Enhanced simulated sunlight photocatalytic reduction of an aqueous hexavalent chromium over hydroxyl-modified graphitic carbon nitride

Journal Pre-proofs Full Length Article Enhanced simulated sunlight photocatalytic reduction of an aqueous hexavalent chromium over hydroxyl-modified graphitic carbon nitride Xinyue Wang, Lingyu Li, Jiaqi Meng, Peiyu Xia, Yuxin Yang, Yihang Guo PII: DOI: Reference:

S0169-4332(19)32997-6 https://doi.org/10.1016/j.apsusc.2019.144181 APSUSC 144181

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

30 May 2019 7 September 2019 24 September 2019

Please cite this article as: X. Wang, L. Li, J. Meng, P. Xia, Y. Yang, Y. Guo, Enhanced simulated sunlight photocatalytic reduction of an aqueous hexavalent chromium over hydroxyl-modified graphitic carbon nitride, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144181

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Enhanced simulated sunlight photocatalytic reduction of an aqueous hexavalent chromium over hydroxyl-modified graphitic carbon nitride Xinyue Wang, Lingyu Li, Jiaqi Meng, Peiyu Xia, Yuxin Yang*, Yihang Guo* School of Environment, Northeast Normal University, Changchun 130117, P.R. China

*Corresponding authors. Tel./fax:

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Abstract

Hydroxyl-modified graphitic carbon nitride (HUCN) with interconnected open-framework is fabricated by hydrolysis of urea-derived graphitic carbon nitride (UCN) in an aqueous NaOH solution followed by a self-assembly in dialysis process. Compared with bulk graphitic carbon nitride (UCN), HUCN possesses more advantages such as plentiful exposed active sites, fast photogenerated charge carrier separation rate and more negative conduction band edge potential. These advantages bestow the HUCN remarkably higher simulated sunlight photocatalytic reduction ability towards an aqueous Cr(VI) as compared with UCN. By the combination of the experimental results including photoelectrochemistry and electron scavenger with the DFT calculated electrostatic potential (ESP) distribution, a possible reaction mechanism for the simulated sunlight photocatalytic reduction of highly toxic and carcinogenic Cr(VI) to non-toxic Cr(III) is put forward tentatively.

Keyword: Graphitic carbon nitride; Chemical self-modification; Simulated sunlight; Photocatalytic reduction; Hexavalent chromium

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1. Introduction

Heavy metal contamination is one of the most noteworthy environmental problems in this century. Hexavalent chromium (Cr(VI)), one of hazardous heavy metals, widely exists in various wastewaters from different industrial production such as electroplating, tanning, dyes and pigments, film and photography [1,2]. It is considered to be of urgency to develop efficient strategy for the removal of Cr(VI) because of its unignorable pollution to water resources and serious harm to human health [3,4]. Adsorption [5], ion exchange [6], chemical precipitation [7] and electrolysis [8] have been applied to remove Cr(VI) from water. Compared with these conventional methods, photocatalytic reduction of highly toxic and carcinogenic Cr(VI) to nontoxic Cr(III) is recognized as one of the advanced technologies because of the significant advantages such as high efficiency, simple handling and environmentally-friendly [9-11], and the key of photocatalytic reduction of Cr(VI) to Cr(III) is to develop abundant photocatalysts with strong photoreducibility. Graphitic carbon nitride (g-C3N4), a metal-free polymeric semiconductor with a graphitelike structure, has gained tremendous attention owing to its unique properties including nontoxicity, low-cost, desirable band gap (2.7 eV) and facile synthetic methods [12,13]. In particular, the relatively more negative conduction-band edge potential endows g-C3N4 with strong reduction activity, and it is strongly advocated as a promising photocatalyst for the reduction of Cr(VI) [10,11]. Unfortunately, the efficiency of the photocatalytic reduction of aqueous Cr(VI) over g-C3N4 is low because the pristine g-C3N4 photocatalyst suffers from its intrinsic drawbacks including: i) fast recombination of the photogenerated charge carriers 3

reduces the number of electrons involved in Cr(VI) reduction reaction; and ii) bulk layered structure of g-C3N4 severely hinders the adequate contact of Cr(VI) with the photogenerated electrons migrating to the photocatalyst surface [14-16]. To inhibit the recombination of photogenerated charge carriers of g-C3N4, the fabrication of g-C3N4-based heterojunctions via noble metal deposition (e.g. Ag/g-C3N4 [17]) and coupling with other semiconductors (e.g. gC3N4-TiO2 [18], g-C3N4-Cu2WS4 [19], g-C3N4-MoS2 [20], g-C3N4-Bi4O7 [9], g-C3N4-Co9S8 [21] and g-C3N4-MoO3 [22]) offers a promising approach to efficiently migrate and separate the photogenerated charge carriers, thus optimizing the photocatalytic reduction activity towards Cr(VI). However, these g-C3N4-based heterojunctions are usually metal-containing, which may raise the potential toxic metal leaching and thereby associated secondary contamination [23]. Hole scavenger, such as formic acid [24,25] and citric acid [26,27], is also used to promote the separation of photogenerated charge carriers by trapping the photogenerated holes and leaving a large number of electrons involved in Cr(VI) reduction reaction. However, these hole scavengers are dissolved in the reaction media, and no substantial improvements have been made to photocatalyst itself. Therefore, developing a metal-free g-C3N4-based photocatalyst with hole scavenger as a part of itself will be benefit for photocatalytic reduction of Cr(VI) to Cr(III) efficiently. In the other strategy, g-C3N4 with various morphologies, such as quantum dots [28-31], nanosheets [32-34], hollow sphere [35] and hydrogel network [36], have been designed to improve adequate contact between the reactants and photogenerated charge carriers transferred to the photocatalyst surface. Among them, carbon nitride with interconnected openframework is one of the most outstanding candidates, thanks to more exposed reactive sites providing convenient mass transfer channels and then promote the surface chemical reaction. 4

The fabrication of this material begins with the exfoliation/tailoring of bulk g-C3N4 into nanosheets or quantum dots followed by a self-assembly process [37]. However, this process usually comes with significant energy consumption. Therefore, a simple and energy saving method is needed to prepare carbon nitride with interconnected open-framework. The present work intend to adopt a simple and moderate method to achieve the optimization of the photogenerated charge carrier separation, photogenerated electrons reducibility, and the contact between reactants and photogenerated electrons simultaneously. In this case, a metalfree hydroxyl-modified carbon nitride (HUCN) with interconnected open-framework is fabricated via moderate hydrolyzing of urea-derived bulk g-C3N4 (UCN) under alkaline condition followed by a self-assembly in dialysis process. During the hydrolysis process, hydroxyl groups are successfully introduced into the edges of HUCN, and they act as the hole scavenger to promote the fast photogenerated charge carrier separation [38-40]. Meanwhile, the conduction band (CB) edge potential of HUCNs become more negative, which endows the photogenerated electron with stronger reduction ability. The HUCN is therefore anticipated to possess greatly enhanced photocatalytic activity towards the reduction of Cr(VI) to Cr(III) in an aqueous solution with respect to UCN. To confirm the excellent photocatalytic reduction activity of the HUCNs, photoreduction of an aqueous p-nitroaniline (4-NA) over the HUCNs is also tested. By the combination of the testing results of photoelectrochemistry and indirect chemical probe with the DFT calculated ESP distribution, the photocatalytic reduction mechanism of Cr(VI) over the HUCNs is tentatively put forward.

2. Experimental 5

2.1. Preparation of hydroxyl-modified g-C3N4 (HUCN)

At first, bulk g-C3N4 (UCN) powder was prepared by a one-step thermal condensation of urea in a muffle furnace under the temperature programming (ramp rate of 2 oC min1) of 250 oC

for 1 h, 350 oC for 2 h and a final temperature 550 oC for 2 h. The obtained light-yellow

powder was washed successively by HNO3 (0.1 mol L1) and water to neutral, and then the powder was centrifuged followed by being dried at 80 oC for 12 h. Subsequently, HUCN was prepared by hydrolyzing UCN in aqueous NaOH solution. Typically, UCN powder (500 mg) was dispersed in an aqueous NaOH solution (20 mL) with the concentration of 0.5, 3.0 and 8.0 mol L1, respectively. The resulting mixture was treated by ultrasonic for 2 h followed by being stirred at 60 oC overnight. The HUCN thus obtained was purified to remove excess NaOH by dialysis in a membrance of molecular weight cutoff of 3500 Da against water until neutral. Finally, the HUCN was suffered from freeze-drying under vacuum for 48 h, and the dried HUCNs were denoted as c-HUCN with c representing initial concentration of NaOH (mol L1) used for modification of UCN.

2.2. Characterization of hydroxyl-modified g-C3N4

Transmission electron microscope (TEM) images were recorded on a JEM-2100F high resolution transmission electron microscope at an accelerating voltage of 200 kV. Atomic force microscopy (AFM) measurements were carried out on an Asylum Research Cypher. Scanning electron microscope (SEM) observation was performed on a XL-30 ESEMFEG field emission scanning electron microscope. Nitrogen gas porosimetry measurement was obtained using a 6

Micromeritics ASAP 2020 PLUS HD88 surface area and porosity analyzer, and the surface areas were calculated using the Brunauer–Emmett–Teller (BET) equation. X-ray diffraction (XRD) patterns were identified by using a Japan Rigaku D/max 2000 X-ray diffractometer. Fourier transform infrared (FT-IR) spectra were recorded by using a Thermo Scientific Nicolet iS10 spectrophotometer. Raman spectroscopic measurements were performed on a Horiba Jobin Yvon LabRAM Aramis, using a 785 nm excitation laser. X-ray photoelectron spectroscopy (XPS) was identified by a VG-ADES 400 instrument with Mg Ka-ADES source at a residual gas pressure of lower than 108 Pa. UV-Vis diffuse reflectance spectra (UVVis/DRS) were tested on a Cary 500 UV-Vis-NIR spectrophotometer.

2.3. Photocatalytic reduction performance of hydroxyl-modified g-C3N4

Photocatalytic reduction of Cr(VI) (20 mg L1) and p-nitroaniline (4-NA 10 mg L1) was conducted in a self-made quartz reactor that was fitted with a circulation water system to maintain the reaction temperature of 20 ± 2 oC, and external simulated sunlight irradiation (320 <  < 680 nm) was provided by a PLS-SXE300 Xe lamp (300 W) with an IR cut filter. The average optical power density was about 170 mW/cm2. In a typical photocatalytic test, the catalyst (100 or 50 mg) was suspended into an aqueous solution containing K2Cr2O7 (100 mL) or 4-NA (50 mL), respectively. Before light irradiation, the suspension containing the catalyst and aqueous K2Cr2O7 or 4-NA was sonicated for 10 min and then stirred in dark for 60 min to reach the adsorption-desorption equilibrium. Fixed amounts of the reaction solution were taken out at certain irradiating intervals and centrifuged, followed by passed through a 0.45 m filter. Changes of the concentrations of Cr(VI) were monitored by diphenylcarbazide method using 7

a Cary 60 UV-Vis-NIR spectrometer at λ = 540 nm [40], while the concentrations of 4-NA were determined using a Cary 60 UV-Vis-NIR spectrometer at λ = 379 nm.

2.4. Photoelectrochemical measurement

All electrochemical measurements were performed on a conventional three-electrode electrochemical workstation (CHI 660E, China) equipped with an Ag/AgCl (saturated KCl) and a Pt wire as the reference electrode and the counter electrode, respectively. FTO coated with the photocatalyst film (area of 2 cm2) was used as the working electrode for electrochemical impedance spectroscopy (EIS) Nyquist analysis and photocurrent measurement. L-type glassy carbon electrode covered with the catalyst film on the black round area (diameter of 3.0 mm and area of 7.06 mm2) served as working electrode for the MottSchottky plot determination. The simulated sunlight provided by the Xe lamp irradiated the working electrode from the side.

2.5. Theoretical calculation

The calculation was performed on Gaussian 09 program [41]. The B3LYP/6-31g method was used to optimize organic models. ESP surface distribution of UCN and HUCN model were constructed with Gview program on the basis of the B3LYP/6-31g-optimized results.

3. Results and discussion

3.1. Preparation and characterization of hydroxyl-modified g-C3N4

8

Hydroxyl-modified g-C3N4 (HUCN) is prepared by hydrolysis of UCN in aqueous NaOH solution followed by self-assembly in dialysis process. First, the UCN powders derived from the calcination of urea disperse in NaOH solution and full contact with OH ion and H2O under the ultrasonic for 2 h. Under the stirring at 60 oC, OH ion and H2O act as scissor to tailor the polymeric UCN framework into nanosheets. The chemical cleavage of UCN happens in the CN bond belonging to N(C)3 groups. Simultaneously, the hydroxide ions from NaOH and the protons from H2O are connected to the sp2 hybridized carbon (NC=N) and bridged tertiary nitrogen [N(C)3] at the edge of UCN, respectively (Scheme 1). During the hydrolysis process, part of bridged tertiary nitrogen atoms converts to NH3 and escape to the system. The continuously increased offensive smell implies that more NH3 gas is yielded as the increase of initial NaOH concentration from 0.5, 3.0 to 8.0 mol L1, indicating the increased tailoring degree of NaOH to UCN. Subsequently, the dialysis is applied to remove the excess NaOH. During this process, the hydroxyl-modified g-C3N4 nanosheets undergo a self-assembly via hydrogen bonding between NH and OH bonds or OH and OH bonds, and the suspension transfers to the sol-like solution. After freeze-drying the sol-like solution, white cotton-like HUCN is obtained. Microscopically, UCN undergoes the hydrolysis into hydroxyl-modified nanosheets, and eventually converts to the nanofiber structure and constructs an interconnected open-framework.

3.1.1. Morphological characteristics, textural properties and structural information As shown in Fig. 1a, UCN displays folded sheet-like morphology. After hydrolysis and dialysis treatment, UCN is firstly fragmented into nanosheets with the thickness of ca. 4 nm, 9

estimated by AFM (Fig. S1), and then it self-assembles into nanofibers with a width of ca. 50 nm and a length of several hundred nanometer (Fig. 1b and Fig. S2). The nanofibers look smooth, and they interwoven into an interconnected open-framework. This open-framework makes the HUCN with much larger volume as compared with UCN (see digital images presented in Scheme 1). Nitrogen gas absorption-desorption isotherms and pore size distribution curves of as-prepared UCN and 3.0-HUCN samples are shown in Fig. S3, and the determined BET surface area of 3.0-HUCN is 40.5 m2 g1, slightly smaller than that of UCN (52.5 m2 g1), which may attribute to the distraction of the aggregated pores in HUCN and the construction of nanofibers with fewer pores in it. But the unique nanofiber-constructed interconnected open-framework of HUCN is expected to improve the electrons transfer to the surface and the accessibility to the active sites. Both advantages are favorable to enhance the photocatalytic reduction activity of HUCN as compared with UCN. The phase structure and phase purity of UCN and as-prepared HUCNs are tested by XRD analysis. As shown in Fig. 2, UCN displays an amorphous melon-based CN structure, and the peaks at 13.2 (100) and 27.5 (002) are originated from in-planar structural packing motif of tri-s-triazine and interlayer stacking of g-C3N4 aromatic units, respectively [42]. With the increase of the alkalinity of the preparation system, the (100) and (002) peak intensities become weaker than those of UCN, which is probably due to the decreased order degree of the in-plane structural packing and the destruction of layered structures. Increasing the concentration of NaOH also leads to the (002) peaks of HUCNs shift to higher diffraction angle, indicating the decrease in the distance between the layers [43]. After treatment of UCN in high NaOH concentration (8.0 mol L1), the (100) and (002) peak intensities are continuously weaker for 10

the resulting 8.0-HUCN, accompanying with the appearance of some unidentified new diffraction peaks, implying that the graphitic structure of UCN is destroyed partly owing to the treatment by high concentration NaOH solution [37]. The detailed chemical structure of HUCNs are investigated by FT-IR spectroscopy shown in Fig. 3. The strong band at 800 cm1 represents the bending vibration of tri-s-triazine ring of plane, while the peaks at 12001700 cm1 are attributed to the stretching vibration of aromatic UCN heterocycles. The broad IR absorption band located in the range of 3000–3300 cm−1 is assigned to the stretching vibration of the N–H bonds [44,45]. Compared with UCN, the aforementioned vibrational bands can still be found in various HUCNs, suggesting the primary framework of tri-s-triazine structure is retained after hydrolysis under alkaline condition. In addition, a new peak positioned at 1088 cm1 corresponding to the stretching vibration of C–O bond is observed in HUCNs [37], and the peak intensity becomes stronger with increasing initial concentration of NaOH. Compared with UCN, the peaks appearing in the range of 3000– 3300 cm–1 become broader and weaker after modification with NaOH, implying the stretching vibration of the O–H bond may also be involved in these peaks. The result therefore provides a powerful evidence that OH groups are introduced into the edges of HUCNs. Raman scattering spectra of UCN and 3.0-HUCN are recorded by using a laser of 785 nm to overcome the fluorescence interference. As shown in Fig. 4, several characteristic peaks at 1236, 1214, 982, 775, 706, 591 and 484 cm−1 are observed for UCN, corresponding to the typical vibration modes of g-C3N4 heterocycles [46,47]. Among them, the peak at 706 cm−1 matches well with the in-plane bending vibrations of the N(C)3 unit linked to heptazine units [48,49]. The aforementioned characteristic Raman scattering peaks still can be found in the 11

3.0-HUCN, however, the peak at 706 cm−1 for UCN has somewhat redshift (702 cm−1 for 3.0HUCN) after treatment with 3.0 mol L1 of NaOH. The result clearly indicates that the chemical environment of the N(C)3 unit has a tiny changes, and the hydroxyl group may be attached to the broken unsaturated carbon atoms belonging to N(C)3. XPS analysis is conducted to study the surface chemical states of UCN and HUCNs. XPS survey spectra shown in Fig. 5a indicate that both UCN and HUCNs have three elements including O, N and C, and the atomic ratio and element content estimated from XPS analysis are shown in Table S1. The peak intensity of O 1s in HUCNs is higher than that of UCN, indicating higher oxygen content in the HUCNs. Additionally, C-to-N atom ratio in the HUCNs increases compared with UCN, confirming that part of bridged tertiary nitrogen atoms converts to NH3 and escape to the system. Fig. 5b shows XPS spectra of bulk UCN and HUCNs in the C 1s binding energy region. For the UCN, the peaks centering at 284.6, 286.1, 288.2 and 289.1 eV are assigned to the adsorbed carbon, carbon atoms from C−NH2 groups, sp2 carbon atoms bonded to nitrogen atoms in aromatic rings [(N)2−C=N] and C−O bonds, respectively [34,50]. The result confirms that the UCN exhibits graphite-like structure. After modification of UCN by dilute NaOH, the aforementioned four peaks can also be detected in the C 1s XPS spectra of the HUCNs; moreover, the ratio of carbon atoms from C−NH2 (and C−O) bonds to the total carbon atoms in the HUCNs gradually increases as increasing initial NaOH concentration from 0, 0.5 to 3.0 mol L1, which indicates that more CNH2 and CO bonds may be formed in the HUCNs compared with UCN. N 1s XPS spectra of UCN and HUCNs shown in Fig. 5c display three characteristic peaks 12

centering at 398.1 eV, 399.4 eV and 400.9 eV, corresponding to nitrogen atoms from C–N=C bonds (sp2 N nitrogen atoms involved in tri-s-triazine rings), bridging nitrogen atoms in N– (C)3 and C–NH2 bonds (nitrogen atoms bonded with H atoms in amino groups), respectively [51]. However, the peak intensity of nitrogen atoms from C–NH2 bonds of the HUCNs is much stronger than that of the UCN, further evidencing that more CNH2 bonds are formed in the HUCNs. O 1s XPS spectra of the UCN and HUCNs shown in Fig. 5d are fitted into two peaks with binding energies at 531.4 and 532.8 eV, respectively, and they correspond to oxygen atoms from COH bonds and the adsorbed H2O molecule [34,51]. Additionally, the peak intensity of oxygen atoms from COH bonds gradually increases with the increase of initial NaOH concentration in the preparation systems. The ratio of COH groups in the composite is ca. 0.68, 1.85 and 3.72% for UCN, 0.5-HUCN and 3.0-HUCN, respectively. The above XRD, FT-IR, Raman and XPS analysis results indicate that hydroxyl-modified graphitic carbon nitride is successfully fabricated via hydrolysis of UCN in an alkaline condition at low temperature (60 oC) followed by a self-assembly in dialysis process, and the active hydroxyl groups are successfully introduced into the edge of HUCNs; meanwhile, the primary tri-s-triazine-based graphitic framework of UCN retains intact after treatment with aqueous NaOH at suitable concentrations.

3.1.2. Optical absorption properties and band structures The optical absorption properties of UCN and HUCNs are studied by UV-Vis/DRS analysis. As shown in Fig. 6a, UCN exhibits a semiconductor absorption in the range of 200460 nm, 13

originating from the charge transfer from valence band (VB) populated by N 2p orbitals to the CB formed by C 2p orbitals [52,53]. The absorption edge of UCN is located at ca. 460 nm. Compared to UCN, the absorption edges of HUCNs show a gradually blue-shift with increasing initial NaOH concentrations in the preparation system, which is originated from the quantum size effect of the HUCNs with nanosheet assembled nanofiber structure. The estimated absorption edges of the UCN, 0.5-HUCN, 3.0-HUCN and 8.0-HUCN from the absorption spectra are 460, 441, 420 and 396 nm, respectively; meanwhile, the corresponding bandgaps (Eg) of the samples estimated by the plots of (αhυ)1/2 vs. hυ are 2.70 (UCN), 2.81 (0.5-HUCN), 2.95 (3.0-HUCN) and 3.13 eV (8.0-HUCN), respectively (Fig. 6b). The flat-band potentials of the UCN and HUCNs are estimated by Mott-Schottky plots, which are applied to estimate the CB edge potential of n-type semiconductor [54]. As shown in Fig. 7, the CB of UCN, 0.5-HUCN, 3.0-HUCN and 8.0-HUCN are 1.15, 1.23, 1.62 and 1.88 V (vs NHE, pH = 7.0), respectively. The CB edge potentials of the HUCNs are more negative than that of the UCN, indicating that the photogenerated electrons of HUCNs possess stronger reduction ability than the UCN. On the basis of the determined bandgaps and CB edge potentials of UCN and HUCNs, the band structures of UCN, 0.5-HUCN, 3.0-HUCN and 8.0HUCN are illustrated (Fig. 8).

3.1.3. Separation and migration ability of the photogenerated charge carriers The separation and migration performance of the photogenerated charge carriers for UCN and HUCNs are evaluated by photocurrent response experiment and electrochemical impedance spectroscopy (EIS) measurement. As shown in Fig. 9a, all the tested 14

photoelectrodes show stable and reproducible photocurrent response during five on-off intermittent cycles, and the determined photocurrent density of the UCN, 0.5-HUCN, 3.0HUCN and 8.0-HUCN working electrodes is 1.3, 1.9, 2.5 and 2.7 A cm2, respectively. Therefore, four HUCN/FTO electrodes present remarkably enhanced transient photocurrent density than that of the UCN/FTO, and the photocurrent density gradually increases with continuous increasing initial NaOH concentration in the preparation systems. The higher transient photocurrent density of HUCN samples suggests the enhanced separation efficiency of the photogenerated charge carriers, which can be further supported by the EIS Nyquist measurements (Fig. 9b). Obviously smaller arc radius of EIS Nyquist plots of HUCNs compared with those of UCN imply the decreased charge carrier migration resistance with respect to the UCN. The fitted EIS parameters based on the equivalent circuit are summarized in Table 1, which consists of the series resistance (Rs) and interfacial charge migration resistance (Rct) [55,56]. The fitting results of Rs for different photoanodes are almost similar, while the Rct value of the HUCNs change dramatically due to the decoration with the different concentration hydroxyl groups. The HUCNs afford lower resistance for interfacial charge transfer than the UCN. The result further implies that HUCNs systems possess excellent photogenerated charge carrier separation ability and high quantity of free electrons (eCB) [5759]. DFT calculated ESP distribution of the optimized UCN and HUCN models provide a reasonable electron migration pathway at the edge of photocatalyst. As shown in Fig. 10, red and blue color in ESP maps represent the electron rich (negative potentials) and poor (positive potentials) regions, respectively. The electron rich regions of UCN locate at the bridged tertiary 15

nitrogen atoms and triangular edge nitrogen atoms in melem units, whereas the electron poor regions are at the other parts of melem units (Fig. 10a). During the hydrolysis process, the hydroxyl groups are introduced at the edges of HUCN and act as the hole accepter, leaving more electrons locate at the triangular edge nitrogen atoms in melem units (Fig. 10b). Therefore, it is deduced that the reduction sites have a large shift from internal flat part of UCN to the edges of HUCN. Based on the above result it is confirmed that gradually increased photocurrent density or decreased electrochemical impedance reflect faster photogenerated charge carrier separation and migration ability of the HUCNs than UCN. This is due to the introduction of hydroxyl groups to the edges of UCNs, and these hydroxyl groups can serve as the hole scavengers to promote the separation of the photogenerated charge carriers. In addition, narrow nanofiber with the width of less than 50 nm will benefit for the electrons transfer to the surface and improve the accessibility of the active sites.

3.2. Photocatalytic reduction performance of hydroxyl-modified g-C3N4

3.2.1. Photocatalytic reduction activity The simulated sunlight photocatalytic reduction activity of the HUCNs is firstly evaluated by the reduction of an aqueous Cr(VI) to Cr(III) [60,61]. As shown in Fig. 11a, blank test confirms the reduction of Cr(VI) is negligible in the absence of the catalyst under simulated sunlight irradiation. The adsorption-desorption equilibrium between Cr(VI) and the catalyst is reached after strring the suspension for 60 min, and the catalysts show low adsorption capacity towards Cr(VI). After simulated sunlight irradiating the UCN for 45 min, only 30.8% of Cr(VI) 16

is removed. Significantly increased removal of Cr(VI) is obtained over the simulated sunlight irradiating various HUCNs under the same conditions, for example, the removal efficiency of Cr(VI) is 81.9%, 99.8% and 70.7% for 0.5-HUCN, 3.0-HUCN and 8.0-HUCN, respectively. Correspondingly, the solar-to-Cr(III) efficiency (STC) of UCN, 0.5-HUCN, 3.0-HUCN and 8.0-HUCN is calculated, and it is ca. 0.04, 0.11, 0.13 and 0.09%, respectively (see the details presented in supplementary information). Accordingly, the 3.0-HUCN exhibits the highest STC for photoreduction of Cr(VI) to Cr(III). Fig. 11b presents apparent rate constant (k) for Cr(VI) removal, which is determined based on the formula of ln(ct/c0) = kt by supposing the photocatalytic reduction of Cr(VI) follows pseudo first-order model. The rate constants of UCN, 0.5-HUCN, 3.0-HUCN and 8.0-HUCN are estimated to be 0.0085, 0.0262, 0.1137 and 0.0166 min1, respectively. Accordingly, the photoreduction of Cr(VI) over the 3.0-HUCN proceeds the most rapidly, and its k value is almost 13 times higher than that of the UCN. In order to provide direct evidence for the photocatalytic reduction of Cr(VI) over the 3.0-HUCN, the time-resolved UV-Vis absorption spectra of Cr(VI) during 3.0-HUCN-mediated simulated sunlight catalytic process are displayed in Fig. 11c. As expected, the characteristic absorption of Cr(VI) at 347 nm gradually decreases, and the peak is hardly observed after light irradiation for 45 min. Interestingly, the color of the spent catalyst finally turns to light green, which is consistent with the color of Cr(III) (inset of Fig. 11d). XPS spectrum of the spent 3.0-HUCN catalyst show two typical Cr(III) peaks centering at 577.1 and 586.8 eV [62,63], further confirming Cr(VI) is reduced to Cr(III) by 3.0-HUCN (Fig. 11d). Wavelength-dependent photocatalytic reduction of Cr(VI) over 3.0-HUCN using band-pass 17

filters (350, 420, 500 nm, respectively) is studied to observe the spectral response of photocatalytic reduction reaction. As shown in Fig. 12a, conversion of Cr(VI) over the 3.0HUCN reaches 52.3, 5.1 and 1.7% under the irradiation with the incident wavelength of 350, 420 and 500 nm, respectively, indicating the photocatalytic activity of 3.0-HUCN towards the reduction of Cr(VI) matches well with the light absorption behavior of 3.0-HUCN. Additionally, photocatalytic reduction of Cr(VI) over the selected wavebands of Xe lamp irradiated 3.0-HUCN is also conducted. Under the light irradiation of 320 nm    680 nm, the conversion of Cr(VI) over the 3.0-HUCN is higher than that of 400 nm    680 nm (Fig. 12b). The conversion of Cr(VI) is hardly observed at the incident light waveband of 420 nm    680 nm. These results indicate that the majority of photons at a wavelength shorter than 420 nm contribute to the reduction of Cr(VI). Influence of initial pH of the reaction media on the removal efficiency of Cr(VI) over the UCN and 3.0-HUCN is shown in Fig. 13. It can be seen that the photocatalytic reduction efficiency of Cr(VI) over the UCN and 3.0-HUCN decreases obviously as increasing pH value from 2.3 to 6.8. For example, the removal efficiency of Cr(VI) over 3.0-HUCN is 99.9% (pH 2.3), 51.0% (pH 3.8), 43.0% (pH 5.5) and 28.0% (pH 6.8), respectively, after simulated sunlight irradiating for 60 min, while the removal efficiency of Cr(VI) over the UCN reaches 41.0% (pH 2.3), 20.0% (pH 3.8), 11.0% (pH 5.5) and 7.0% (pH 6.8), respectively, under the same reaction conditions. Under acidic condition, Cr(VI) exists mainly in the form of Cr2O72 with strong oxidization ability, and it is therefore easily to be reduced. However, CrO42 with less oxidization ability is predominant under neutral solutions, and it is difficult to be photoreduced [63-65]. Moreover, when the pH value is higher than 6, Cr(III) precipitates on the surface of 18

the photocatalyst as Cr(OH)3, leading to coverage of some active sites and thus remarkably inhibited photocatalytic reduction activity. In order to further study the photocatalytic reduction ability of the HUCNs, the photocatalytic reduction of 4-NA over the UCN and HUCNs is also tested. Similarly, 3.0HUCN still exhibits the highest photocatalytic reduction activity among UCN and other HUCNs. After simulated sunlight irradiating for 20 min, 99.0% of 4-NA is reduced by 3.0HUCN, while only 96.0%, 58.2% and 49.5% of 4-NA are reduced by 0.5-HUCN, 8.0-HUCN and UCN, respectively (Fig. 14a). Fig. 14b presents the apparent rate constants (k) for the photocatalytic reduction of 4-NA by supposing the photocatalytic reduction reaction follows pseudo first-order model. The photoreduction rate of 4-NA over the 3.0-HUCN is the most rapid among all tested catalysts with k value of 0.3179 min1. Fig. 14c shows temporal evolution of the UV-Vis absorption spectra of 4-NA during simulated sunlight photocatalytic reduction process over the 3.0-HUCN, showing that the characteristic absorption peak of 4NA (379 nm) promptly declines over time until it disappears within the test period. Based on the aforementioned results it is inferred that the factors affecting the photocatalytic reduction activity of HUCNs include the crystal structure, surface properties, separation and migration efficiency of the photogenerated charge carriers as well as the CB edge potential of the photocatalysts. During the preparation process, UCN is hydrolyzed into nanosheets and constructing an interconnected open-framework, accompanying with the introduction of hydroxyl groups on the edge of HUCN. The higher concentration of NaOH, the more hydroxyl groups generates. When the UCN hydrolyzes in lower alkali concentration, e.g., 0.5 and 3.0 mol L1, the crystal structure of g-C3N4 completely maintain. Compared to the UCN and 0.519

HUCN, the enhanced photocatalytic reduction activity of the 3.0-HUCN is mainly contributed to the following reasons. Firstly, the increased amount of hydroxyl groups can function as a hole accepter to accelerate the separation of the photogenerated electron-hole. Secondly, HUCNs with the interconnected open-framework will benefit for the electrons transfer to the surface of HUCN, which can improve the accessibility of the active sites. Thirdly, the upward CB edge position (or more negative CB edge potential) endows the photogenerated electrons of 3.0-HUCN the stronger reduction ability than UCN and 0.5-HUCN. Finally, the increased hydrophilicity owing to the presence of abundant OH and NH2 groups at the edges of the 3.0-HUCN may facilitate the miscibility between aqueous reactant and the catalyst. Although the 8.0-HUCN exhibits the fastest photogenerated charge carrier separation and migration rate, its tri-s-triazine-based graphitic framework is partly destroyed during the process of hydrolysis of UCN in much higher NaOH concentration (8.0 mol L1). This is the direct reason that results in the 8.0-HUCN lower photocatalytic reduction activity than its other two counterparts.

3.2.2. Photocatalytic reusability and stability The reusability and stability of the HUCNs is evaluated by photocatalytic reduction of aqueous Cr(VI) and 4-NA for five catalytic cycles by selecting the 3.0-HUCN as the representative photocatalyst. After each catalytic cycle, the catalyst is separated and then washed completely with water and ethanol, and the recovered catalyst is applied to the next cycle. As shown in Fig. 15a and b, the 3.0-HUCN shows considerably high photocatalytic reusability and stability, and the loss of the photocatalytic activity is negligible during five consecutive runs. To further confirm the excellent photocatalytic reusability and stability, the 20

structure and morphology of the fifth time spent 3.0-HUCN are tested by XRD and TEM. As shown in Fig. 15c and d, the spent 3.0-HUCN still remains the perfect crystalline structure and interconnected open-framework after five consecutive catalytic cycles.

3.2.3. Mechanism consideration In order to understand the enhanced photocatalytic reduction activity of the HUCN with respect to bulk UCN in depth, electron scavenging experiment is carried out by using K2S2O8 as a scavenger [66]. As shown in Fig. 16a, after simulated sunlight irradiating the Cr(VI)-UCN system without and with K2S2O8 (1 mmol L−1) for 45 min, the removal efficiency of Cr(VI) is 30.8% and 19.3%, respectively, and the corresponding apparent reaction rate constant is 0.0085 and 0.0048 min1 (Fig. 16b). The result indicates that K2S2O8 can obviously inhibit the photocatalytic reduction ability of UCN to Cr(VI). As for the Cr(VI)-3.0-HUCN system in the absence and presence of K2S2O8, the removal efficiency of Cr(VI) is 99.8% and 40.2%, respectively, after simulated sunlight irradiating for 45 min. Correspondingly, the apparent reaction rate constants for photocatalytic reduction of Cr(VI) are 0.1137 and 0.0089 min1. Therefore, the inhibition effect of K2S2O8 to the photocatalytic reduction activity of the 3.0-HUCN is more seriously than that of UCN, implying stronger photocatalytic reducibility of the 3.0-HUCN. Similar result is also found in the 4-NA-UCN and 4-NA-3.0-HUCN photocatalytic reduction system (Fig. 16c and d). After simulated sunlight irradiating for 10 min, the inhibition effect to photocatalytic reduction of 4-NA over the 3.0-HUCN is nearly four times higher than that of the UCN. The results suggest that a larger number of free electrons are generated during simulated sunlight 21

irradiating the HUCN than UCN, originating from the faster electron-hole separation efficiency; meanwhile, the photogenerated electrons of the 3.0-HUCN possess stronger photocatalytic reduction ability, attributing to the more negative CB edge potential. On the basis of the testing results including photocatalytic reduction, photoelectrochemical experiment and electron scavenging experiment as well as the DFT calculated ESP distribution, a reasonable mechanism of simulated sunlight photocatalytic reduction of Cr(VI) over the HUCN is revealed and illustrated in Scheme 2. In the HUCN photocatalytic reduction of Cr(VI) system, Cr(VI) exists in the form of Cr2O72 at pH 2.3, which possesses strong oxidization ability and is easily to be reduced. With the help of hydrophilic functional groups of NH2 and OH at the edges of HUCN and the interconnected open-framework, Cr2O72 ions can easily adsorb on the surface of the HUCN. Under simulated sunlight irradiation, the photogenerated electron-hole pairs are generated in the HUCN. The introduced hydroxyl groups at the edges of HUCN can act as the hole scavenger to accelerate the separation of electrons and holes. Meanwhile, the nanosheet constructed nanofiber structure and open-framework of HUCN will benefit for the electrons transfer to the surface and improve the accessibility of the active sites. Therefore, more free photogenerated electrons with higher reduction ability participate in the reduction of Cr(VI) to Cr(III) effectively.

4. Conclusions

Hydroxyl-modified g-C3N4 prepared by hydrolysis of bulk g-C3N4 (UCN) in suitable concentrations of aqueous NaOH solutions followed by self-assembly in dialysis process exhibit the enhanced simulated sunlight photocatalytic reduction activity toward Cr(VI) as 22

compared with the UCN. Such excellent photocatalytic reduction activity is ascribed to the following reasons: i) the hydroxyl groups introduced on the edges of HUCN can not only promote the separation of the photogenerated electron-hole but also increase the miscibility between Cr(VI) and the HUCNs; ii) the nanosheet constructed nanofiber structure and the open-framework constructed of HUCN is benefit for the electrons transfer to the surface and the improvement of the accessibility of the active sites; and iii) the upward CB edge potential endows the photogenerated electrons stronger reduction ability.

Acknowledgements

This work is supported by NSFC of China (21573038 and 51608102) and Jilin province science and technology development projects (20160520177JH) and the Fundamental Research Funds for the Central Universities (2412019FZ018).

23

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33

Table 1 EIS fitted results of Rct for UCN and HUCNs. UCN

0.5-HUCN

3-HUCN

8-HUCN

Rs/

47.8

45.8

51.3

51.2

Rct/

214290

156360

90838

44561

34

Fig. 1 TEM images of UCN (a) and 3.0-HUCN (b).

35

Fig. 2 XRD patterns of UCN and HUCNs.

Intensity (a.u.)

8.0-HUCN 3.0-HUCN

0.5-HUCN UCN

10

20

30

2 (degree)

36

40

50

Fig. 3 FT-IR spectra of UCN and HUCNs.

1088 cm

Transmittance %

8.0-HUCN

-1

3.0-HUCN 0.5-HUCN UCN

4000 3500 3000 2500 2000 1500 1000 -1

Wavenumbers (cm )

37

500

Intensity (a.u.)

Fig. 4 Raman scattering spectra of UCN and 3.0-HUCN.

UCN

red shift

3.0-HUCN

500

1000

1500 -1

Raman Shift (cm )

38

Fig. 5 XPS survey (a) and high-resolution spectra in C 1s (b), N 1s (c) and O 1s (d) binding energy regions of the UCN and HUCNs.

(a)

C 1s XP Intensity (a.u.)

UCN

Intensity (a.u.)

C1s

O1s

(N)2-C=N

(b)

N 1s

0.5-HUCN

3.0-HUCN

absorbed C

C-NH2

C-O

UCN

0.5-HUCN

3.0-HUCN 1200 1050 900

600

750

450

292

300

290

C-N=C

(d)

N-(C)3

O 1s XP Intensity (a.u.)

N 1s XP Intensity (a.u.)

C-NH2

286

284

UCN

0.5-HUCN

402

400

398

396

280

H2O C-OH

UCN

0.5-HUCN

3.0-HUCN

3.0-HUCN 404

282

Binding Energy (eV)

Binding Energy (eV)

(c)

288

394

Binding Energy (eV)

538

536

534

532

530

Binding Energy (eV)

39

528

Fig. 6 UV-Vis DRS (a) and plots of (αhυ)1/2 vs. hυ (b) of the UCN and HUCNs.

Absorbance (a.u.)

(a) 1.0

UCN 0.5-HUCN 3.0-HUCN 8.0-HUCN

0.8 0.6 0.4 0.2 0.0 200

300

400

700

600

500

800

Wavelength (nm)

(b) 1.0

UCN 0.5-HUCN 3.0-HUCN 8.0-HUCN

1/2

(ah) (eV)

1/2

0.8 0.6 0.4

2.70 eV 2.81 eV 2.95 eV 3.13 eV

0.2 0.0

2

3

4

h (eV)

40

5

6

Fig. 7 Mott-Schottky plots of UCN and HUCNs in an aqueous solution of Na2SO4 (0.5 mol L1) at a fixed frequency of 2 kHz or 3 kHz.

(a) 1.5

UCN CB=-1.15 V

2.0 kHz 3.0 kHz

(b) 2.0

2.0 kHz 3.0 kHz

0.5-HUCN CB=-1.23 V

1.5 8

C /F *10

-2

1.0

-2

-2

-2

C /F *10

7

1.0

0.5

0.5

0.0 -1.5

-1.0

-0.5

0.0

0.0

0.5

E (V vs NHE) (c)

5

CB=-1.62 V

-0.5

-1.0

0.0

0.5

E (V vs NHE) 2.0 kHz 3.0 kHz

3.0-HUCN

-1.5

(d) 1.5

2.0 kHz 3.0 kHz

8.0-HUCN CB= -1.88 V

C /F *10

2

-2

-2

1.0

-2

3

-2

C /F *10

8

9

4

0.5

1 0

-1.5

-1.0

-0.5

0.0

0.0

0.5

E (V vs NHE)

-2.0

-1.5

-1.0

-0.5

E (V vs NHE)

41

0.0

0.5

Fig. 8 Band structure of UCN and HUCNs.

42

Fig. 9 Photocurrent responses (a) and EIS Nyquist plots (b) of UCN and HUCNs electrodes in aqueous Na2SO4 electrolyte solution under Xe lamp irradiation. Inset in (b): equivalent circuit model (Rs refers to the series resistance, Rct and CPE are the charge transfer resistance and the constant phase element).

(a) 3.0 -2

Current (Acm )

2.5 2.0

UCN

1.5

0.5-HUCN 3.0-HUCN 8.0-HUCN

1.0 0.5 0.0

on

0

off

50

100

150

200

Time (s) (b)

100

Z" (Kohm)

80 60 40 UCN 0.5-HUCN 3.0-HUCN 8.0-HUCN

20 0

0

20

40

60

Z' (Kohm)

43

80

100

Fig. 10 ESP surface distribution of optimized UCN (a) and HUCN (b) model.

44

Fig. 11 (a) Photocatalytic reduction of an aqueous Cr(VI) under simulated sunlight irradiating UCN and HUCNs. (b) The corresponding apparent rate constants (k) for Cr(VI) removal. (c) Temporal evolution of the UV-Vis absorption spectra of Cr(VI) during simulated sunlight photodegradation process over the 3.0-HUCN. (d) XPS spectrum of Cr 2p adsorbed on the 3.0HUCN after photocatalytic Cr(VI) reduction reaction. Catalyst amount 100 mg; c0 = 20 mg L1; volume 100 mL; pH = 2.3.

1.0

(a)

0.8

In Dark

U C N

H U C

60

0.4

0.2

0.0 300 320 340 360 380 400 420 440

Cr(III)

45

2p3/2

2p1/2

592

Wavelength (nm)

N

(d) Intensity (a.u.)

Absorbance (a.u.)

0.6

Initial Cr(VI) solution Adsorption for 60 min Irradiation for 15 min Irradiation for 30 min Irradiation for 45 min Irradiation for 60 min

UC

Catalyst

Time (min) (c)

-H

N

N

30

0

0.0166

8. 0

5-

C

-30

0.0085 U

0.0 -60

0.0262 0.

0.2

UCN 0.5-HUCN 3.0-HUCN 8.0-HUCN Photolysis

H

0.4

0-

-1

3.

k (min )

0.6

c t /c 0

0.1137

(b)

588

584

580

Binding Energy (eV)

576

572

Fig. 12 (a) Overlay of the UV-Vis spectra and wavelength-dependent photocatalytic reduction of Cr(VI) over the 3.0-HUCN using band-pass filters. (b) Photocatalytic reduction of Cr(VI) over the 3.0-HUCN under the selected wavebands of Xe lamp irradiating.

(a)

1.0

Absorbance (a.u.)

Conversion (%)

100

350 nm

50

0.5

420 nm

0 200

500

400

300

500 nm

600

0.0 700

Wavelength (nm) 1.0

(b)

420<  < 680 nm

c t /c 0

0.8 400<  < 680 nm

0.6 0.4 0.2

320<  < 680 nm 0.0

-60

-30

0

30

60

Time (min)

46

90

120

Fig. 13 Influence of pH on the photocatalytic removal efficiency of Cr(VI) over the UCN and 3.0-HUCN. Irradiation time 60 min; Catalyst amount 100 mg; c0 = 20 mg L1; volume 100 mL.

100

UCN

Conversion rate (%)

3.0-HUCN 80 60 40 20 0

2.3

3.8

pH

47

5.5

6.8

Fig. 14 (a) Photocatalytic reduction of an aqueous 4-NA under simulated sunlight irradiating UCN and HUCNs. (b) The corresponding apparent rate constants (k) for 4-NA removal. (c) Temporal evolution of the UV-Vis absorption spectra of 4-NA during simulated sunlight photocatalytic reduction process over the 3.0-HUCN. Catalyst amount 50 mg; c0 = 10 mg L1; volume 50 ml.

1.0

(b)

(a)

In Dark

0.8

-1

k (min )

H U C N

0

15

N

-15

C

-30

N

30

Catalyst

Time (min)

(c)

Initial 4-NA Adsorption for 60 min Irradiation for 5 min Irradiation for 10 min Irradiation for 15 min Irradiation for 20 min Irradiation for 30 min

Absorbance (a.u.)

1.0

U

-45

UC

H

-60

0.0166

5-

0.2

0.1372

0.

UCN 0.5-HUCN 3.0-HUCN 8.0-HUCN Photolysis

0-

0.4

3.

c t /c 0

0.6

0.0

0.3179

0.8 0.6 0.4 0.2 0.0

300

350

400

450

Wavelength (nm)

48

500

550

0.0345 8. 0 -H UC N

Fig. 15 Recycling experiments of simulated sunlight photocatalytic reduction of (a) Cr(VI) (Catalyst amount 100 mg; c0 = 20 mg L1; volume 100 ml) and (b) 4-NA (Catalyst amount 50 mg; c0 = 10 mg L1; volume 50 ml) over the 3.0-HUCN. XRD pattern (c) and TEM image (d) of the fifth time used 3.0-HUCN.

(a)1.0

1st

2nd

3rd

4th

(b)1.0

5th

0.8

0.6

0.6

c t /c 0

c t /c 0

0.8

0.4 0.2

1st

2nd

3rd

4th

5th

0.4 0.2

0.0

-60 0

60

0 60

0 60

Time (min)

0 60

0.0 -60

0 60

(c)

Intensity (a.u.)

Used 3.0-HUCN

3.0-HUCN 10

20

30

40

50

60

70

80

2 (degree)

49

0 -60

0 -60

0 -60

Time (min)

0 -60

0

Fig. 16 Simulated sunlight photocatalytic activity of the UCN and 3.0-HUCN toward Cr(VI) (a) and 4-NA (c) reduction in the presence of K2S2O8 (1 mmol L1) as an electron scavenger. The corresponding apparent rate constants (k) for Cr(VI) (b) and 4-NA (d) photocatalytic reduction reactions. (a) 1.0

(b)

Cr(VI)

0.1137

Cr(VI) 3.0-HUCN

0.8

UCN

-1

k (min )

ct/c0

0.6 0.4 UCN 3.0-HUCN UCN+K2S2O8

0.2 0.0

0.0085

3.0-HUCN+K2S2O8

-60

30

0

-30

60

No scavenger

Time (min)

(c) 1.0

(d)

4-NA

0.3179

K 2S 2O 8

4-NA UCN

-1

k (min )

0.6

ct/c0

0.0048

3.0-HUCN

0.8

0.4 UCN 3.0-HUCN UCN+K2S2O8

0.2 0.0

0.0089

0.0170

3.0-HUCN+K2S2O8

-60

-30

0

30

No scavenger

Time (min)

50

0.0213 0.0128

K 2S 2O 8

Scheme 1 Structure evolution of bulk UCN to HUCN treated by aqueous NaOH solution. The insets are digital images of UCN and HUCN.

100 mg

100 mg

51

Scheme 2 Schematic illustration of the separation mechanism of electron-hole pairs during simulated sunlight photocatalytic reduction of an aqueous Cr(VI) over the 3.0-HUCN.

52

Graphical abstract

53

Highlights Enhanced simulated sunlight photocatalytic reduction of an aqueous hexavalent chromium over hydroxyl-modified graphitic carbon nitride

Xinyue Wang, Lingyu Li, Jiaqi Meng, Peiyu Xia, Yuxin Yang*, Yihang Guo*

School of Environment, Northeast Normal University, Changchun 130117, P.R. China



Hydroxyl-modified g-C3N4 is successfully prepared by base-hydrolyzing treatment.



HUCN exhibits excellent simulated sunlight photocatalytic reduction activity.



Presence of COH groups at the edges of HUCN facilitates charge separation.



More negative CB edge potential of HUCN promotes the photocatalytic reduction ability.



The catalyst also possesses excellent reusability.

54