbon nitride and hydrogen peroxide

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