Easy dispersion and excellent visible-light photocatalytic activity of the ultrathin urea-derived g-C3N4 nanosheets

Accepted Manuscript Title: Easy dispersion and excellent visible-light photocatalytic activity of the ultrathin urea-derived g-C3 N4 nanosheets Authors: Yuxin Yang, Lei Geng, Yingna Guo, Jiaqi Meng, Yihang Guo PII: DOI: Reference:

S0169-4332(17)31978-5 http://dx.doi.org/doi:10.1016/j.apsusc.2017.06.323 APSUSC 36523

To appear in:

APSUSC

Received date: Revised date: Accepted date:

26-5-2017 29-6-2017 30-6-2017

Please cite this article as: Yuxin Yang, Lei Geng, Yingna Guo, Jiaqi Meng, Yihang Guo, Easy dispersion and excellent visible-light photocatalytic activity of the ultrathin urea-derived g-C3N4 nanosheets, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.06.323 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Easy dispersion and excellent visible-light photocatalytic activity of the ultrathin urea-derived g-C3N4 nanosheets Yuxin

Yang1,

Lei

Geng2,

Yingna

Guo3,

Jiaqi

Meng1,

Yihang

Guo1*

[email protected]

1

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

2

CCCC-TDC Environmental Engineering Co., LTD, Tianjin 300461, P.R. China

3

School of Chemistry, Northeast Normal University, Changchun 130024, P.R. China

*Corresponding authors at: Tel./Fax: +86 431 89165626.

Graphical abstract

Highlights 

Ultrathin g-C3N4 nanosheets are obtained by HCl-assisted hydrothermal treatment.



The g-C3N4 nanosheets exhibit excellent visible-light photocatalytic activity.



Presence of COH and C=O groups at the edge of g-C3N4 facilitates charge separation.



More positive potential of VB in g-C3N4 nanosheets promotes the oxidation ability.



The g-C3N4 nanosheets can disperse in aqueous solution easily and uniformly.

Abstract

Ultrathin two-dimensional (2D) g-C3N4 nanosheets (UCNS) are facilely prepared by liquid-exfoliation of the urea-derived g-C3N4 via an HCl-assisted hydrothermal treatment method. The UCNS are served as the novel visible-light-driven photocatalyst to degrade a typical organic pollutant, p-nitrophenol (PNP), in an aqueous solution. By the combination of the advantages of plentiful exposed active sites, fast photogenerated charge carrier separation rate, aligned energy levels and easy dispersion in an aqueous solution, the UCNS exhibit considerably enhanced visible-light photocatalytic activity toward the degradation of the target compound in comparison of their bulk g-C3N4 counterpart, melamine-derived g-C3N4 nanoparticles or Degussa P25 TiO2. The indirect chemical probe experimental results indicate that both the superoxide radicals and the photogenerated holes are responsible for the complete decomposition of PNP. Additionally, the UCNS can be reused at least four times without obvious activity loss and morphology change, exhibiting the excellent reusability in photodegradation aqueous organic pollutants.

Keywords: Visible-light photocatalysis; Graphitic carbon nitride; Nanosheet; Hydrothermal treatment; Organic pollutant

1. Introduction

Sunlight-driven chemical fuels production and pollutants elimination through a photocatalytic process has been regarded as an attractive and promising strategy to alleviate the global energy crisis and environmental contamination [1,2]. Among various photocatalysts applied in the aforementioned processes, graphitic carbon nitride (g-C3N4) has emerged as an ideal metal-free polymeric semiconductor, mainly due to its desirable band gap (2.7 eV), robustness, abundance and eco-friendly [3-13]. However, the practical applications of bulk g-C3N4 still face to the problems of inefficiency, originating from its low surface area, fast recombination rate and weak redox ability of the photogenerated electrons (eCB) and holes (hVB+) [14]. Encouraged by the pioneering work on graphene [15], the delamination of the layered bulk g-C3N4 into 2D thinner counterparts has successfully achieved. Meanwhile, the morphology and band gap structure of bulk g-C3N4 can be well regulated, which endows the resulting 2D g-C3N4 nanosheets with unique physicochemical properties including open-up surface, ultrahigh charge carrier mobility and enlarged band gap with strong redox ability of eCB and hVB+. Therefore, 2D g-C3N4 nanosheets exhibit enhanced photocatalytic performance in pollutant degradation and hydrogen generation in comparison of bulk g-C3N4 [16-24]. The layered bulk g-C3N4 structure could be exfoliated into nanosheets by the liquid-exfoliation process when it meets with the following two conditions: i) allowing the incorporation of proper molecules such as H2O [17], organic solvent [25], acid [26-28] or alkali [29] into the interlayer gallery of g-C3N4 to swell the bulk matrix and

make it ready for delamination; and ii) assisting with sonication [25,28,30], heating [27] or hydrothermal treatment [31], which can provide enough power to break up the layered structure. It has been experimentally proved that bulk g-C3N4 could be exfoliated into nanosheets by acid-assisted sonication treatment, and the enhanced photocatalytic activity of g-C3N4 nanosheets is attributed to the enlarged surface area and increased exposed active edges. Compared with sonication, hydrothermal treatment could provide higher temperature and higher pressure to completely exfoliate bulk g-C3N4 into ultrathin or even monolayer nanosheets, leading to it easy dispersion in water and thereby much higher photocatalytic activity [31]. Unfortunately, most of the modification focus on the bulk g-C3N4 derived from dicyandiamide [26,29,32,33] or melamine [18,34,35] rather than urea. Although the dicyandiamide- or melamine-derived g-C3N4 nanosheets exhibit obviously increased photocatalytic activity than their bulk counterparts, the activity is hard to exceed that of urea-derived bulk g-C3N4 (UCN) [14]. As a consequence, it is exciting and challenging to achieve g-C3N4 nanosheets with further enhanced photocatalytic activity via the delamination of urea-derived bulk g-C3N4. In the present work, ultrathin 2D g-C3N4 nanosheets (UCNS) are successfully prepared by liquid-exfoliation of urea-derived g-C3N4 via an HCl-assisted hydrothermal treatment method. The UCNS with small size can provide more exposed active sites and shortened diffusion distance, which can facilitate the mass and charge transfer. At the same time, the electrophilic groups of COH and NCO at the edge of g-C3N4 are introduced, which can prevent the eCB and hVB+ from the direct

recombination. The UCNS also exhibit enlarged band gap with lowered valence band, endowing hVB+ with stronger oxidizing ability. Most importantly, well dispersion of UCNS in an aqueous solution ultimately brings the above properties into full play. As expected, the UCNS exhibit considerably enhanced visible-light photocatalytic activity toward the degradation of a typical organic pollutant, p-nitrophenol (PNP). By the combination of the testing results of photoelectrochemistry and indirect chemical probe, direct photooxidation of O2− and hVB+ for the decomposition of PNP is revealed.

2. Experimental

2.1. Chemicals and materials

Urea (99%), HNO3 (85%) and HCl (37%) were obtained from Beijing chemical works (PR China). PNP was obtained from Tianjin guangfu fine chemical research institute. Degussa P25 TiO2 was obtained from Degussa (Germany).

2.2. Catalyst preparation

Bulk g-C3N4 (UCN) was prepared according to the previously reported procedure [36]. Urea (50 g) was heated in a muffle furnace under the temperature programming of 250 oC for 1 h, 350 oC for 2 h and a final temperature 550 oC for 2 h at a heating rate of 2 oC min1. The yielded yellow powder was washed with nitric acid (0.1 mol L1) and distilled water, followed by dried at 80 oC for 12 h. Two-dimensional g-C3N4 nanosheets (UCNS) were prepared following the steps below. First, UCN powder (0.9 g) was dispersed into an aqueous HCl solution (150 mL)

with the concentration of 7.4, 14.8 and 18.9%, respectively. The resulting suspension was sonicated for 1 h followed by magnetically stirring for 24 h. The resulting suspension was transferred to Teflon-lined stainless steel autoclaves and heated to 110 o

C for 5 h. The obtained suspension was pump filtrated and washed for at least 5 times

to remove HCl. Finally, the product was dried at 80 oC for 6 h, and they are denoted as UCNS-1, UCNS-2 and UCNS-3, respectively, representing HCl concentration of 7.4, 14.8 and 18.9% used in the hydrothermal treatment process. For comparison, bulk melamine-derived g-C3N4 (TCN) and its g-C3N4 nanoparticles (TCNP, HCl concentration of 14.8%) were prepared via the same method except for the precursors used in the fabrication of g-C3N4.

2.3. Catalyst characterization

TEM was recorded on a JEM-2100F high resolution transmission electron microscope at an accelerating voltage of 200 kV. XRD patterns were obtained on a Japan Rigaku D/max 2000 X-ray diffractometer (Cu K,  = 1.5418 Å). FT-IR spectra were recorded on a Nicolet Magna 560 IR spectrophotometer. XPS was performed on a VG-ADES 400 instrument with Mg K-ADES source. Nitrogen porosimetry measurement was performed on a Micromeritics ASAP 2020M surface area and porosity analyzer. UV–Vis/DRS were recorded on a Cary 500 UV-Vis-NIR spectrometer.

2.4. Photocatalytic tests

The photocatalytic activity of as-prepared photocatalysts (100 mg) was evaluated by the degradation of an aqueous PNP (10 mg L1, 100 mL) in a self-made quartz reactor fitted with a circulation water system to maintain a constant reaction temperature. The light source was supplied by a 300 W Xe lamp equipped with an IR cut filter and a 400 nm cut filter. The suspension was magnetically stirred in the dark for 60 min to reach the adsorption-desorption equilibrium between PNP and the catalyst, and then it was irradiated under visible-light for 120 min. Fixed amounts of the reaction solution were taken out at given time intervals, followed by centrifugation to remove the photocatalyst powder completely. Changes of the concentrations of PNP during the photocatalytic process were monitored by a Cary 60 UV-Vis-NIR spectrometer at  = 317 nm and an Agilent 1200 HPLC: C18 column; UV detector ( = 320 nm); methanol/water (60/40, v/v); 1.0 mL min1.

2.5. Electrochemistry and photoelectronchemical measurements

Both electrochemistry and photoelectronchemical measurements were carried out in a three electrode setup connected to an electrochemical station (CHI 630E, Shanghai Chenhua, China). The prepared catalyst/Ti or catalyst/FTO sheet was used as the working electrodes, a Pt wire was used as the counter electrode and an Ag/AgCl electrode (saturated KCl) was used as the reference electrode. Na2SO4 aqueous solution (0.01 mol L1, 100 mL) was used as the electrolyte. Details for the preparation of working electrodes are described in electronic supplementary information (ESI). For the photocurrent measurement, a 300 W Xe lamp served as a light source, and the

measurements were carried out at a constant potential of +1.0 V to the working electrode. Mott-Schottky tests were performed with the potentials ranged from 1.5 to 0.5 V at the selected frequencies of 1.0 kHz. Electrochemical impedance spectroscopy (EIS) tests were performed at open circuit potential over frequency range between 1000 to 50 mHz with an AC voltage magnitude of 5 mV, using 12 points/decade.

3. Results and discussion

3.1 Preparation and characterization of g-C3N4 nanosheets

The UCN was prepared by thermal-induced self-polymerization of urea, which resulted in a laminar stacked g-C3N4 with mesoporosity (Scheme 1 and Fig. 1a). It is generally accepted that tri-s-triazine is the primary building block of g-C3N4, which are linked by strand nitrogen atoms to form stable “melon” units [37,38]. The melon units stack together though hydrogen bonds between the strand nitrogen atoms and NH/NH2 groups [27]. During the process of hydrothermal treatment of UCN under acidic environment, the protons can easily intercalate into its layered structure and selectively attach to the strand nitrogen atoms in a manner similar to a Brnsted acid/based interaction. Accordingly, the UCN matrix is greatly swelled, making it ready for delamination [27] (Scheme 1). Moreover, under hydrothermal condition, high temperature and high pressure provide powerful energy to break the hydrogen bonding between melon units, and thereby facilitating the delamination of UCN and forming ultrathin UCNS (Fig. 1b-d). At the same time, various oxygen-containing functional groups such as COH and NCO are introduced into the CN structure of the UCNS,

which may play an important role to prevent the stacking of 2D meterials [34] and inhibit the direct recombination of electrons and holes [32]. The morphology of UCN and UCNSs is revealed by TEM observations (Fig. 1). As shown in Fig. 1a, the UCN displays the folded platelet-like morphology with mesoporosity. After hydrothermal treatment under acidic environment, the UCN undergoes a delamination process and displays nearly transparent morphology, implying the ultrathin nanostructures of the UCNS (Fig. 1b-d). The UCNSs become thinner and smaller with increasing the acidity of the preparation system. However, obvious aggregation of the sheet-like g-C3N4 nanoparticles (e.g. UCNS-3) is observed after treatment of UCN with higher HCl concentration (e.g. 18.5%), and the nanoparticle aggregation may occur during the drying process. The thinner and smaller the UCNS are, the more serious aggregation occurs. A typical AFM image shows that the UCNS-2 is laminar in small size, and the estimated thickness is 15 nm (Fig. S1 of ESI). The peculiar nanostructure endows the UCNS with open-up surface, providing plentiful active sites and shortened diffusion distance. Therefore, the mass transfer and charge separation in nanodomains is expected to be promoted, which can further optimize the photocatalytic activity of g-C3N4. The morphologies of melamine-derived g-C3N4 (TCN) and g-C3N4 nanoparticles (TCNP) are presented in Fig. S2 of ESI. Different from the UCN, the TCN exhibits thick and block morphology, just like the solid rocks; meanwhile, owing to heavy aggregation of TCN particles its size is hardly estimated (Fig. S1a). Compared with urea, melamine is easier to polymerize together accompanying with less NH3 and CO2

gases releasing, and therefore TCN exhibits bulk and solid morphology. Unfortunately, TCN is hardly exfoliated into ultrathin nanosheets. After hydrothermal treatment of TCN under acidic condition, the resulting TCNP exhibits spherical morphology with particle size of ca. 150 nm, and obvious particle aggregation is also observed (Fig. S1b). The presence of C, N and O elements in UCN and UCNS-2 is observed in energy dispersive X-ray spectroscopy (EDS) spectra (Fig. 1e and f), and elemental mapping images verify the uniform distribution of C, N and O elements throughout the UCN and UCNS-2 (Fig. 1g and h). In addition, the content of O element in the UCNS-2 is obviously higher than that of the UCN, indicating the presence of more oxygen element in the UCNS after HCl-assistant hydrothermal treatment. The structural information of as-prepared UCN and UCNSs is provided by XRD, FT-IR and XPS measurements. Firstly, XRD measurement presents the phase structure of the samples, showing the graphitic stacking structure of the UCN and UCNSs (Fig. 2). The peak at 13.1o represents in-plane structural packing motif with a period of 0.675 nm and can be indexed as the (100) crystal plane of g-C3N4, while the typical (002) interlayer-stacking peak at 27.5o corresponds to an interlayer distance of d = 0.33 nm for g-C3N4 [39,40]. The result also indicates that the diffraction intensity of the peak at 27.5o becomes stronger with the acidity of the preparation system increasing, originating from more serious aggregation of the tiny g-C3N4 nanosheets. FT-IR spectra of UCN and UCNSs are shown in Fig. 3. For the UCN, the sharp peak at 810 cm–1 is originated from the bending vibration of s-triazine rings, while a series

of peaks from 1639 to 1208 cm1 are typical stretching vibration modes of CN heterocycles [14,34]. In addition, the broad absorption band located in the range from 3300 to 3000 cm1 is assigned to the stretching vibration of N−H and O−H bonds [41,42]. After hydrothermal treatment of the UCN in HCl solution, all characteristic vibrational peaks related to the g-C3N4 can also be found, indicating that the structural integrity of g-C3N4 remains intact after the delamination. However, the peak intensities in the range from 1639 to 1208 cm1 decreases, indicating that part of CN heterocycles are broken during the process of acid-assisted hydrothermal treatment. The high-resolution XPS surface probe technique can further confirm the local structure of the g-C3N4. In addition, the inner structure transformation during the process of hydrothermal treatment with HCl can also be detected (Fig. 4). As shown in Fig. 4a, three elements of C, N and O are detected in both UCN and UCNS-2. The peak intensity of O1s in the UCNS-2 is stronger than that of the UCN, showing a higher density of oxygen-containing species on the surface of UCNS-2, in accord with the result of EDS spectra. The peak of N1s in UCNS-2 is weaker than that of the UCN, which is probably caused by the replacement of N element with O element in the UCNS-2. Fig. 4b shows the XPS of UCN and UCNS-2 in the O 1s binding energy region. For the UCN sample, the peaks centering at 531.9 and 532.6 eV are assigned to oxygen species from the C=O groups of g-C3N4 and the adsorbed water molecules [43]. As for the UCNS-2 sample, two new O 1s peaks appearing at 531.4 and 533.7 eV are also found besides the aforementioned two peaks, indicating that the additional oxygen

species, NCO and COH groups, present in the UCNS-2 [34,44,45]. Both C 1s spectra of the UCN and UCNS-2 show four characteristic peaks centering at 284.6, 286.1, 287.8 and 289.1 eV (Fig. 4c), corresponding to carbon species from the adsorbed carbon, CNH2 group, sp2 carbon atoms bonded to nitrogen atoms in an aromatic ring [(N)2C=N] and CO bond, respectively [34]. Compared with UCN, the ratio of CNH2 (and CO) bonding to the total C in the UCNS-2 increases. The N 1s spectra of the samples are fitted into three peaks (Fig. 4d). The peak at 398.1 eV is assigned to sp2 hybridized aromatic nitrogen atoms bonded to carbon atoms (CN=C). The peak at 398.9 eV relates to tertiary nitrogen N(C)3 groups linking structural motif (C6N7). The last peak at 400.2 eV corresponds to –NH2 groups [43]. Compared with the UCN, the peak intensity of N(C)3 groups in the UCNS-2 decreases, while the peak intensity of the –NH2 groups increases. On the basis of the above discussion, the layered bulk g-C3N4 is successfully disintegrated under hydrothermal protonation, and the functional groups such as –NH2, COH and NCO are situated at the surface of the resulting UCNS; meanwhile, the primary tri-s-triazine-based framework remained intact. The textural property of as-prepared UCN and UCNSs is characterized by the nitrogen gas porosimetry measurement, and the obtained sorption isotherms and BJH pore-size distribution curves are shown in Fig. 5. All tested samples exhibit type IV isotherms with H3 hysteresis loops, indicating their mesoporosity (Fig. 5a) [36]. Two kinds of mesopores centering at 3.7 and 33.0 nm are found in Fig. 5b, originating from the soft-templates of the released NH3 and CO2 gases during the course of urea

polymerization and the fold of platelet-like g-C3N4, respectively [46]. This excellent porosity property leads to the urea-derived g-C3N4 much larger BET surface area (71 m2 g1) and higher pore volume (0.23 cm3 g1). After hydrothermal protonation of g-C3N4 with HCl, the formed UCNSs also show two types of porous structure, but the pore-size distribution region is different from the UCN. The UCNS-1 and UCNS-2 possess the similar smaller pore size (3.7 nm) to the UCN, however, their larger pores mainly situate in the range of 60 to 70 nm. After hydrothermal protonation and drying process, the UCN is firstly delaminated to the UCNS, and then the sheet-like UCNS begin to aggregation in some extent, which forms a large amount of folded pores with a wider pore size distribution region. Such acidity could not thoroughly destroy the smaller pores belonged to the interior structure of the UCNSs, accordingly, some of the smaller pores of the UCNS-1 or UCNS-2 remain intact. Due to the aggregation among UCNS, they possess smaller BET surface area and lower pore volume compared with the UCN. For examples, the BET surface area and pore volume is 48.2 m2 g1 and 0.15 cm3 g1 for the UCNS-1 and 47.9 m2 g1 and 0.16 cm3 g1 for the UCNS-2. Continuous increasing the acidity, the UCN undergoes much stronger delamination effect, and the process even destroys most of smaller pores, leading to more g-C3N4 fragments in the UCNS-3. These fragments gather tightly to form hard g-C3N4 particles, resulting in the UCNS-3 much smaller BET surface area (18.7 m2 g1) and pore volume (0.06 cm3 g1); meanwhile, it has two kinds of mesopores with the size of 3 and 7 nm, respectively. UV-Vis/DRS measurement is applied to study the light absorption property of the UCN and UCNSs (Fig. 6). As shown in Fig. 6a, UCN shows a typical semiconductor

absorption in the region of 200 to 450 nm, originating from the charge transfer from the valence band (VB) populated by N 2p orbitals to the conduction band (CB) formed by C 2p orbitals [37,38]. As for the UCNS-1 and UCNS-2, they show a slight blueshift of absorption edge, i.e. from 450 to 430 nm. The result is attributed to the quantum confinement effect, implying the reduced particle size of the UCNS-1 or UCNS-2 with respect to the UCN [47,48]. In the case of the UCNS-3, it shows somewhat redshift of the absorption edge in comparison of the UCN, and the absorption edge extends to 550 nm. This is due to the larger particle size of the UCNS-3, originating from the aggregation of the finely divided UCNS-3 nanosheets. During the aggregation process, the nanosheets stacked with each other tightly by intermolecular van der waals force or electrostatic force to form a large π-conjugation system, which results in the redshift of absorption edge of the UCNS-3. For an indirect band-gap semiconductor, the optical absorption near the band edge follows Kubelka–Munk formula (Eq. 1) αhν = A (hν−Eg)1/2

(1)

where α, ν, Eg and A are the absorption coefficient, the light frequency, the band gap and a constant, respectively. The Eg of the UCN and UCNSs are therefore estimated to be 2.82 (UCN), 2.86 (UCNS-1), 2.89 (UCNS-2) and 2.85 eV (UCNS-3), respectively, from the onsets of the absorption edges (Fig. 6b). Besides a suitable band gap, the proper CB and VB levels of the semiconductor are also important for the photocatalytic reaction. Herein, Mott-Schottky plots are applied to verify the flat-band of the UCN and UCNS-2 (the representative UCNS sample),

which can be used to estimate the CB level [49]. As shown in Fig. 7a, both the UCN and UCNS-2 exhibit the positive slope in the linear region, implying n-type semiconductor structure of the g-C3N4. The estimated flat band potentials (0.93 V) of the UCN and UCNS-2 are the same, indicating that the eCB generated on the UCN and UCNS-2 exhibit similar reduction capability. Combined the result of band gap energy and flat band potentials of the UCN and UCNS-2, the VB of UCNS-2 is calculated to be 1.96 V, which is 0.07 V higher than that of the UCN (Fig. 7b). Such enhanced VB position endows hVB+ of the UCNS-2 with stronger oxidizing ability compared with the UCN. The photogenetated charge separation and transfer performance were evaluated by the photoelectrochemistry test and electrochemical impedance spectroscopy (EIS). Fig. 8 shows the photocurrent responses of all tested working electrodes, which are reproducible and stable during three on-off intermittent irradiation cycles. Under visible-light irradiation, all tested UCNS/Ti electrodes present higher photocurrent responses with respect to the UCN/Ti electrode, moreover, the UCNS-2/Ti electrode can produce the highest photocurrent intensity among three UCNS/Ti electrode. The higher photocurrent response reflects the faster separation and transportation of the photogenerated carriers on the surface of the working electrodes. The small sized and sheet-like nanostructures of the UCNS can provide short migration distance for the photogenerated carriers, facilitating them to transfer to the surface of the nanosheets more easily and rapidly. The other reason of higher photocurrent response of the UCNS/Ti electrode than the UCN/Ti electrode is the presence of electrophilic groups

(COH and C=O) at the edge of UCNS, which can efficiently trap the photogenerated electrons and thereby inhibiting the direct recombination of electrons and holes efficiently. The experimental Nyquist impedance plots for the UCN and UCNS-2 are shown in Fig. 9. The Nyquist semicircle of the UCNS-2 shows a smaller diameter compared with the UCN, indicating the enhanced conductivity for electron transfer of the UCNS-2 than the UCN. In summary, urea-derived g-C3N4 with good physiochemical properties is further optimized by HCl-assisted hydrothermal treatment, and the resultant g-C3N4 nanosheets exhibit ultrathin sheet-like morphology with open-up surface, lowered VB level and enhanced conductivity properties. All of the advantages are expected to improve the photocatalytic activity of bulk g-C3N4.

3.2 Photocatalytic studies

The visible-light photocatalytic activity of the UCN and UCNSs is evaluated by the degradation of an aqueous PNP, a light insensitive compound with light response in the UV region. For comparison, TCN and TCNP as well as Degussa P25 TiO2 are also tested under the same conditions. Before light irradiation, the suspensions are magnetically stirred in dark for 60 min to study the adsorption behaviors of the tested photocatalysts (Fig. 10a). It shows that the adsorption-desorption equilibrium between PNP molecules and the photocatalyst reaches after stirring the suspension for 30 min, and the adsorption capacity of P25 TiO2 ,

UCN, UCNS-1, UCNS-2 and UCNS-3 to PNP molecules is 2, 3, 15, 18 and 19%, respectively. The stronger adsorption ability of the UCNSs than that of the UCN is closely related to their ultrathin sheet-like nanostructures and electrostatic interaction between the UCNSs and PNP molecules, which compensates the disadvantage of smaller BET surface area. Since the photocatalytic reaction takes place on the surface of the catalyst, the stronger adsorption ability of the catalyst to the substrate is favorable to improve its photocatalytic activity [50]. Additionally, in the absence of the photocatalyst, the decomposition of PNP is negligible after visible-light irradiation for 120 min, ruling out the direct photolysis of PNP under visible-light irradiation. Over period of 60 min visible-light irradiation of the UCN, UCNS-1, UCNS-2 and UCNS-3, the conversion of PNP reaches to 41, 77, 86 and 79%, respectively. However, under the identical conditions, the conversion of PNP only reaches to 6% by using P25 TiO2 as the photocatalyst. Poor visible-light photocatalytic activity of TiO2 in degradation of PNP is due to its lacking of the light response in visible-light region. The higher visible-light photocatalytic activity of the UCNSs than that of the UCN is mainly attributed to their ultrathin sheet-like nanostructures, which can provide abundant active sites and shortened diffusion length. Accordingly, the accessibility of the active sites to the substrates is improved; meanwhile, mass transfer and charge separation is promoted. In the cases of three tested UCNSs, the visible-light photocatalytic activity enhances with the increasing HCl concentration used in hydrothermal treatment. UCNS-2 possesses the highest photocatalytic activity. However, further increasing the concentration of HCl, the photocatalytic activity of the UCNS-3 begins to decrease. This is due to the serious

aggregation of the ultrathin UCNS-3 nanosheets, resulting in it poor dispersion in an aqueous solution. “Easy dispersion” and the formation of uniform and stable suspension can prevent the UCNS nanosheets from aggregation, a prerequisite to ensure g-C3N4 with excellent photocatalytic activity in the degradation of PNP. Firstly, homogeneous g-C3N4 nanosheets suspension can significantly improve the accessibility of the active sites to PNP molecules. Secondly, the photogenerated electrons and holes can rapidly transfer to the surface of UCNS owing to the short migration path, inhibiting their direct recombination efficiently. Lastly, “easy dispersion” can weaken the intermolecular van der waals force or electrostatic force between nanosheets, reducing the formation of π-conjugation system. Accordingly, the enhanced bandgap with more positive VB level is obtained for the UCNS, endowing hVB+ of the UCNS with stronger oxidizing ability compared with that of the UCN. To quantitatively investigate the reaction kinetics of the PNP photocatalytic degradation, the experimental data are fitted by applying a first order model (Eq. 2) − ln

=

(2)

where c0 and ct are the concentrations of PNP at initial and t time, respectively, and k is the apparent first-order rate constant. As shown in Fig. 10b, the plot of –ln(ct/c0) vs the irradiation time (t) is nearly straight. Additionally, the UCNSs exhibit obviously enhanced photodegradation efficiency with k values are about 4152 and 2.93.7 folds higher than that of P25 TiO2 (k = 0.00070) and the UCN (k = 0.00995), respectively; moreover, the UCNS-2 with kUCNS-2 = 0.03651 shows the highest photocatalytic efficiency compared with UCNS-1 (kUCNS-1 = 0.02874) or UCNS-3 (kUCNS-3 = 0.02344).

Fig. 10c shows the characteristic absorption of PNP at 317 nm gradually decreases during the UCNS-2-photocatalyzed PNP degradation, which is hardly observed after 90 min visible-light irradiation, consistent with the result shown in Fig. 10a. The adsorption performance and photocatalytic activity of the melamine-derived TCN and TCNP are presented in Fig. 10d. The adsorption capacity of the TCN to PNP is 3%, which is similar to that of the UCN. As for the TCNP, the adsorption capacity to PNP reaches to 9%, which is lower than that of the UCNSs. Over period of 60 min visible-light irradiation of the TCN and TCNP, the conversion of PNP reaches to 16% and 45%, respectively. The result indicates that the TCN or TCNP shows much lower photocatalytic activity than that of the UCN or UCNSs. The insert of Fig. 10d shows the plot of –ln(ct/c0) vs t in TCN- and TCNP-PNP system, and the determined kTCN = 0.00278 and kTCNS = 0.01403. Poor photocatalytic activity of the TCNP in comparison of its UCNS counterpart is mainly due to the less exposed active sites. The above result further confirms the important contribution of the ultrathin sheet-like nanostructures of the UCNSs to its excellent photocatalytic activity. The photostability and reusability of the photocatalyst is an important issue for its practical photocatalytic applications. Herein, the photostability and reusability of the UCNS is evaluated by the photocatalytic degradation of PNP for four times by selecting the UCNS-2 as the representative photocatalyst. As shown in Fig. 11a, UCNS-2 shows considerably stable photocatalytic activity toward the degradation of PNP during four consecutive runs, and the conversion of PNP is 98% (1st run), 96% (2nd run), 94% (3rd run) and 93% (4th run), respectively, after 120 min visible-light irradiation. Additionally,

the morphology of the fourth time used UCNS-2 is observed by TEM. As displayed in Fig. 11b, the UCNS-2 still remains perfect ultrathin sheet-like nanostructure without particle agglomeration after four catalytic cycles. Therefore, the UCNS exhibits excellent photostability and reusability, and it can perform as the genuine heterogeneous photocatalysts for the decomposition of aqueous organic pollutants.

3.3 Discussion

In order to better understand the enhanced photocatalytic activity of the UCNS with respect to the UCN in depth, free radical and hole scavenging experiments are carried out to identify the active species generated during the process of the UCN- and UCNS-2-photocatalyzed PNP degradation. In this study, tert-butyl alcohol (t-BuOH), ethylene diamine tetraacetic acid disodium salt (EDTA-2Na) and 1,4-benzoquinone (BQ) are used as the scavenger of hydroxyl radical (OH), hVB+ and superoxide radical (O2), respectively [51,52]. As shown in Fig. 12a, after visible-light irradiation of the UCN for 90 min, the conversion of PNP is 60.9%. The degradation rate of PNP is hardly inhibited after the addition of t-BuOH (1 mmol L1) in the above system, and the conversion of PNP is 58.6% in this case. However, in the presence of EDTA-2Na (1 mmol L1), the degradation rate of PNP is obviously decelerated, and under the same visible-light irradiation time, the conversion of PNP only reaches to 46.0%. The degradation of PNP is significantly inhibited in the BQ (1 mmol L1)UCN system, and only 4.2% of PNP is decomposed after 90 min visible-light irradiation. The above results indicate that both hVB+ and O2 are the main active species that are responsible

for the photooxidization of PNP completely, while OH radicals are hardly generated in current photocatalytic system. The absence of OH radicals is due to the fact that the VB potential of g-C3N4 (1.89 V vs. NHE) is more negative than the standard redox potential of OH/OH (1.99 V vs. NHE) [36], and therefore hVB+ cannot oxidize OH to generate 

OH radicals in the g-C3N4 photocatalytic system (Scheme 2). Similar to the UCN system, O2 radicals and hVB+ are the main active species

generated in the UCNS-2 system, and OH radicals are still hardly found in Fig. 12b. Nevertheless, different from the UCN system, the inhibition effect of EDTA-2Na to the photocatalytic activity of the UCNS is more distinct. After visible-light irradiation of the UCNS-2 for 90 min, the conversion of PNP is 97.0% and 60.9%, respectively, in the scavenger-free and EDTA-2Na system. The result suggests that more hVB+ species exist in the UCNS-2-photocatalytic system, owing to more efficient separation of the eCB and hVB+ at the surface of the UCNS-2. As demonstrated by the XPS analysis, the oxygen-containing functional groups such as COH and C=O are presence in the CN structure of the UCNS, which can not only act as the electrophilic groups to inhibit the direct recombination of eCB and hVB+ [32,53] but also prevent the stacking of 2D materials [54], resulting in the ultrathin g-C3N4 nanosheets with shortened charge diffusion distance. Since the hVB+ possesses extremely strong oxidation ability, more hVB+ species with stronger oxidizing ability in the UCNS-photocatalytic system is one of the important factors that ensures the sheet-like g-C3N4 excellent visible-light photocatalytic activity. Additionally, the ultrathin sheet-like nanostructures of the UCNS with plentiful mesopores can provide more exposed active sites, which can improve the accessibility

of PNP molecules to the g-C3N4. This is another important factor that makes the UCNS enhanced visible-light photocatalytic activity with respect to its bulk counterpart. Last but not least, due to the ultrathin sheet-like nanostructure and the presence of hydrophilic functional groups of NH2 and OH at the edges, the UCNSs can disperse in an aqueous solution easily and homogeneously, which is a prerequisite to bring their unique properties into full play. On the basis of the above discussion, a reasonable mechanism of visible-light photocatalytic degradation of PNP over the ultrathin g-C3N4 nanosheets is revealed and illustrated in Scheme 2. With the help of the hydrophilic functional groups of NH2 and OH at the edges, the g-C3N4 nanosheets can disperse in an aqueous solution easily and homogeneously, which can improve the accessibility of PNP molecules to the g-C3N4. After visible-light irradiation of the g-C3N4 nanosheets, eCB and hVB+ generate on the surface of g-C3N4. Due to small particle size of the g-C3N4 nanosheets and the presence of electrophilic groups (e.g., COH and C=O) at the edge of the g-C3N4 nanosheets, efficient separation of eCB and hVB+ are realized. Subsequently, the eCB react with O2 to produce O2 radicals. Both O2 and hVB+ are responsible for complete degradation of PNP to NO3, H2O and CO2. 4. Conclusions The

ultrathin

g-C3N4

nanosheets

are

successfully

developed

by

hydrothermal-exfoliation of the urea-derived g-C3N4 under acidic condition, and they exhibit obviously enhanced visible-light photocatalytic activity toward the degradation of an aqueous PNP compared with bulk g-C3N4, melamine-derived g-C3N4

nanoparticles or Degussa P25 TiO2. The excellent photocatalytic performance is attributed to the synergic effect of the ultrathin g-C3N4 nanosheets: i) the UCNSs with abundant exposed active sites and shortened diffusion distance is benefit for facilitating the mass and charge transport; ii) the presence of the electrophilic groups of COH and C=O at the edge of the UCNSs can promote the separation of the photogenerated electrons and holes; iii) enlarged band gap of the UCNSs endows hVB+ with stronger oxidizing ability; and iv) easy and homogeneous dispersion of the UCNSs in aqueous solution ultimately brings the above unique properties into full play. The UCNSs also show excellent reusability, and they are the promising visible-light-driven photocatalyst candidate that may find wide applications in environment remediation.

Acknowledgements

This work was supported by the Natural Science Fund Council of China (51608102; 21573038), the Fundamental Research Funds for the Central Universities (2412016KJ032) and Jilin province science and technology development projects (20160520177JH).

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Figures Legend Fig. 1 TEM images of (a) UCN; (b) UCNS-1; (c) UCNS-2 and (d) UCNS-3. EDS spectra and corresponding elemental mapping of UCN (e, g) and UCNS-2 (f, h). The areas marked in (a) and (c) are magnified in (g) and (h).

Fig. 2 XRD patterns of the UCN and UCNSs.

Fig. 3 FT-IR spectra of the UCN and UCNSs.

Fig. 4 XPS survey (a) and high-resolution spectra in O 1s (b); C 1s (c) and N 1s (d) binding energy regions of the UCN and UCNS-2.

Fig. 5 Nitrogen gas adsorption-desorption isotherms (a) and BJH pore-size distribution curves (b) of the UCN and UCNSs.

Fig. 6 UV-Vis/DRS of UCN and UCNSs (a) and the corresponding band gap plots (b).

Fig. 7 Mott-Schottky plots (a) and schematic illustration of the band structure (b) of the UCN and UCNS-2.

Fig. 8 Photocurrent responses of the UCN and UCNS electrodes in 0.01 mol L1 Na2SO4 electrolyte solution under Xe irradiation. The working electrode potential is constant at +1.0 V.

Fig. 9 EIS Nyquist plots of the UCN and UCNS-2.

Fig. 10 (a) Adsorption property and photocatalytic activity of UCN and UCNSs towards the degradation of PNP under visible light irradiation; (b) the kinetic fit for PNP degradation and (c) the UV-Vis absorption spectra of PNP solution as a function of irradiation time during the UCNS-2-photocatalyzed PNP degradation process; (d) adsorption property and photocatalytic activity of TCN and TCNP towards the degradation of PNP under visible light irradiation and the kinetic fit for PNP degradation (insert of Fig. 9d). Catalyst amount 100 mg; c0 = 10 mg L1; volume 100 mL.

Fig. 11 (a) Recycling experiments of visible-light photocatalytic degradation of PNP over UCNS-2. Catalyst amount 100 mg; c0 = 10 mg L1 ; volume 100 mL; (b) TEM image of the fourth used UCNS-2.

Fig. 12 Influence of various scavengers on the visible-light photocatalytic activity of (a) UCN and (b) USNS-2 towards the degradation of PNP. Catalyst amount 100 mg; c0 = 10 mg L1; volume 100 mL; illumination time 90 min.

Scheme 1 Illustration of the route of the preparation of ultrathin g-C3N4 nanosheets.

Scheme 2 Mechanism illustration of visible-light photocatalytic degradation of an aqueous PNP over the g-C3N4 nanosheets.