Enhanced photocatalytic performance of polymeric carbon nitride through combination of iron loading and hydrogen peroxide treatment

Journal Pre-proof Enhanced photocatalytic performance of polymeric carbon nitride through combination of iron loading and hydrogen peroxide treatment Vu Viet Thang, Stephan Bartling, Tim Peppel, Henrik Lund, Carsten Kreyenschulte, Jabor Rabeah, Nikolaos G. Moustakas, Annette-Enrica Surkus, Ta Hong Duc, Norbert Steinfeldt

PII:

S0927-7757(19)31381-0

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124383

Reference:

COLSUA 124383

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

13 September 2019

Revised Date:

18 December 2019

Accepted Date:

21 December 2019

Please cite this article as: Thang VV, Bartling S, Peppel T, Lund H, Kreyenschulte C, Rabeah J, Moustakas NG, Surkus A-Enrica, Duc TH, Steinfeldt N, Enhanced photocatalytic performance of polymeric carbon nitride through combination of iron loading and hydrogen peroxide treatment, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124383

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Enhanced photocatalytic performance of polymeric carbon nitride through combination of iron loading and hydrogen peroxide treatment

Vu Viet Thang1, Stephan Bartling2, Tim Peppel2, Henrik Lund2, Carsten Kreyenschulte2, Jabor Rabeah2, Nikolaos G. Moustakas2, Annette-Enrica Surkus2, Ta Hong Duc1, Norbert Steinfeldt2

Hanoi University of Science and Technology, No. 1 Dai Co Viet Str., Hanoi, Vietnam

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Leibniz Institute for Catalysis, Albert-Einstein-Straße 29a, D-18059 Rostock, Germany

Dr. Norbert Steinfeldt

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Corresponding author:

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Tel: +049 (0)381 1281 319

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e-mail: [email protected]

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Graphical abstract

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Abstract Polymeric carbon nitride (p-C3N4) based materials have shown great potential as photocatalysts for degradation of pollutants from wastewater. In this work p-C3N4 was synthesized from urea or ferric chloride/urea by pyrolysis followed by a post-treatment with hydrogen peroxide (H2O2). The prepared materials were characterized by physico-chemical

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methods in order to study their structural and electronic features. The characterization results revealed that iron was present in p-C3N4 as a Fe3O4 phase. The H2O2 treatment did not only

modify the p-C3N4 framework through the generation of oxygen functional groups and defect

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sites but it also affected the size, distribution, and oxidation state of iron oxide. The

photocatalytic activity of the catalysts was tested through the degradation of methyl orange

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under visible and white light irradiation. In comparison, the iron oxide containing p-C3N4 post-treated with H2O2 sample shows superior photocatalytic performance towards methyl

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orange degradation. This might be attributed to the valence and conduction band levels of the composite material, an inhibited recombination of photogenerated charge carriers as well as a

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present in p-C3N4.

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synergistic effect of iron species and the oxygen containing functional groups or defects

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Keywords: p-C3N4; iron oxide; hydrogen peroxide; photocatalysis; methyl orange; degradation

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1. Introduction Nowadays, water pollution is becoming one of the most emerging environmental concerns [1]. One of the most important sources of such pollution comes from organic wastes and residues in wastewater of the textile, dying, printing and other related industries. Between 1– 20% of the total world production of dyes is lost during the dyeing process and subsequently released in the textile effluents [2]. These effluent streams have to be treated to eliminate the poisonous dye residues. Moreover, an effective dye wastewater decolorization is usually

treatment of the raw dye waste water is often necessary [3].

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required due to legal regulations. To achieve the stringent disposal limits, an extended

For the treatment of dye wastewater, semiconductors have shown great potential as

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photocatalysts. Graphitic carbon nitride (g-C3N4), a 2-D π-conjugated polymeric, metal free, narrow band gap semiconductor (about 2.7 to 3.0 eV) is considered an attractive candidate for

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practical applications due to its visible light absorption, graphene like structure and a relatively high thermal and chemical stability [4]. The photocatalytic performance of g-C3N4

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is affected by many factors e.g., crystal structure, optical, electronic and surface physiochemical properties as well as its defected structure [4-6]. Decoration of g-C3N4 with

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metallic species like alkali, transition or noble metals has been identified as an effective method to enhance its photocatalytic activity [7-9]. However, for sustainable applications the

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development of highly active, noble-metal-free photocatalytic systems is of utmost importance [6].

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Iron is an abundant, inexpensive, environmentally benign transition metal. Recently, it

was shown that the photocatalytic activity of g-C3N4 towards dye degradation can be significantly improved by the decoration of g-C3N4 with iron or iron oxide [10-17]. Enhancement of photoactivity of the iron(oxide)/g-C3N4 composite material was also observed for hydrogen evolution in the presence of H2O2 or Pt [18, 19], for CO2 [20], and Cr(VI) [21] reduction. The enhanced photoactivity of iron(oxide)/g-C3N4 was explained by 3

synergistic effects including improved optical properties [11, 14, 19], higher surface area [10, 12, 14, 19], and more efficient separation of photo-induced charge carriers [11-14, 17, 19]. However, an enhancement of the photocatalytic performance of pristine g-C3N4 can also be achieved by its treatment with hydrogen peroxide (H2O2). The H2O2 treatment leads to the formation of oxygen functional groups [22-24] and/or active surface species [25]. The oxygen functional groups were assumed to enhance light absorption and accelerate electron transfer [22], trapping photo-generated electrons [23], and inducing intrinsic electronic and

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band structure modulation [24]. Also the blocking of surface sites responsible for the adsorption of particular substrates as a result of treating the g-C3N4 with H2O2 has been discussed [25].

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The aim of the present work was to study the effect of iron (oxide) loading and H2O2 treatment on photocatalytic activity of g-C3N4. Therefore, g-C3N4 with and without an iron

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precursor was synthesized directly from urea and post-treated with H2O2. The obtained

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materials were characterized by complementary methods in order to evaluate the influence of iron loading and H2O2 treatment on the structure, morphology, texture as well as the optical and electronic properties of the formed composite materials. The photocatalytic performance

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of the prepared materials was investigated through methyl orange (MO) degradation under

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irradiation with white and visible light (λ ≥ 420 nm). 2. Experimental

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2.1 Chemicals

Urea (98 %), iron(III) chloride hexahydrate (FeCl3·6H2O, 97 %), p-benzoquinone (98 %), terephthalic acid, and hydrogen peroxide (H2O2, 30 % aqueous solution) were purchased from Sigma Aldrich. Methyl orange was obtained from Acros Organics. Silver nitrate (>99.5 %) was purchased from Fluka. Nanocrystalline Fe3O4 powder was obtained from Io-Li-Tec.

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2.2 Material synthesis Pure p-C3N4 was synthesized using the pyrolysis method. In brief, 6 g of urea (100 mmol) were put in a sealed ceramic crucible, transferred to a muffle furnace and heated at 550 °C over 2 hours in presence of air. After reaching 550 °C the crucible was taken out from the furnace and the sample was left to cool down to room temperature at ambient conditions. One gram (1 g) of the as-prepared p-C3N4 was suspended in 50 mL H2O2 solution under vigorous stirring and refluxed at 75 °C for 6

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hours. Afterwards, the solid material was collected by centrifugation. The reaction with H2O2 was repeated 3 times. At the end, the material was washed with 100 mL of water, centrifuged and ovendried at 80 °C for 16 hours. The sample was designated as p-C3N4-O.

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The Fe-p-C3N4 catalyst was synthesized by dissolving 6 g of urea and 19.46 mg of FeCl3 in distilled water (molar ratio urea/Fe = 0.12 %). After the solvent was evaporated, the solid compound

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was heated in the muffle furnace under the same conditions as the ones used to prepare the pristine p-C3N4. For the treatment with H2O2, 200 mg of Fe-p-C3N4 were added into a three-necked flask

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and mixed with 10 mL of H2O2 solution (Caution! Very fast decomposition of H2O2). An ice bath was used to control the temperature of the mixture and slowing down the reaction. After 3 hours the

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suspension was centrifuged and in a subsequent second step the solid was treated again with 10 mL H2O2 for another 3 hours. Afterwards, the material, designated as Fe-p-C3N4-O, was centrifuged and

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dried in a drying chamber at 80 °C for 16 hours.

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2.3 Characterization

X-ray diffraction (XRD) powder patterns were recorded on a Panalytical X'Pert diffractometer (40 kV, 40 mA) equipped with a Xcelerator detector using automatic divergence slits and Cu kα1radiation (λ = 0.15406 nm) in a 2 scan range between 5 and 80°. The step size was 0.0167° and each single point of the diffraction data was collected for 400 s. The samples were mounted on silicon zero background holders. The obtained intensities were converted 5

from automatic to fixed divergence slits (0.25°) for further analysis. Peak positions and profile were fitted with Pseudo-Voigt function using the HighScore Plus software package (Panalytical). Phase identification was done by using the PDF-2 database of the International Center of Diffraction Data (ICDD). Scanning transmission electron microscopy (STEM) measurements were performed in an aberration-corrected JEM-ARM200F instrument (JEOL, Corrector: CEOS) operated at 200 kV. The microscope was equipped with a JED-2300 (JEOL) energy dispersive X-ray

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spectrometer (EDXS) for elemental analysis and high angle annular dark field (HAADF) and annular bright field (ABF) detectors for imaging. The samples were supported on holey carbon Cu (mesh 300) and transferred into the microscope without any pretreatment.

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Scanning electron microscopy (SEM) measurements were done using a Merlin VP

compact device (Zeiss, Oberkochen, Germany) and additional EDX investigations were performed

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using a Quantax 400 device (Bruker, Billerica, MA, USA). Acceleration voltage was between 2.5

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and 5.0 kV. In order to minimize charging of the samples, p-C3N4-O, Fe-p-C3N4 and Fe-pC3N4-O were sputtered with carbon prior to the respective SEM measurements. Attenuated total reflection infrared (ATR-IR) spectra were acquired using a Bruker

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ALPHA FTIR-spectrometer. The data collection consisted of 64 scans per spectrum with a resolution of 4 cm-1. The powder sample was deposited on the ATR-crystal without any

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further pretreatment.

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UV/Vis diffuse reflectance spectra of the solid material were measured with a PerkinElmer Lamda 365 spectrometer equipped with a reflectance sphere using pure calcium sulphate CaSO4 powder as white reflection standard. The optical band gap (Eg) energies were calculated from the Tauc’s plot [26], using the relationship between (F(R) hν)1/2 and hν. F(R) is the Kubelka-Munk function derived from reflectance spectra where F(R) = (1-R)2/2R and hν is the photon energy.

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Nitrogen (N2) adsorption-desorption isotherms were collected at –196 °C on the apparatus ASAP 2020 (USA). The Brunauer-Emmett-Teller (BET) surface area and the pore size distribution were calculated from the adsorption and desorption branches of the isotherm, respectively, applying the BET equation for the N2 relative pressure range of 0.05 < p/p0 < 0.30 and the Barrett-Joyner-Halenda (BJH) method for the pressure range of 0.30 < p/p0 < 0.99. The samples were pretreated before measure by heating to 200 °C. Photoluminescence (PL) spectra were obtained by a Cary Eclipse Fluorescence

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spectrophotometer (Agilent Technologies Inc., Mulgrave, Australia) with an excitation wavelength of 370 nm.

X-ray Photoelectron Spectroscopy (XPS) measurements were performed on an

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ESCALAB 220iXL (Thermo Fisher Scientific) with monochromated AlKα radiation

(E = 1486.6 eV). Samples were prepared on a stainless steel holder with conductive double

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sided adhesive carbon tape. The electron binding energies were obtained with charge

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compensation using a flood electron source and referenced to the C1s core level of adventitious carbon at 284.8 eV (C-C and C-H bonds). For quantitative analysis the peaks were deconvoluted with Gaussian-Lorentzian curves using the software Unifit 2020. The peak

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areas were normalized by the transmission function of the spectrometer and the element specific sensitivity factor of Scofield [27].

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Carbon hydrogen nitrogen (CHN)-analysis was performed with a TruSPec CHNS Micro

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(Leco). The amount of iron in solid p-C3N4 as well as in the reaction mixture was analyzed with a AAS-contrAA800D (Analytic Jena) spectrometer. The total organic carbon (TOC) content was obtained using the multi N/C 3100 (Analytic Jena) analyzer from the solution after an irradiation time of 180 min. X-band Electron Paramagnetic Resonance (EPR) spectra were recorded on a Bruker EMX CW-micro EPR spectrometer equipped with an ER4119HS high-sensitivity resonator with a microwave power of ca 6.9 mW, modulation frequency of 100 kHz and modulation 7

amplitude up to 5 G. g values were calculated using hν = gβB0 equation (ν frequency, β Bohr magneton, and B0 resonance field). The 2,2-Diphenyl-1-picrylhydrazyl (DPPH) was used as a standard (g = 2.0036 ± 0.0004) for the calibration of the g values. EPR spectra were recorded at 20 oC using 10 mg of each individual p-C3N4 catalyst. 2.4 Photocatalytic experiments Photocatalytic experiments were carried out in a 50 ml double jacket cylindrical glass reactor. The solution was irradiated with a 1000 W Xenon lamp (LAX 1000, Oriel) equipped with a

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90° deflection reflector system (MS 90) containing a dichroic mirror. The distance between reactor and the deflection reflector system was 12 cm. Experiments were performed with and without a UV cut-off filter (λ ≥ 420 nm). The light intensity near the reactor was 35.2 mW/cm2 when using white

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light (without filter) and 1.4 mW/cm2 at visible light irradiation. The reaction was thermostated at

25 °C. Oxygen was fed into the reactor with a flow rate of 10 mL/min. Thirty milligrams (30 mg) of

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the catalyst were dispersed in 30 mL of aqueous MO solution (15 mg/L). Prior to the illumination,

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the MO containing suspension was stirred in the dark for 30 min. UV/Vis spectra from the aqueous solution during irradiation with visible (  420 nm) or white (without filter) light were obtained

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using an AVASpec-2048-USB2 spectrometer (Avantes) equipped with a fiber optic probe FDP7UV200-2-VAR-Pk. About 2 mL of the suspension were taken from the reaction mixture at

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equidistant time intervals, centrifuged, filtered and finally the UV/Vis spectrum of the clear solution was measured. After the UV/Vis measurement the solution and the solid catalyst were re-added to

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the reaction mixture. MO substrate conversion was monitored with HPLC using a lichrosorb RP 18 column (Merck).

The formation of hydroxyl radicals was detected by the photoluminescense (PL) technique

using terephthalic acid as a probe molecule. In these experiments, the catalyst was suspended in 30 mL of an aqueous solution containing terephthalic acid (5·10-4 mol/L) and NaOH (2·10-5 mol/L).

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Subsequently, the suspension was collected, centrifuged and filtered to collect the photoluminescense spectra in the 350 – 600 nm range with an excitation wavelength of 318 nm.

2.5 Photoelectrochemical characterization The p-C3N4 samples were deposited as a thin layer on the surface of cleaned conductive fluorine doped tin oxide (FTO) glasses (see also Supplementary Material (SM)). The prepared catalyst-loaded FTO-electrode (WE) was immersed into an aqueous solution of 0.5 M Na2SO4 (pH

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= 7.2) together with a Pt rod as the counter electrode (CE) and Ag/AgCl/3 M KCl as a reference electrode (RE). The electrolyte was purged with argon to displace oxygen. Electrochemical

impedance spectroscopy (EIS) was carried out using an Autolab PGSTAT 302N potentiostat

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(Metrohm). The EIS measurements for the application of the Mott-Schottky equation were

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performed at potentials between +1.0 V and 0 V vs Ag/AgCl in 50-mV-steps using the frequencies

3. Results and discussion 3.1 Structure and Morphology

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1000 Hz, 1500 Hz and 2000 Hz (5 mV of amplitude (rms)).

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Materials obtained from pyrolysis of CN-containing compounds at temperatures above 525 °C were recently referred to polymeric carbon nitride (p-C3N4) instead of graphitic carbon

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nitride (g-C3N4) [28]. In p-C3N4 the discernible basal planes are held together by hydrogen bonding between individual strands of the 1-D polymers instead of covalent bonds like in

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graphite [28]. The XRD powder patterns of the four prepared p-C3N4 samples are shown in Figure 1a. Two broad diffraction peaks located near 2θ = 13.0° and 27.0° are to be found in all patterns. The reflection at 2θ = 13.0° was related to in-plane repeats between heptazine units (C6N7) and the reflection near 2θ = 27.0° was related to the - stacking motif of the heptazines on top of each other [28]. Treatment of the materials with H2O2 led to a slight shift of this diffraction peak towards a larger 2θ value from 27.1° to 27.4°. Reasons for this shift 9

might be a removal of intermediates probably not included in the p-C3N4 structure, the formation of planarized oxidized p-C3N4 layers [29], the introduction of oxygen heteroatoms [24], or the formation of oxygen containing groups (hydroxyl group and/or nitrogen oxide) [23]. The powder pattern of Fe-p-C3N4 showed, additionally, a low intense diffraction peak at 2θ = 35.5° which can be assigned to the (311) crystal plane of Fe3O4 (PDF 01-084-2782, ICDD, 2016) that was not detected in the Fe-p-C3N4-O sample. Powder XRD pattern of pure Fe3O4 is shown in Figure S8a. The disappearance of the reflection at 2θ = 35.5° after H2O2

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treatment indicates a loss of long-range order within the crystallites of the iron oxide phase. The iron content of the Fe-p-C3N4 sample was only slightly affected by the H2O2 treatment (Table 1) which indicates that such treatment has little influence on the stability of the

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composite.

Figure 1a) XRD powder pattern and b) ATR-IR spectra of the as-prepared p-C3N4 samples which differ in composition and/or post-treatment.

The ATR-IR spectra of the samples are shown in Figure 1b. The absorption bands located between 1200 – 1600 cm-1 are characteristic for the stretching mode of typical C–N heterocycles [23]. Additionally, the characteristic breathing mode of the tri-s-triazine units at 10

807 cm-1 was also observed [30, 31]. The broad band between 3000 and 3600 cm-1 originate from NH, NH2 (primary and secondary), and OH stretching vibration modes [11, 31] indicating that the amino functions still exist in the material post-treated with hydrogen peroxide. A peak located at 2150 cm-1, previously observed in Fe doped p-C3N4 and attributed to a CN vibration [32] that arises due to Fe induced surface defects, was not detected. SEM images of the p-C3N4–materials are presented in Figure S1 (see Supplementary Material). The pristine sample is predominantly composed of porous stacked layers (Figure

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S1a and b) exhibiting the typical morphology of p-C3N4 [33]. Previously, it was mentioned that the layer structure and the pore channels become discerned when the sample is heated to 550 °C [34]. The images taken after H2O2 treatment showed no obvious difference between

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the structure of the p-C3N4 and p-C3N4-O materials, thus suggesting that the H2O2 treatment only slightly affects the morphology of the p-C3N4 (Figures S1c and d) which is in good

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accordance with the XRD results. The sample prepared in presence of FeCl3 (Figures S1e and

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f) shows a similar morphology like the pristine p-C3N4. Here also, the material is composed of porous stacked layers, indicating that the presence of iron has only minor effects on size and shape of the formed p-C3N4 structures. The morphology of the Fe-p-C3N4-O (see Figures S1g

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and h) sample is quite similar compared to p-C3N4-O, proving that the reaction of iron ions

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with H2O2 does not change the morphology of the p-C3N4.

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Figure 2 HAADF-STEM images of p-C3N4-O (a,b) and Fe-p-C3N4–O (c,d) with different magnifications (the white arrows show the position of iron containing nanoparticles).

The morphology of the material exposed to H2O2 was further studied by STEM. HAADF-STEM images (Figure 2) illustrate the presence of meso- and macropores in both

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materials. The single pores differ in size and shape. Obvious differences in the pore structure

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between p-C3N4-O and Fe-p-C3N4-O were not identified. However, the iron containing sample shows the presence of sub-10 nm iron based nanoparticles well dispersed inside the interlayer gallery of p-C3N4 and on the p-C3N4 surface (see also Figure S2). N2 sorption results are presented in Figure S3a. All samples show type IV isotherms with small hysteresis loops typical of the presence of meso- and macropores [20]. The BET surface area was higher for materials containing iron (see Table 1). While the BET surface

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area of the pristine p-C3N4 did not change during post-treatment with H2O2, the BET surface of the iron oxide containing sample slightly increased. One reason for the higher BET surface area might be the presence of the iron containing nanostructures. The pore size distribution is presented in Figure S3b. Differences between the samples are mainly in the wide pore diameter. Generally, samples containing iron have a higher pore volume.

Table 1. Summary of general characteristics of the different p-C3N4 samples Iron wt% -

SSA m2/g 39

Pore volume cm3/g 0.1

Band gap eV 2.8

Fe-p-C3N4

0.60

1.85

51

0.1

2.8

p-C3N4-O

0.61

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39

0.3

Fe-p-C3N4-O

0.60

1.65

69

0.3

2.8 2.8

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*obtained by CHN analysis

3.2 Optical and electronic properties

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p-C3N4

Molar C/N ratio* 0.60-0.65

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Sample

The optical properties of the materials were investigated by UV/Vis diffuse reflectance

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spectroscopy (Figure 3a). Treatment of pristine p-C3N4 with H2O2 had only a relatively small effect on light absorption. The absorbance in the visible range (λ  420 nm) increases when

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iron oxide is present and was highest for Fe-p-C3N4. Band gaps for the p-C3N4 samples calculated from the Tauc plot using the value 0.5 as exponent [22, 35, 36] are shown in Figure

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3b. All four samples have a band gap of approximately 2.8 eV which is close to the value

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given in literature for the band gap of the g-h-heptazine phase [5, 31]. The long tail in the visible light absorption of the sample containing iron might be attributed to light absorption of the iron oxide phase [37, 38] or from interaction of the iron oxide with p-C3N4. The UV/Vis diffuse reflectance spectrum of pure Fe3O4 is presented in Figure S8b. The band gap of the Fe3O4 was estimated to be 1.5 eV.

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Figure 3a) UV-Vis diffuse reflectance spectra (the inset shows a zoom of the absorption in visible range), b) the corresponding Tauc plots, and c) PL spectra of the different samples (a: p-C3N4, b: p-C3N4-O, c: Fep-C3N4, and d: Fe-p-C3N4-O)

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Photoluminescence (PL) spectra were monitored to obtain information about the

separation and recombination of photogenerated electrons and holes (Figure 3c). Pristine pC3N4 has an intense emission peak centered at 465 nm. This emission peak is assigned to

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direct charge carrier recombination of band transitions. The intensity of the PL light

significantly decreased in the modified p-C3N4. This indicates that both the presence of iron

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oxide as well as treatment of p-C3N4 with H2O2 can efficiently hamper the recombination of

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photogenerated charge carrier pairs. Pure Fe3O4 does not show photoluminescense (see Figure S8c).

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Moreover, electron paramagnetic resonance (EPR) spectroscopy was employed to obtain further insight into the electronic properties. The pristine p-C3N4 (Figure 4a) shows a weak signal at g = 2.005 which is related to the unpaired electrons on the carbon atoms of the

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aromatic rings [39-41]. After treatment with H2O2 this signal disappeared and a new signal

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centered at g = 2.008 corresponding to the nitrogen centered radicals appearing instead. Previously, such signal was attributed to a negatively charged oxygen atom in an amine oxide group [23]. The EPR spectrum of the Fe-p-C3N4 sample shows a relatively weak, broad signal at g  2.44 (Figure 4b). This signal is attributed to Fe3+ ions within a Fe3O4 phase [42], as previously confirmed by XRD. For the pure Fe3O4 a g-value of 2.47 was estimated (Fig S8d). In the sample treated with H2O2, the ESR signal was shifted to a lower g-value and the 14

intensity of the signal increased considerably. This behavior indicates the oxidation of Fe2+ to Fe3+ ions by reaction with H2O2 [43, 44]. Furthermore, a new low signal at g = 4.31 was detected which was previously attributed to isolated Fe3+ ions in an orthorhombic environment [44]. Moreover, the appearance of the signal at g = 2.008 suggested

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simultaneous oxidation of the p-C3N4 framework already observed in the p-C3N4-O sample.

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Figure 4 EPR spectra of a) p-C3N4 and p-C3N4-O and b) Fe-p-C3N4 and Fe-p-C3N4-O (10 mg sample, RT, modulation amplitude: 5 G, intensity of spectra presented in (a) was multiplied by a constant factor of 2.5).

3.3 Surface chemical state

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The surface chemical state of the materials was explored by XPS. The full survey spectra are presented in Figure S4. All samples contain the elements C, N, O. Samples synthesized with

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FeCl3 contain additionally an iron -but no chlorine- signal. The presence of oxygen in the spectra is attributed to the presence of air during the materials’ synthesis. High resolution C1s spectra of the four samples are shown in Figure 5a. The C1s signal can be fitted with five peaks at binding energies between 284.8 and 294.0 eV. The peak at 284.8 eV corresponds to C–C, C–H coordinated surface adventitious carbon. The weak peak around 286.2 was previously ascribed to C–O from adsorbed CO2 [30], sp3-coordinated carbon bonds from 15

defects in p-C3N4 [45], and C–OH bonds from hydroxyl groups [46]. The main component located at 288.3 eV was attributed to sp2 bonded carbon in a N-containing aromatic structure (N–C=N) [30, 45] and the peak near 289.5 eV was assigned to N–C–O [24, 47], C–O–C [22, 48, 49] (O atoms doped into the C3N4 matrix) or C=O [22, 46] bonds. Finally, the peak at 293.6 eV is characteristic of the π to π* transition [31]. When the samples were exposed to H2O2 the intensity of the sub-band located at 286.2 eV slightly decreased, thus indicating the

288.2

293.6

284.8

Intensity / a.u.

286.0

p-C3N4

290

288

286

284

282

531.8

534.4

Intensity / a.u.

415

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533.0

c)

p-C3N4-O

410

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Binding Energy / eV

d)

Fe-p-C3N4-O

405

p-C3N4

400

395

Binding Energy / eV

725.2

712.0

Fe-p-C3N4-O

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Fe-p-C3N4

p-C3N4-O

Intensity / a.u.

292

398.7

Fe-p-C3N4

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p-C3N4-O

294

401.0 399.9

Fe-p-C3N4-O

Fe-p-C3N4

296

404.7

b)

Fe-p-C3N4-O

Intensity / a.u.

289.5

a)

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removal of labile surface species in the catalyst.

Fe-p-C3N4

p-C3N4

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540 538 536 534 532 530 528 526 524

Binding Energy / eV

745 740 735 730 725 720 715 710 705

Binding Energy / eV

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Figure 5 XPS spectra of the as-prepared materials a) C1s, b) N1s, c) O1s, and d) Fe2p.

The N1s spectra of all four samples (Figure 5b) were fitted with four peaks. The peak

at around 398.7 eV was attributed to sp2 bonded N atoms (C–N=C) in the triazine ring [5052], the sub-band between 399.8 and 400.1 eV to tertiary N–(C)3 groups of the polymeric network [20, 46], and the peak with binding energy around 401.0 eV was assigned to N–H2, N–H and N-O groups [31, 46, 51, 52]. The peak at 404.7 eV probably results from  excitation, differential charging effects or terminal nitrate groups [50, 53]. The peak area of 16

N–(C)3 (399.9 eV) and N–Hx groups (401.0 eV) is higher compared to C–N=C groups (398.7 eV) for samples without iron (see also Figure S5 for details). In contrast, the peak at 404.7 eV has a higher peak area in the iron containing samples. A relatively high integral intensity of N–(C)3 and NH2 or NH peaks has already been reported for p-C3N4 synthesized from urea and explained with the formation of polymer segments containing only few heptazine units bearing high numbers of NH/NH2 groups at the terminal end [46]. Table 2. Near surface composition of the different catalysts obtained by XPS N at% 54.9 53.5 53.6 56.6

O at% 2.1 3.5 2.7 2.5

Fe at% 0.14 0.16

C/N

0.78 0.80 0.82 0.72

-p

p-C3N4 p-C3N4-O Fe-p-C3N4 Fe-p-C3N4-O

C at% 43.0 42.9 43.8 40.7

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Sample

The O1s spectra (Figure 5c) are fitted with three peaks located at around 531.8 eV,

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533.0 eV and 534.4 eV. These binding energies correspond to N–C–O, C–O–C and adsorbed

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O2, respectively [54]. XP spectra of the samples without iron show a higher peak area of adsorbed oxygen (534.4 eV) compared to the samples which do not contain iron (see Figure

such species.

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S5 for details), a fact that indicates that the presence of iron can suppress the formation of

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The Fe2p spectra of the Fe-p-C3N4 sample exhibited a Fe2p3/2 peak at 712.0 eV and a Fe2p1/2 peak at 725.2 eV (Figure 5d) which are slightly higher compared to the typical iron or

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iron oxide binding energies found in literature [55, 56]. Due to the faint signal (mainly originating from the low Fe loading) a thorough identification of the iron oxidation states was not possible. Anyhow, after treatment with H2O2 the binding energy of the Fe2p3/2 peak increased to 712.3 eV. Together with the disappearance of the diffraction peak of the Fe3O4 phase and the strong increase of EPR intensity of Fe+3 species after treatment with H2O2, this

17

shift might indicate an oxidation process to Fe3+ [54, 55] and/or an increased interaction between iron oxide and the p-C3N4 [57]. Valence band (VB) XP spectra of the four materials are presented in Figure 6. The treatment with H2O2 leads to a negative shift of about 0.4 eV of the VB edge position in comparison to the untreated samples, which may result from the incorporation of O atoms into the triazine units [58]. The O doping into these units may have led to a shift of the VB

Fe-p-C3N4-O

2.53

p-C3N4-O

2.42

p-C3N4

-6

-4

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2.12

2.14

-2

0

2

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Intensity / a.u.

Fe-p-C3N4

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position due to the higher O 2p orbital energy than that of N 2p [59].

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8

10

12

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Binding Energy / eV

Figure 6 Valence band XPs spectra of the different samples

3.4 Photocatalytic results

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The photocatalytic activity of these p-C3N4 based materials was investigated by methyl orange (MO, 15 mg/L) degradation under visible (λ ≥ 420 nm) and white light irradiation. The

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process was followed by UV-Vis spectroscopy, as shown in Figure 7 and Figure 8. Without a

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catalyst MO was stable both under visible and white light irradiation (see Figure S6). After 30 min stirring in the dark to achieve the adsorption and desorption equilibrium, only p-C3N4-O sample showed a clear decrease in UV/Vis absorbance indicating enhanced MO adsorption on this material. The higher MO adsorption ability of p-C3N4-O might be due to the presence of oxygen functional groups formed during the H2O2 treatment. Those functional groups are hydrophilic, suggesting that they could not only improve the overall hydrophilicity of the material, but also the interaction with hydrophilic organic molecules [23]. Under visible light 18

irradiation the slowest MO degradation was obtained for pristine p-C3N4 followed by the sample containing iron (Fe-p-C3N4). Degradation increased considerably when the samples were exposed to H2O2. Besides, a decreasing absorbance of both p-C3N4-O and Fe-p-C3N4-O showed a clear change of the shape of the UV/Vis spectrum with increasing irradiation time, indicating an enhanced transformation of MO to intermediate degradation products. In order to follow the transformation of the MO substrate, HPLC analysis was applied. Figure 7e shows the effect of irradiation time on the MO conversion. For all four catalysts studied, MO

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conversion was faster than the decrease in UV/Vis absorbance, and the materials exposed to H2O2 showed a significantly enhanced MO transformation compared to the untreated ones.

First order rate constants of MO conversion are given in the SM (Table S1). Rate constants of

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samples treated with H2O2 were more than six times higher than those of the untreated

materials. In cases, where the chemical composition of the solution is unknown, kinetic plots

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cannot be obtained from UV/Vis absorbance. However, UV/Vis can be used to follow

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decolorization of the solution [60]. Figure 7f shows a plot of decolorization efficiency versus time (for calculation of the decolorization efficiency see SM). The plots show that decolorization was enhanced when the samples were exposed to H2O2 but the degradation

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pathway seems to be different for both treated samples. When using p-C3N4-O, the rate of MO degradation strongly decreases already at 30 min of irradiation. The combination of iron oxide

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loading and H2O2 treatment gave the most active material in terms of MO decolorization

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under visible light irradiation. TOC values obtained after 3 hours of irradiation for all catalysts were similar to the value obtained before irradiation indicating that MO was not mineralized. It was also tested whether iron has leached into the liquid phase during irradiation with visible light. Therefore, the reaction mixture obtained after 180 min of irradiation was filtered to remove the particles (< 200 nm) and subsequently analyzed by AAS. According to AAS analysis no iron was detected in the liquid phase (detection limit: 0.34 mg/L). Furthermore, the effect of materials’ exposure time to H2O2 on their photoactivity 19

was investigated. Results presented in Figure S7 show that the decolorization efficiency increased with increasing H2O2 exposure time, however, the enhancement of MO degradation

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compared to the untreated samples already occurred at shorter H2O2 treatment times.

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Figure 7 The UV-Vis spectra of MO degradation process at irradiation with visible light: (a) p-C3N4, (b) pC3N4 – O, (c) Fe-p-C3N4, (d) Fe-p-C3N4 -O, (e) plot of MO conversion versus time and (f) plot of decolorization efficiency versus time (  420 nm, mcat = 30 mg, V = 30 mL, c(MO) = 4.6·10 -5 mol/L).

Differences in temporal evolution of UV/Vis absorbance were also observed in case of white

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light (Figure 8). In general, decolorization with white light was much faster compared to visible light probably because of the higher amount of charge carrier generated under

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irradiation with white light. Surprisingly, decolorization with pristine p-C3N4 seems to be faster than with Fe-p-C3N4. A significant difference in temporal evolution of UV/Vis absorbance between white and visible light was also observed for p-C3N4-O. Under irradiation with white light, MO degradation continuously proceeded with increasing irradiation time contrary to the one with visible light where MO degradation decreased with the increase of irradiation time (compare Figure 7c and 8c). The rate of MO conversion was again 20

significantly higher for the samples post-treated with H2O2 (Table S1) as already observed in the case of visible light (Figure 7e) irradiation. The fastest decolorization using white light was observed again for the Fe-p-C3N4-O catalyst. Interestingly, after 180 min irradiation with white light, the UV/Vis absorbance of the solutions containing p-C3N4, p-C3N4-O, or Fe-pC3N4-O was similar, leading to identical decolorization levels (Figure 8f). When using a nanocrystalline Fe3O4 phase instead of p-C3N4 materials as photocatalyst MO degradation was very low (Figure S8e and f). TOC content after 3 hours was 38 % (Fe-p-C3N4-O), 8 % (p-

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C3N4), and 4 % (p-C3N4-O) lower than the TOC content at the beginning which indicates the

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mineralization of MO under white light irradiation.

Figure 8 UV-Vis spectra of MO degradation process at irradiation with white light: (a) p-C3N4, (b) p- C3N4 –O, (c) Fe-p-C3N4, (d) Fe-p-C3N4 -O, (e) plot of MO conversion versus time, and (f) plot of decolorization efficiency versus time (mcat = 30 mg, V = 30 mL, c(MO) = 4.6·10 -5 mol/L).

3.5 Mechanistic considerations The Mott-Schottky technique was used to determine the flat band potential (VFB), which can be regarded as the Fermi level (EF) [61]. The corresponding plots are shown in Figure 9a-d. All curves have a positive slope, which indicates that all four catalysts are n-type 21

semiconductors. The VFB values were derived from the X-axis intercept in the Mott-Schottky plots (mean value of the three measurements) and were -1.38, -1.30, -1.18, and -1.30 V vs Ag/AgCl for p-C3N4, p-C3N4-O, Fe-p-C3N4, and Fe-p-C3N4-O, respectively. The potentials vs RHE are -0.75 (p-C3N4), -0.67 (p-C3N4-O), -0.55 (Fe-p-C3N4), and -0.67 V (Fe-p-C3N4-O) (the Ag/AgCl potential vs NHE was assumed as 0.21 V [62]). The relative potential of the valance band (VB) edge vs the EF can be estimated using data obtained from the VB XPS spectra [35]. Finally, the relative position of the conduction band (CB) edge can be obtained

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using the Eg which was 2.8 eV. Figure 9e shows a schematic of the band alignment for the four catalysts. Treatment with H2O2 caused a shift of the VB and CB band edge position to a more positive potential compared to pristine p-C3N4. TV43

8,0E+09

C-2 / F-2

C-2 / F-2

6,0E+09 4,0E+09 2,0E+09 -1.38 V 0,0E+00 -1,5

C-2 / F-2

6,0E+09 4,0E+09

4,0E+09

1,0

0,0E+00 -1,5

c)

8,0E+09

1000 Hz 1500 Hz 2000 Hz

Fe-p-C3N4

6,0E+09

-1,0 -0,5 0,0 0,5 1,0 potential (vs. Ag/AgCl) / V

4,0E+09 2,0E+09 -1.18 V 0,0E+00 -1,5

-1,0 -0,5 0,0 0,5 1,0 potential (vs. Ag/AgCl) / V

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8,0E+09

1000 Hz 1500 Hz 2000 Hz

6,0E+09

p-C3N4-O

2,0E+09 -1.30 V

-1,0 -0,5 0,0 0,5 potential (Ag/AgCl) / V

d)

1,0E+10

1000 Hz 1500 Hz 2000 Hz

Fe-p-C3N4-O

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1,0E+10

b)

-p

p-C3N4

C-2 / F-2

8,0E+09

1,0E+10 1000 Hz 1500 Hz 2000 Hz

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a)

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1,0E+10

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2,0E+09 -1.30 V 0,0E+00 -1,5

-1,0 -0,5 0,0 0,5 1,0 potential (vs. Ag/AgCl) / V

Figure 9 Mott-Schottky plots of p-C3N4 a), p-C3N4-O b), Fe-p-C3N4 c), Fe-p-C3N4-O d), and their band alignment e).

The degradation of MO might involve h+, ·O2- or ·OH radicals as active species. To gain insight into the role of these species in MO degradation on Fe-p-C3N4 under visible light 22

irradiation, experiments were performed using ammonium oxalate, silver nitrate, and pbenzoquinone as quencher for holes (h+), electrons (e-), and superoxide radicals (·O2-), respectively [63]. Figure 10a shows the effect of different scavengers on the decolorization efficiency (the corresponding UV/Vis spectra are shown in Figures S9-S11). In the presence of ammonium oxalate the decolorization efficiency was similar to the experiments without scavenger indicating that a direct transformation of h+ to MO is not the main path for MO degradation. However, when the solution contained silver ions the rate of MO degradation

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increased. Under visible light irradiation the added silver ions might be reduced and

photodeposited on the carbon nitride surface where they form silver centers which can act as a trap for conduction band electrons eCB-, leading to an improved separation of charge carriers

-p

and their migration at the interphase [64]. On the other hand, MO degradation decreased significantly when the solution contained benzoquinone (BQ), used as ·O2- scavenger.

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Moreover, a control test in the absence of oxygen shows a very low MO degradation rate (see

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Figure S12). Thus, it can be concluded that super oxide radicals are involved in the MO

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degradation pathway on Fe-p-C3N4-O under visible light irradiation.

Figure 10a) Plot of decolorization efficiency versus irradiation time at presence of a scavenger using Fe-pC3N4-O; (  420 nm, mcat = 30 mg, c(MO) = 2.3·10-5 mol/L, c((NH4)2C2O4) = 4.4·10-5 mol/L, c(AgNO3) = 6.0·10-5 mol/L, c(benzoquinone) = 6.9·10-5 mol/L, without scavenger: c(MO) = 4.6·10-5 mol/L) and b) ·OH trapping PL spectra of Fe-p-C3N4-O in terephthalic acid solution at irradiation with visible ((  420 nm) or white light (mcat = 15 mg, V = 30 mL, c(TPA) = 5·10 -4 mol/L, c(NaOH) = 2·10-5 mol/L).

23

Finally, it was investigated whether ·OH radicals were generated during the photocatalytic reaction using terephthalic acid (TPA) as a probe molecule [16, 57]. TPA can readily react with ·OH radicals to produce the highly fluorescent product 2hydroxyterephthalic acid. Figure 10b shows fluorescent spectra of the TPA containing solution after irradiation with visible or white light. The low fluorescence intensity of spectra detected from the solution irradiated with visible light indicates that ·OH radicals were formed only in traces. Otherwise, at irradiation with white light, fluorescence spectra of 2-

irradiation time indicating the formation of ·OH radicals.

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hydroxyterephthalic acid can be detected and the fluorescence intensity increases with

Based on the presented results a possible mechanism for the photocatalytic

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degradation of MO on Fe-p-C3N4-O is proposed (Figure 11). Under visible light irradiation,

photo-generated electrons and holes are formed. At the presence of iron oxide nanostructures

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on the surface of p-C3N4, the photo-generated electrons will rapidly move to iron oxide

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nanostructures due to the intimate contact between the iron oxide and the p-C3N4 and the high number of defects on the iron oxide phase [19] which greatly benefits the separation of charge

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carriers and promotes the interfacial electron transfer. Moreover, charge carrier separation will be further promoted by the defects in the p-C3N4 structure. Because of the low iron

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content, charge carriers which are generated on FeOx were not be considered. The trapped electrons can react with solved oxygen molecules to superoxide radicals which are assumed to

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be acting as the main reactive species under visible light irradiation.

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Figure 11 Possible photocatalytic mechanism for MO degradation on Fe-p-C3N4-O under visible light irradiation.

4. Conclusions

The presence of FeCl3 at the synthesis of p-C3N4 by pyrolysis of urea in air has only a

-p

marginal effect on p-C3N4 structure, morphology, and band gap. However, it leads to the

formation of a crystalline Fe3O4 phase. The decoration of the p-C3N4 with Fe3O4 particles

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increases the BET surface area and affects the optical and electronic properties of the p-C3N4.

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Post-treatment of pristine or Fe3O4 decorated p-C3N4 with H2O2 affects the p-C3N4 bulk structure and morphology only slightly but leads to the formation of oxygen functional groups and defects in the p-C3N4 framework. Furthermore, H2O2 treatment strongly affects the size,

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oxidation state and composition of the FeOx species. Moreover, the interaction between iron oxide species and p-C3N4 seems to be enhanced during H2O2 treatment.

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The presence of Fe3O4 leads to an increase in photoactivity of methyl orange

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degradation under visible light compared to pristine p-C3N4. This might be attributed to an improved charge carrier separation and an inhibited rate of charge carrier recombination. These capabilities, the shift in band edge positions as well as a synergistic effect of iron oxide species and oxygen functional groups or defects on the p-C3N4 are assumed to be responsible for the superior photocatalytic performance of the catalyst obtained after treatment of the Fep-C3N4 composite with H2O2 in MO degradation under visible and white light compared to

25

the other materials studied. Moreover, the superoxide radical ·O2- seems to be the main active species in MO degradation on this catalyst under visible light irradiation. Acknowledgments This work has been supported by the RoHan Project funded by the German Academic Exchange Service (DAAD, No. 57315854) and the Federal Ministry for Economic Cooperation and Development (BMZ) inside the framework "SDG Bilateral Graduate school

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programme". Dr. Armin Springer (Arbeitsbereich Medizinische Biologie und Elektronenmikroskopisches Zentrum (EMZ), Universitätsmedizin Rostock) is gratefully acknowledged for SEM and Reinhard Eckelt for BET measurements. Igor Medic is acknowledged for final

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reading of the manuscript and providing language help. Dr. T. Peppel and Dr. N. Moustakas

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thanks the BMBF (CO2Plus, Project-No. 033RC003, PROPHECY) for financial support.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal

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CRedit

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relationships that could have appeared to influence the work reported in this paper.

V.V.T. performed photocatalytic experiments and synthesized the materials. S.B., T. P., H.L., C.K., J.R., N.G.M., A.-E.S., T.H.D. performed characterization experiments. N.S. conceived and directed this project.

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All authors analyzed the data and were involved in writing the manuscript.

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All authors discussed the results and revised the manuscript.

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References

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[1] G. Odling, N. Robertson, Bridging the gap between laboratory and application in photocatalytic water purification, Catal. Sci. Technol., 9 (2019) 533-545. [2] U.G. Akpan, B.H. Hameed, Parameters affecting the photocatalytic degradation of dyes using TiO2-based photocatalysts: a review, J. Hazard. Mater., 170 (2009) 520-529. [3] J.-S. Wu, C.-H. Liu, K.H. Chu, S.-Y. Suen, Removal of cationic dye methyl violet 2B from water by cation exchange membranes, J. Membr. Sci., 309 (2008) 239-245. [4] G. Mamba, A. Mishra, Graphitic carbon nitride (g-C3N4) nanocomposites: a new and exciting generation of visible light driven photocatalysts for environmental pollution remediation, Appl. Catal. B: Environ., 198 (2016) 347-377. [5] J. Wen, J. Xie, X. Chen, X. Li, A review on g-C3N4-based photocatalysts, Appl. Surf. Sci., 391 (2017) 72-123. [6] D. Masih, Y. Ma, S. Rohani, Graphitic C3N4 based noble-metal-free photocatalyst systems: A review, Appl. Catal. B: Environ., 206 (2017) 556-588. [7] G. Li, N. Yang, W. Wang, W. Zhang, Synthesis, photophysical and photocatalytic properties of Ndoped sodium niobate sensitized by carbon nitride, J. Phys. Chem. C, 113 (2009) 14829-14833. [8] Y. Wang, Y. Wang, Y. Li, H. Shi, Y. Xu, H. Qin, X. Li, Y. Zuo, S. Kang, L. Cui, Simple synthesis of Zr-doped graphitic carbon nitride towards enhanced photocatalytic performance under simulated solar light irradiation, Catal. Commun., 72 (2015) 24-28. [9] Z. Li, J. Wang, K. Zhu, F. Ma, A. Meng, Ag/g-C3N4 composite nanosheets: Synthesis and enhanced visible photocatalytic activities, Mater. Lett., 145 (2015) 167-170. [10] Q. Wang, A. Chen, X. Wang, J. Zhang, J. Yang, X.a. Li, Fe-species-loaded graphitic carbon nitride with enhanced photocatalytic performance under visible-light irradiation, J. Mol. Catal. A, 420 (2016) 159-166. [11] K.C. Christoforidis, T. Montini, E. Bontempi, S. Zafeiratos, J.J.D. Jaén, P. Fornasiero, Synthesis and photocatalytic application of visible-light active β-Fe2O3/g-C3N4 hybrid nanocomposites, Appl. Catal. B: Environ., 187 (2016) 171-180. [12] X. Liu, J. Zhong, J. Li, W. Hu, Q. Wang, G. Wang, J. Song, K. Liu, K. Huang, S. Wu, Enhanced Simulated Sun Light Photocatalytic Activity of α-Fe2O3 Modified g-C3N4 prepared In-situ, J. Adv. Oxid. Techn., 19 (2016) 27-33. [13] Y. Xu, S. Huang, M. Xie, Y. Li, H. Xu, L. Huang, Q. Zhang, H. Li, Magnetically separable Fe2O3/g-C3N4 catalyst with enhanced photocatalytic activity, RSC Adv., 5 (2015) 95727-95735. [14] S. Kumar, T. Surendar, B. Kumar, A. Baruah, V. Shanker, Synthesis of magnetically separable and recyclable g-C3N4–Fe3O4 hybrid nanocomposites with enhanced photocatalytic performance under visible-light irradiation, J. Phys. Chem. C, 117 (2013) 26135-26143. [15] S. Hu, R. Jin, G. Lu, D. Liu, J. Gui, The properties and photocatalytic performance comparison of Fe3+-doped g-C3N4 and Fe2O3/g-C3N4 composite catalysts, RSC Adv., 4 (2014) 24863-24869. [16] S. Tonda, S. Kumar, S. Kandula, V. Shanker, Fe-doped and-mediated graphitic carbon nitride nanosheets for enhanced photocatalytic performance under natural sunlight, J. Mater. Chem. A, 2 (2014) 6772-6780. [17] J. Theerthagiri, R. Senthil, A. Priya, J. Madhavan, R. Michael, M. Ashokkumar, Photocatalytic and photoelectrochemical studies of visible-light active α-Fe2O3–g-C3N4 nanocomposites, RSC Adv., 4 (2014) 38222-38229. [18] Z. Li, C. Kong, G. Lu, Visible photocatalytic water splitting and photocatalytic two-electron oxygen formation over Cu-and Fe-doped g-C3N4, J. Phys. Chem. C, 120 (2015) 56-63. [19] R. Cheng, L. Zhang, X. Fan, M. Wang, M. Li, J. Shi, One-step construction of FeOx modified gC3N4 for largely enhanced visible-light photocatalytic hydrogen evolution, Carbon, 101 (2016) 62-70. [20] H. Guo, M. Chen, Q. Zhong, Y. Wang, W. Ma, J. Ding, Synthesis of Z-scheme α-Fe2O3/g-C3N4 composite with enhanced visible-light photocatalytic reduction of CO2 to CH3OH, J. CO2 Utiliz., 33 (2019) 233-241.

28

Jo

ur

na

lP

re

-p

ro of

[21] D. Xiao, K. Dai, Y. Qu, Y. Yin, H. Chen, Hydrothermal synthesis of α-Fe2O3/g-C3N4 composite and its efficient photocatalytic reduction of Cr(VI) under visible light, Appl. Surf. Sci., 358 (2015) 181-187. [22] S. Liu, D. Li, H. Sun, H.M. Ang, M.O. Tadé, S. Wang, Oxygen functional groups in graphitic carbon nitride for enhanced photocatalysis, J. Colloid Interface Sci., 468 (2016) 176-182. [23] G. Dong, Z. Ai, L. Zhang, Efficient anoxic pollutant removal with oxygen functionalized graphitic carbon nitride under visible light, RSC Adv., 4 (2014) 5553-5560. [24] J. Li, B. Shen, Z. Hong, B. Lin, B. Gao, Y. Chen, A facile approach to synthesize novel oxygendoped g-C3N4 with superior visible-light photoreactivity, Chem. Commun., 48 (2012) 12017-12019. [25] M. Ilkaeva, I. Krivtsov, E. Bartashevich, S. Khainakov, J.R. García, E. Díaz, S. Ordóñez, Carbon nitride assisted chemoselective C–H bond photo-oxidation of alkylphenolethoxylates in water medium, Green Chem., 19 (2017) 4299-4304. [26] R. López, R. Gómez, Band-gap energy estimation from diffuse reflectance measurements on sol– gel and commercial TiO2: A comparative study, J. Sol-Gel Sci. Technol., 61 (2012) 1-7. [27] J.H. Scofield, Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV, Journal of Electron Spectroscopy and Related Phenomena, 8 (1976) 129-137. [28] F.K. Kessler, Y. Zheng, D. Schwarz, C. Merschjann, W. Schnick, X. Wang, M.J. Bojdys, Functional carbon nitride materials—design strategies for electrochemical devices, Nature Reviews Materials, 2 (2017) 17030. [29] R. Kumar, M.A. Barakat, F.A. Alseroury, Oxidized g-C3N4/polyaniline nanofiber composite for the selective removal of hexavalent chromium, Scientific reports, 7 (2017) 12850. [30] B. Zhu, P. Xia, W. Ho, J. Yu, Isoelectric point and adsorption activity of porous g-C3N4, Appl. Surf. Sci., 344 (2015) 188-195. [31] F.R. Pomilla, M.A. Cortes, J.W. Hamilton, R. Molinari, G. Barbieri, G. Marci, L. Palmisano, P.K. Sharma, A. Brown, J.A. Byrne, An Investigation into the Stability of Graphitic C3N4 as a Photocatalyst for CO2 Reduction, J. Phys. Chem. C, 122 (2018) 28727-28738. [32] W.-D. Oh, V.W. Chang, Z.-T. Hu, R. Goei, T.-T. Lim, Enhancing the catalytic activity of g-C3N4 through Me doping (Me= Cu, Co and Fe) for selective sulfathiazole degradation via redox-based advanced oxidation process, Chem. Eng. J., 323 (2017) 260-269. [33] J. Liu, T. Zhang, Z. Wang, G. Dawson, W. Chen, Simple pyrolysis of urea into graphitic carbon nitride with recyclable adsorption and photocatalytic activity, J. Mater. Chem., 21 (2011) 1439814401. [34] H.-B. Fang, Y. Luo, Y.-Z. Zheng, W. Ma, X. Tao, Facile large-scale synthesis of urea-derived porous graphitic carbon nitride with extraordinary visible-light spectrum photodegradation, Ind. Eng. Chem. Res., 55 (2016) 4506-4514. [35] C. Yao, A. Yuan, Z. Wang, H. Lei, L. Zhang, L. Guo, X. Dong, Amphiphilic two-dimensional graphitic carbon nitride nanosheets for visible-light-driven phase-boundary photocatalysis, J. Mater. Chem. A, 7 (2019) 13071-13079. [36] R. Shen, J. Xie, H. Zhang, A. Zhang, X. Chen, X. Li, Enhanced solar fuel H2 generation over gC3N4 nanosheet photocatalysts by the synergetic effect of noble metal-free Co2P cocatalyst and the environmental phosphorylation strategy, ACS Sustainable Chem. Eng., 6 (2017) 816-826. [37] S. Chidambaram, B. Pari, N. Kasi, S. Muthusamy, ZnO/Ag heterostructures embedded in Fe3O4 nanoparticles for magnetically recoverable photocatalysis, J. Alloys Compd., 665 (2016) 404-410. [38] T. Wang, S. Zhou, C. Zhang, J. Lian, Y. Liang, W. Yuan, Facile synthesis of hematite nanoparticles and nanocubes and their shape-dependent optical properties, New J. Chem., 38 (2014) 46-49. [39] Q. Liu, T. Chen, Y. Guo, Z. Zhang, X. Fang, Grafting Fe(III) species on carbon nanodots/Fedoped g-C3N4 via interfacial charge transfer effect for highly improved photocatalytic performance, Appl. Catal. B: Environ., 205 (2017) 173-181. [40] G. Dong, D.L. Jacobs, L. Zang, C. Wang, Carbon vacancy regulated photoreduction of NO to N2 over ultrathin g-C3N4 nanosheets, Appl. Catal. B: Environ., 218 (2017) 515-524. [41] Y. Jiang, Z. Sun, C. Tang, Y. Zhou, L. Zeng, L. Huang, Enhancement of photocatalytic hydrogen evolution activity of porous oxygen doped g-C3N4 with nitrogen defects induced by changing electron transition, Appl. Catal. B: Environ., 240 (2019) 30-38.

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-p

ro of

[42] S. Pal, P. Dutta, N. Shah, G. Huffman, M. Seehra, Surface Spin Disorder in Fe3O4 Nanoparticles Probed by Electron Magnetic Resonance Spectroscopy and Magnetometry, IEEE transactions on magnetics, 43 (2007) 3091-3093. [43] Y. Georgiou, E. Mouzourakis, A. Bourlinos, R. Zboril, M. Karakassides, A. Douvalis, T. Bakas, Y. Deligiannakis, Surface decoration of amine-rich carbon nitride with iron nanoparticles for arsenite (AsIII) uptake: The evolution of the Fe-phases under ambient conditions, J. Hazard. Mater., 312 (2016) 243-253. [44] S. Neubert, D. Mitoraj, S.A. Shevlin, P. Pulisova, M. Heimann, Y. Du, G.K. Goh, M. Pacia, K. Kruczała, S. Turner, Highly efficient rutile TiO2 photocatalysts with single Cu(II) and Fe(III) surface catalytic sites, J. Mater. Chem. A, 4 (2016) 3127-3138. [45] F. Chang, C. Li, J. Luo, Y. Xie, B. Deng, X. Hu, Enhanced visible-light-driven photocatalytic performance of porous graphitic carbon nitride, Appl. Surf. Sci., 358 (2015) 270-277. [46] Y. Zheng, Z. Zhang, C. Li, A comparison of graphitic carbon nitrides synthesized from different precursors through pyrolysis, J. Photochem. Photobio. A: Chem, 332 (2017) 32-44. [47] J. Fu, B. Zhu, C. Jiang, B. Cheng, W. You, J. Yu, Hierarchical porous O‐ doped g‐ C3N4 with enhanced photocatalytic CO2 reduction activity, Small, 13 (2017) 1603938. [48] Z.-F. Huang, J. Song, L. Pan, Z. Wang, X. Zhang, J.-J. Zou, W. Mi, X. Zhang, L. Wang, Carbon nitride with simultaneous porous network and O-doping for efficient solar-energy-driven hydrogen evolution, Nano Energy, 12 (2015) 646-656. [49] J.-W. Zhang, S. Gong, N. Mahmood, L. Pan, X. Zhang, J.-J. Zou, Oxygen-doped nanoporous carbon nitride via water-based homogeneous supramolecular assembly for photocatalytic hydrogen evolution, Appl. Catal. B: Environ., 221 (2018) 9-16. [50] D.J. Martin, K. Qiu, S.A. Shevlin, A.D. Handoko, X. Chen, Z. Guo, J. Tang, Highly efficient photocatalytic H2 evolution from water using visible light and structure‐ controlled graphitic carbon nitride, Angew. Chem. Int. Ed., 53 (2014) 9240-9245. [51] H. Bicalho, J. Lopez, I. Binatti, P. Batista, J. Ardisson, R. Resende, E. Lorençon, Facile synthesis of highly dispersed Fe(II)-doped g-C3N4 and its application in Fenton-like catalysis, Mol: Cat., 435 (2017) 156-165. [52] V.W.-h. Lau, M.B. Mesch, V. Duppel, V. Blum, J.r. Senker, B.V. Lotsch, Low-molecular-weight carbon nitrides for solar hydrogen evolution, J. Am. Chem. Soc., 137 (2015) 1064-1072. [53] A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.-O. Müller, R. Schlögl, J.M. Carlsson, Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts, J. Mater. Chem., 18 (2008) 4893-4908. [54] F. Wei, Y. Liu, H. Zhao, X. Ren, J. Liu, T. Hasan, L. Chen, Y. Li, B.-L. Su, Oxygen self-doped gC3N4 with tunable electronic band structure for unprecedentedly enhanced photocatalytic performance, Nanoscale, 10 (2018) 4515-4522. [55] S. Poulin, R. Franca, L. Moreau-Bélanger, E. Sacher, Confirmation of X-ray photoelectron spectroscopy peak attributions of nanoparticulate iron oxides, using symmetric peak component line shapes, J. Phys. Chem. C, 114 (2010) 10711-10718. [56] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials, Appl. Surf. Sci., 254 (2008) 2441-2449. [57] B. Liu, L. Tian, R. Wang, J. Yang, R. Guan, X. Chen, Pyrrolic-N-doped graphene oxide/Fe2O3 mesocrystal nanocomposite: efficient charge transfer and enhanced photo-Fenton catalytic activity, Appl. Surf. Sci., 422 (2017) 607-615. [58] P. Qiu, C. Xu, H. Chen, F. Jiang, X. Wang, R. Lu, X. Zhang, One step synthesis of oxygen doped porous graphitic carbon nitride with remarkable improvement of photo-oxidation activity: Role of oxygen on visible light photocatalytic activity, Appl. Catal. B: Environ., 206 (2017) 319-327. [59] Y. Zeng, X. Liu, C. Liu, L. Wang, Y. Xia, S. Zhang, S. Luo, Y. Pei, Scalable one-step production of porous oxygen-doped g-C3N4 nanorods with effective electron separation for excellent visible-light photocatalytic activity, Appl. Catal. B: Environ., 224 (2018) 1-9. [60] S. Chong, G. Zhang, H. Tian, H. Zhao, Rapid degradation of dyes in water by magnetic Fe0/Fe3O4/graphene composites, J. Environ. Sci., 44 (2016) 148-157. [61] H. Zhang, X. Han, H. Yu, Y. Zou, X. Dong, Enhanced photocatalytic performance of boron and phosphorous co-doped graphitic carbon nitride nanosheets for removal of organic pollutants, Separation and Purification Technology, 226 (2019) 128-137.

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Jo

ur

na

lP

re

-p

ro of

[62] T. Giannakopoulou, I. Papailias, N. Todorova, N. Boukos, Y. Liu, J. Yu, C. Trapalis, Tailoring the energy band gap and edges’ potentials of g-C3N4/TiO2 composite photocatalysts for NOx removal, Chem. Eng. J., 310 (2017) 571-580. [63] P. Parnicka, T. Grzyb, A. Mikolajczyk, K. Wang, E. Kowalska, N. Steinfeldt, M. Klein, P. Mazierski, A. Zaleska-Medynska, J. Nadolna, Experimental and theoretical investigations of the influence of carbon on a Ho3+-TiO2 photocatalyst with Vis response, J. Colloid Interface Sci., 549 (2019) 212-224. [64] Y. Yang, Y. Guo, F. Liu, X. Yuan, Y. Guo, S. Zhang, W. Guo, M. Huo, Preparation and enhanced visible-light photocatalytic activity of silver deposited graphitic carbon nitride plasmonic photocatalyst, Appl. Catal. B: Environ., 142 (2013) 828-837.

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