g-C3N4 nanosheet semiconductors for photocatalytic hydrogen generation

Applied Catalysis B: Environmental 261 (2020) 118249

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Inter-plane heterojunctions within 2D/2D FeSe2/g-C3N4 nanosheet semiconductors for photocatalytic hydrogen generation

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Jia Jiaa, Wenjuan Suna, Qiqi Zhanga, Xiaozhuo Zhanga, Xiaoyun Hub, Enzhou Liua,⁎, Jun Fana,⁎ a b

School of Chemical Engineering, Northwest University, Xi’an, 710069, PR China School of Physics, Northwest University, Xi’an, 710069, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: g-C3N4 FeSe2 2D/2D heterojunction H2 evolution H2O2 decomposition

Developing and designing a robust hydrogen generation photocatalyst for water splitting remains a huge challenge for realizing highly effective conversion of solar energy into chemical fuel. Herein, two-dimensional FeSe2/g-C3N4 inter-plane heterostructures (2D/2D FeSe2/CNNS) were rationally constructed via the in-situ deposition of FeSe2 nanosheets on the g-C3N4 surface. The resulting 15% FeSe2/CNNS 2D/2D composite exhibited an optimal H2 generation rate of 1655.6 μmol∙h−1 ∙g−1 in Na2S/Na2SO3 solution, being nearly 2.65, 1.73 and 1.19 times higher than that of pristine g-C3N4, FeSe2 and corresponding 0D/2D FeSe2/CNNS nanocrystals, respectively. Such remarkably improved photocatalytic performance could be ascribed to efficient charge carrier mobility, acceleration of H2O2 decomposition via a stepwise two-electron/two-step pathway, and the formed 2D heterojunction interfacial contact between g-C3N4 and FeSe2 nanosheets. This work can provide new insight for designing atomic-level structural and interfacial 2D nanojunctions to steer charge separation and transportation in the nanocomposite.

1. Introduction Photocatalysis is considered to be an advanced and promising technology to convert solar energy into high-density hydrogen energy for chemical fuel production and environmental remediation [1–5]. Graphitic carbon nitride (g-C3N4), a two-dimensional (2D) layered polymer semiconductor, has been developed as an ideal candidate material for water splitting driven by visible light because of its appropriate bandgap (∼2.7 eV), abundant exposed active sites, shortened photocarrier migration distance and favorable chemical stability [6,7]. Nevertheless, the intrinsic bottlenecks of single-component g-C3N4 have still severely retarded its practical application in the photocatalytic hydrogen evolution reaction (HER), as follows: i) poor visible light utilization, rapid carrier recombination and low quantum efficiency; ii) sluggish reaction kinetics originating from complex four-electron water oxidation to produce O2; and iii) the kinetically competing two-electron reaction to hydrogen peroxide (H2O2) often poisons the catalysts [8,9]. In this regard, a multitude of modification techniques have been adopted to further conquer these shortcomings and improve its photocatalytic performance, including noble metal or cocatalyst deposition (Pt, Au, Ag and transition-metal oxides/sulfides/hydroxides/phosphides etc.), element doping, morphology control, and heterojunction construction [10,11]. Among them, several kinds of g-C3N4-based 0D/ ⁎

2D heterojunction composites have been extensively designed and fabricated in order to solve the above obstacles to photocatalysis, such as Cu3P/g-C3N4 [12], Bi3TaO7/g-C3N4 [13], NiS/g-C3N4 [14], ZnO/gC3N4 [15]. These heterostructures not only can accelerate photocarrier separation, resulting from an effective transfer channel at their heterojunction interface, but can also provide massive reactive active sites for water splitting, CO2 reduction and pollutant degradation, thus elevating the catalytic efficiency of single catalyst. Noteworthy, a more recent report uncovered that a highly effective electrocatalyst toward HER can also serve as an excellent H2 evolution photocatalyst. And the g-C3N4 with rich pyridine-like nitrogen atoms and nanocavities, can provide abundant pairs of lone electrons prone to capture and stabilize transition metal ions, thereby forming an intimate heterojunction interface and triggering effective photocatalytic reaction [6,11]. Accordingly, coupling of g-C3N4 with various transition metal dichalcogenides (TMDCs, MX2, M = Fe, Co, Ni, Mo, W; X = Se and S), as a family of unique electrocatalysts, has attracted considerable attention owing to their exciting metal active sites, outstanding electrical conductivity and good H+ adsorption [16]. For instance, He et al. employed nanocoral-like NiSe2/g-C3N4 composites as the highly efficient photocatalyst for water splitting under alkaline conditions [17]. Similarly, g-C3N4 nanosheets incorporated with CoSe2 nanorods were also successfully fabricated by a simple hydrothermal route, which can

Corresponding authors. E-mail addresses: [email protected] (E. Liu), [email protected] (J. Fan).

https://doi.org/10.1016/j.apcatb.2019.118249 Received 12 June 2019; Received in revised form 11 September 2019; Accepted 30 September 2019 Available online 01 October 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.

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afford a photocurrent density of -4.89 mA/cm2 at 0 V vs. RHE and favor the H2 generation reaction with excellent durability under visible light [18]. Undoubtedly, the studies above showed the prominent merits of TMDCs-based catalysts for HER. On the other hand, photocatalytic H2 and O2 generation from water splitting, in general, requires a standard free energy of 237 kJ/mol (or 1.23 eV per electron). It refer to four-proton and four-electron transfer for the eventual formation of an OeO bond [19]. Moreover, the O2 generation by a four-electron reaction process (1.23 eV) is thermodynamically more favorable than H2O2 formation by a two-electron process (1.78 eV), while the higher reaction rate could be kinetically achieved by a stepwise two-electron, two-step reaction by oxidizing water to H2O2 and H2, followed by H2O2 decomposition to O2 and H2O [9]. Further investigations also demonstrated that the generated H2O2 on the g-C3N4 surface had a negative effect on the activity of photocatalysts, and was difficult to remove during the photocatalytic water splitting process. Hitherto, there are few reports on suppressing the formation of H2O2 and subsequently promoting its decomposition over the g-C3N4 surface. Particularly, as a member of the TMDCs, FeSe2 not only exhibits good electrical conductivity with a resistivity of less than 10−3 Ω∙cm, which is much more favorable for photocatalytic or electrocatalytic water splitting, but can also be employed as a catalase with abundant [FeFe]-hydrogenase active centers to promote H2O2 decomposition [16]. Considering these facts, we first designed and fabricated 0D/2D FeSe2/g-C3N4 nanocomposites using a successive high-temperature calcination and in situ hot-injection method, exhibiting an increased H2 generation rate of 1395.4 μmol∙ h−1∙ g−1 (detailed analysis see supplementary text, Figs. S1–6 and Table S1). Despite such progress, small point-contact area between 0D nanoparticles and 2D nanosheets will still lead to limited interfacial charge migration and low photocatalytic efficiency (Fig. 1a). Consequently, it is highly desirable to explore and synthesize an abundant and intimate interface contact between two semiconductor materials, yet this remains a grand

challenge. Very recently, the 2D/2D heterojunction nanostructures with vertically stacked van der Waals materials have rendered their special advantages in the fields of photocatalysis and photoelectrocatalysis [20]. This has triggered widespread research interest as a fascinating catalytic system to elevate and realize practicable solar-to-hydrogen conversion efficiency (STH), such as the following (Fig. 1b): first, a large number of intimate contact interfaces can be formed between two semiconductors, being beneficial for exposing high density active sites, dissociating the excition and improving photocatalytic activity. Second, the recombination of photoexcited carriers can be restrained due to a large interfacial contact area obtained from the larger lateral size of the 2D/2D nanojunction. Third, matching band potential as well as alleviating agglomeration and photocorrosion further lead to the enhancement of catalytic activity and stability [21]. Until now, a series of 2D/2D heterojunction photocatalytic systems were successfully constructed and manifested with enhanced catalytic activity. For instance, Yu et al. reported that ultrathin layered 2D/2D WO3/g-C3N4 heterostructures with face-to-face contact were fabricated by a simple and effective electrostatic self-assembly method, exhibiting rapid carrier transfer and better H2 generation activity compared with pure WO3 and g-C3N4 nanosheets [22]. Majima and coworkers also discovered a noble metal-free 2D/2D heterostructured CdS/WS2 nanosheet, in which WS2 can serve as an electron-trapping site and a cocatalyst for efficient HER [23]. Z-scheme heterojunctions of 2D/2D Bi2WO6/BP nanosheets were successfully synthesized and utilized as photocatalysts for NO removal and water splitting [24]. Moreover, other 2D/2D heterostructures were also designed as HER photocatalysts, containing MoS2/Cu-ZnIn2S4 [25], Co(OH)2/g-C3N4 [26] and RGO/TiO2 [27]. Inspired by these explorations mentioned above, the 2D/2D FeSe2/g-C3N4 inter-plane heterostructures with the aims of increased heterojunction contact area and H2O2 decomposition promotion were constructed to further achieve improved HER efficiency. Meanwhile, to the best of our

Fig. 1. Schematic diagram of contact interfaces of (a) 0D/2D and (b) 2D/2D heterostructures. (c) A schematic image for the synthesis procedures of 2D/2D FeSe2/ CNNS inter-plane heterostructures. 2

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respectively.

knowledge, this photocatalytic system has not been reported to date. In this work, both types of composite photocatalysts, namely the 0D/2D and 2D/2D FeSe2/CNNS heterostructures, were rationally designed and fabricated by facile and similar in-situ interfacial engineering routes. Notably, the 2D/2D 15% FeSe2/CNNS composites exhibit superior photocatalytic H2 evolution activity (1655.6 μmol∙ h−1∙ g−1) without any cocatalyst in Na2S/Na2SO3 aqueous solution and good stability for 12 h, as well as simultaneous removal of Cr(VI) and methylene blue under solar light irradiation. Most importantly, compared to the traditional one-step four-electron reaction, FeSe2/CNNS catalyzes water splitting to H2 evolution via a stepwise two-electron, two-step reduction process, based on active free radical trapping and H2O2 detection experimentation. Such 2D/2D inter-plane heterostructures concurrently realize the efficient separation and transportation of photocarriers, acceleration of H2O2 decomposition and surface proton reduction with plenty of surface-active-sites for HER as well as strong visible light harvesting ability. This heterojunction engineering work not only provides a promising strategy in exploiting highly efficient g-C3N4-based photocatalytic materials for hydrogen evolution reaction, but also paves a suitable pathway for designing atomic-level structural and interfacial 2D nanojunctions to steer charge separation and transportation in nanocomposite photocatalytic systems.

2.3. Characterization A set of characterization methods, for example FL (Edinburgh FLS980, Fluorescence spectrophotometer with an F900 Xe lamp), PL (Hitachi F-7000, Photoluminescence spectra), DRS (Shimadzu, UV3600, UV–vis-NIR spectrophotometer), BET (Quantachrome NOVA 2000e, N2 adsorption and desorption isotherm), XPS (Kratos AXIS NOVA spectrometer, X-ray photoelectron spectrometer), TEM (Tecnai G2F20S-TWIN, Transmission electron microscope), SEM and EDS (JEOLJSM-6390 system, Scanning electron microscope with X-ray energy-dispersive spectrometer), AFM (Multimode III, Atomic force microscope), ICP-OES (PerkinElmer 8300, Inductively coupled plasma optical emission spectrometer), XRD (Shimadzu PXRD-6000, Powder Xray diffractomete) and FT-IR (PerkinElmer Frontier, Fourier transform infrared spectra), were utilized to detailedly analyze time-resolved fluorescence decay spectra, steady state photoluminescence spectra, UV–vis-NIR diffuse reflectance spectra, pore structure and specific surface area, morphology and microstructure, crystalline structure, and surface composition and chemical states. Besides, the water contact angle (WCA) of the samples was also measured on a dataphysics (Contact angle system OCA20). All of photoelectrochemical analyses, including transient photocurrent (I-t curve, at 0.5 V potential vs. SCE), electrochemical impedance analysis (EIS, at 0.5 V potential vs. SCE, frequency range from 10−2 to 105 Hz), HER polarization curves (−1 V to 0 V vs. SCE), MottSchottky plots (MS, −1.3 V to 0.7 V vs. SCE), were conducted on an electrochemical workstation (CHI660E) equipped with a typical threeelectrode cell under 300 W Xenon lamp and bubbling with N2 before measurement, in which the working electrode, counter electrode and reference electrode were photoanode, Pt, and saturated calomel electrode, respectively. Typically, the working electrodes were fabricated by dripping the mixed solution directly onto an FTO conductive glass surface and naturally drying at 25 °C; that is, 10 mg of sample was added into 1 mL of mixed solution (the volume ratio of ethanol/water is 1:1) and ultrasonically treated several times. Besides, the electrolyte solution and irradiation area was Na2SO4 solution (0.5 mol/L, pH = 7.0) and 0.785 cm2, respectively.

2. Experimental section 2.1. Synthesis of bulk g-C3N4 and g-C3N4 nanosheets Bulk g-C3N4 was synthesized according to the literature reports, i.e., via a high temperature calcination of 10 g of melamine in a corundum crucible at 550 °C for 3 h [28]. After being cooled to ambient temperature naturally, a light yellow g-C3N4 powder was obtained. Subsequently, the layered g-C3N4 nanosheets were also synthesized according to the following process: typically, the bulk g-C3N4 powder above (approximately 1 g) was dispersed into 100 mL deionized water under continuous stirring and maintained under ultraphonic treatment for 3 h at 30 °C temperature, and then a large number of g-C3N4 nanosheets could be collected after centrifugation and washed three times with a mixed solution of ethanol and water, dried at 60 °C overnight and abbreviated as CNNS.

2.4. Photocatalytic performance experiment 2.2. Synthesis of 2D/2D FeSe2/CNNS and 0D/2D FeSe2/CNNS composites The photocatalytic H2 evolution performance of various catalysts was tested by using 0.15/0.35 mol/L Na2S/Na2SO3 aqueous solution with 30 mg of sample under light irradiation (300 W Xenon lamp, Microsolar 300UV) at ambient temperature. Prior to illumination, the photocatalytic reactant mixture solution was vacuumed several times by bubbling N2 gas in order to take out any gases. Subsequently, at 30 min time intervals, the amount of obtaining H2 generation in reaction system was analyzed by a GC7900 gas chromatography (with a TCD thermal conductive detector and using nitrogen as the carrier gas). Besides, the photocatalytic simultaneous removal of methylene blue (MB, 10 mg/L) and Cr(VI) (20 mg/L) were also carried out under solar light irradiation. Before reaction, 30 mg of catalyst was put into 50 mL of mixed solution including Cr(VI) and MB, and was stirred in darkness for 30 min to achieve the adsorption and desorption equilibrium between catalyst and pollutants. At interval of 20 min, 4 mL of reaction liquid was collected, centrifuged and filtered, then was measured by a UV–vis spectrophotometer at maximum absorption wavelength of Cr (VI) (λmax =354 nm) and MB (λmax =654 nm). The colorimetric method was employed to detect the formation of H2O2 generated on the resulting sample surface. Briefly, a certain amount of N,N-diethyl-p-phenylenediamine (DPD) and horseradish peroxide (POD) mixed solution was added into H2O2-containing solution at room temperature, then subsequently measured by a UV-3600 spectrophotometer. The electron spin resonance spectroscopy (ESR)

First, 0.052 g of Fe(acac)3 (0.1 mmol) was added into a mixed solution of 250 μL of oleic acid (OA), 15 mL of 1-octadecene (ODE) and 10 mL of oleylamine (OLA) in a 50 mL three-neck flask to obtain a welldistributed solution under a N2 atmosphere with vigorous stirring [29]. Moreover, 0.2 g of g-C3N4 nanosheets from the above ultraphonic treatment were also added into mixed solution and stirred for 10 min. The solution was rapidly heated to 175 °C and kept for 5 min to form a light brown solution. After that, 1 mL of OLA solution including 0.0237 g Se powder was quickly injected (within less than 5 s) with a needle tubing and kept for 30 min to generate a substantial amount of black solid precipitate, which demonstrated the formation of 2D/2D FeSe2/CNNS heterostructures. Lastly, the resulting precipitates were collected by centrifugation and washed three times using chloroform and ethanol mixed solution to dislodge the residual reagents, and then dried at 60 °C overnight under N2 airflow. Meanwhile, this sample was named as 15% FeSe2/CNNS. The 2D/2D x%-FeSe2/CNNS heterostructures with different mass contents of FeSe2 were synthesized by a similar preparation method (x = 1, 5, 7.5, 10, 12.5, 17.5, and 20, representing experimental loading amount of FeSe2). In addition, pure FeSe2 nanosheets were prepared by the same methods and conditions without adding CNNS, and 0D/2D FeSe2/CNNS heterostructures were also synthesized when the temperature was heated to 225 °C under N2 atmosphere, 3

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S8), pure g-C3N4 displays a strong single Lorentzian line due to abundant unpaired electrons on sp2-C atoms of the heptazine rings. In contrast, the weaker EPR spin intensity of FeSe2/CNNS heterostructures can be found, implying a significantly reduced concentration of unpaired electrons and that the FeSe2 nanosheets are successfully deposited on the g-C3N4 surface in situ [33,34]. The isotherms of all materials illustrate type II curves with H3 hysteresis loops, affirming the lamellar stacking of nanosheets (Fig. 2c and Table S1) [35]. The BET specific surface area of 15% FeSe2/CNNS composite decreases to 34.58 m2/g, compared with those of pure CNNS (47.83 m2/g) and FeSe2 (39.61 m2/ g), which is probably attributed to the intimate integration and the formation of a large number of nanoheterojunctions between FeSe2 and g-C3N4 nanosheets. Besides, the atomic force microscopy (AFM) image of FeSe2/CNNS composites reveals that the heterojunction nanosheet thickness is ca. 21.1 nm (Fig. 2d) [36]. The morphology and microstructure of obtained samples can be observed by TEM and SEM analysis. As displayed in Fig. 3a and b, bare g-C3N4 shows a typically amorphous 2D ultrathin nanosheet morphology with a smooth surface and transparent features, which agrees with previous reports [37]. TEM image (Fig. 3c) exhibits that FeSe2 crystals consist of a thin nanosheet-like structure. The clear lattice fringe with d-spacing of 0.256 nm can be indexed as the (111) crystal plane of FeSe2 (Fig. 3d) [38]. For FeSe2/CNNS composite (Fig. 3e and f), not only an amorphous g-C3N4 nanosheet, but also the characteristic interplanar spacing of FeSe2 (d = 0.256 nm) can be clearly observed, thus confirming the presence of folded 2D/2D layered heterostructures and tight contact between the two semiconductor materials. Such an intimate interface between FeSe2 and g-C3N4 enables stability and facilitates the separation and migration of photocarriers, promoting photocatalytic HER efficiency. SEM images reveal that pristine FeSe2 possesses a randomly crumpled and aggregated nanosheet structure (Fig. 4a) and that it interlaces closely with g-C3N4 nanosheets (Fig. 4b). It is also difficult to distinguish between FeSe2 and g-C3N4 nanosheets by SEM observation, indicating good 2D heterojunction interface contact and offering more active sites for photocatalytic reactions. Inductively coupled plasma optical emission spectrometry (ICP-OES) indicates that the mass content (mass ratio) of Fe to g-C3N4 in 15% FeSe2/

was analyzed on a Bruker A300 EPR spectrometer at 300 K and 9.86 GHz in order to detect various active radicals during the photocatalytic reaction process. 3. Results and discussion The synthesis procedure of 2D/2D FeSe2/CNNS nanocomposites is schematically described in Fig. 1c. Initially, the layered g-C3N4 nanosheets were synthesized via a successive melamine thermal condensation method, followed by bulk g-C3N4 liquid-exfoliation in aqueous solution. Subsequently, the g-C3N4 nanosheets were transferred into a mixed solution containing oleic acid (OA), 1-octadecene (ODE) and oleylamine (OLA), as well as different amounts of Fe(acac)3 and Se powder, and then quickly heated to 175 °C for 30 min in a N2 atmosphere. After cooling to ambient temperature, the resulting black powders were centrifuged and washed using a chloroform and ethanol mixed solution. As expected, when FeSe2 nanosheets were formed insitu on the surface of g-C3N4 nanosheets, the 2D/2D FeSe2/CNNS interplane heterostructures were successfully constructed, as observed by AFM and TEM analysis. The crystal structures of resulting materials were measured from powder XRD measurements. Fig. 2a exhibits that two characteristic diffraction peaks located at 13.1° and 27.3° can be attributed to the (100) and (002) planes of g-C3N4, corresponding to the planar packing and stacking of the conjugated aromatic systems, respectively [30]. Moreover, new diffraction peaks (31.1°, 34.9°, 36.3°, 48.2°, 49.3°, 54.1°, 57.5°, 59.9° and 64.1°) can be observed in the FeSe2/CNNS composites, respectively, which accurately belong to the (101), (111), (120), (211), (220), (031), (131), (310) and (122) planes of orthorhombic FeSe2 (JCPDS 21-0432) (Fig. S7) [29,31]. The Fourier transform infrared (FTIR) analysis shows that all major absorption peaks at 650–1000 cm−1, 1200–1800 cm−1 and 2900–3500 cm−1 can be discovered in the CNNS and FeSe2/CNNS hybrids, deriving from the tri-triazine ring units, aromatic CeN heterocycles and HOe bands stretching vibration modes of g-C3N4, respectively (Fig. 2b). There is no remarkable difference in the bending and stretching vibrations, validating the effective preservation of the major chemical structures of g-C3N4 [32]. In EPR spectra (Fig.

Fig. 2. (a) Powder XRD patterns and (b) FT-IR spectra of g-C3N4 nanosheets and FeSe2/CNNS composites. (c) N2 adsorption/desorption isotherms of resulting materials. (d) AFM image of FeSe2/CNNS composites. 4

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Fig. 3. TEM images of (a, b) pure g-C3N4, (c, d) FeSe2 nanosheets and (e, f) 15% FeSe2/CNNS composites.

survey spectra (Fig. 5a) illustrate the existence of C 1s and N 1s elements in pure g-C3N4 as well as the detection of Fe 2p and Se 3d in FeSe2/CNNS composites, certifying the successful fabrication of FeSe2/ CNNS. These results are consistent with XRD and TEM (Figs. 2a and 3). In the C 1s spectra (Fig. 5b), two distinct diffraction peaks situated at 288.3 and 284.8 eV are associated with sp2-hybridized carbon in Nincluding aromatic rings (NeCN]) and standard sp2 CeC bonds, respectively. The high-resolution N 1s XPS signals of g-C3N4 can be primarily divided into three peaks at 400.6, 399.7, and 398.1 eV, as depicted in Fig. 5c, which are ascribed to the tertiary nitrogen N-(C)3

CNNS is approximately 7.64 wt%. Moreover, the atomic ratio of Fe, Se, C, N is about 1.08: 2.07: 49.36: 46.85 as determined from EDS analysis. Additionally, EDX elemental mapping images represent that the Fe, Se, C, and N elements can be observed and are homogeneously distributed in the stacked sheets (Fig. 4e–i) [39]. These results strongly indicate that 2D/2D FeSe2/CNNS inter-plane heterostructures have been successfully constructed. Fig. 5 shows the X-ray photoelectron spectroscopy (XPS) of pure gC3N4 and 15% FeSe2/CNNS samples, which are employed to gain insight on the elemental composition and chemical states of surface atoms. The

Fig. 4. SEM images of (a) FeSe2 nanosheets and (b) 15% FeSe2/CNNS. The (c and d) EDS analysis, (e) SEM and (f–i) corresponding SEM-EDS mapping of C, N, Fe and Se elements for the 15% FeSe2/CNNS composites. 5

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Fig. 5. (a) XPS survey spectra of and spectra of (b) C 1s, (c) N 1s, (d) Fe 2p, and (e) Se 3d for pure g-C3N4 and 15% FeSe2/CNNS composites.

groups, charging effects, and sp2-bonded N in the triazine rings (CeNC]), respectively [40]. Fig. 5d exhibits that double peaks at binding energies of 725.0 and 711.5 eV belong to Fe 2p1/2 and Fe 2p3/2 of Fe2+ in FeSe2/CNNS samples, and an additional characteristic peak centering at 714.6 eV can be indexed to a higher oxidation state of iron species. The deconvolution peaks of Se 3d3/2 and 3d5/2 of Se− are observed at 59.1 and 55.7 eV. [41]. Interestingly, the XPS peaks of N 1s at 398.1 eV, 399.7 eV and 400.6 eV for pure g-C3N4 shift gradually to approximately 0.4 eV higher binding energies, meanwhile, XPS peaks of Fe 2p and Se 3d for FeSe2 move to lower energies in 15% FeSe2/CNNS, respectively, indicating that a strong interfacial interaction between two materials has been formed, and FeSe2 species modification causes the slight electron transfer from g-C3N4 to FeSe2 (Figs. 5c and S9) [42]. The photocatalytic HER activities of pure FeSe2, g-C3N4, and FeSe2/ CNNS heterojunction composites with different loading amounts of FeSe2, were determined under solar light irradiation using Na2S/ Na2SO3 (0.15/0.35 mol/L) aqueous solution as a sacrificial agent without any cocatalyst. The blank experiment (no catalyst) shows that there is no H2 evolution in this reaction process, indicating that water splitting for H2 generation is a nonspontaneous process. Pure g-C3N4 and FeSe2 yield low H2 generation rates of 623.7 and 955.3 μmol h−1 g−1, respectively (Fig. 6a and b). However, most of the

composites display a gradually enhanced H2 evolution performance with increasing FeSe2 content, and achieving a maximum for 15% FeSe2/CNNS, and then decrease (Fig. 6c and d). The H2 evolution rate of the 15% FeSe2/CNNS sample is 1655.6 μmol h−1 g−1, which is nearly 2.65 and 1.73 times than those of pure g-C3N4 and FeSe2, respectively. These composites exhibit superior photocatalytic H2 evolution performance compared to other 2D/2D g-C3N4-based materials, for example, phosphorene/g-C3N4 (571.0 μmol h−1 g−1) [43], Bi2MoO6/g-C3N4 (563.4 μmol h−1 g−1) [44], and Ti2C/g-C3N4 (950.0 μmol h−1 g−1) [45]. Moreover, bare g-C3N4 catalyst is also treated by the same solvothermal process, and it shows an unremarkable change in the H2 evolution rate (616.1 μmol h−1 g−1) (Fig. S10). The results above indicate that the positive effects of FeSe2 and 2D/2D inter-plane heterojunctions are able to accelerate the transfer of photoexcited electrons, thus achieving enhanced H2 generation activity. In addition, the stability of the heterojunction catalyst is also an important factor for photocatalytic H2 generation, so two successive cyclic experiments for 15% FeSe2/CNNS composites are performed under the same condition. There is no obvious decrease in amount of the water splitting to generate H2 (Fig. 6e); meanwhile, XRD and XPS analysis also show that the crystal structure and surface composition of samples before and after used are well maintained (Figs. 6f and S11). 6

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Fig. 6. Photocatalytic H2 evolution curves (a and c) and rates (b and d) for pure g-C3N4, FeSe2 and various FeSe2/CNNS heterostructures. The stability experiment of (e) H2 evolution cycles and (f) XRD for used 15% FeSe2/CNNS composites.

fracture of eNeCe and eSeCe bond. Second, it further degrades to structure 3 (m/z = 167.01), structure 4 (m/z = 150.11) and structure 5 (m/z = 114.09), respectively. Third, these benzene derivatives can generate structure 6 at m/z 132.1 with the ring-opening reactions of benzene ring. Finally, with the prolongation of irradiated time, all the intermediate products will be decomposed to H2O and CO2. The total organic carbon measurement (TOC-L, CPN) was also employed to evaluate the extent of mineralization of MB during the photocatalytic degradation process. As shown in Fig. S16, after 2.0 h light irradiation, about 63.4% of organic carbon is removed from the MB solution, which suggests that MB molecules are easily mineralized. Therefore, it can be seen that the resulting composite sample has not only efficient HER activity but is also a promising water treatment catalyst. In general, good interfacial contact between the reactant molecules in water and the catalyst surface is a prerequisite for achieving an effective photocatalytic reaction process [47]. When the contact angle between water and material is less than 90.0°, the surface of this material is hydrophilic. As presented in Fig. 8, the water contact angle of pure g-C3N4 is 13.1°, which means that the water could wet the g-C3N4 surface. After the introduction of FeSe2 nanosheets, the contact angle of the FeSe2/CNNS composites increases to 43.7° (but it is still lower than 90°), indicating that the FeSe2/CNNS is also hydrophilic and beneficial for contact between catalysts and reactive molecules. Based on the results of increased contact angle and decreased surface area (Fig. 2c), the

Furthermore, the simultaneous photocatalytic reduction of Cr(VI) and oxidation of MB over 15% FeSe2/CNNS materials under light irradiation were carried out to further investigate the underlying application value for these photocatalysts. As shown in Fig. 7, pristine g-C3N4 exhibits good adsorption ability in darkness, which is in agreement with BET analysis (Fig. 2c). The composite sample possesses the highest photocatalytic performance, with removal efficiency of 92.6% for Cr (VI) and 99.8% for MB molecules in 120 min under solar light, versus that of g-C3N4 (44.7% for Cr(VI) and 66.1% for MB). Moreover, the corresponding apparent pseudo-first-order kinetic equation: -ln(C/Co) = Kappt could be used to fit these experimental data, in which Co and C are the initial concentration of corresponding reactants and the concentration with different light illumination time (t), and Kapp is the removal rate constants of Cr(VI) and MB molecules (min−1), respectively [46]. Additionally, the removal rate constant (Kapp) of Cr(VI) and MB are also calculated as 0.0128 and 0.0310 min−1 for 15% FeSe2/ CNNS catalyst, respectively, which are 4.41 and 4.77 times higher than that of g-C3N4 (0.0029 and 0.0065 min−1, Fig. S12). The intermediate products from degradation of MB were further identified by Liquid Chromatography–Mass spectroscopy. The predicted products and possible degradation pathway of MB over 15% FeSe2/CNNS composite were illustrated in Table S2 and Figs. S13–15. First, it can be seen that the structure 1 protonated ion peak at m/z 284.12 is corresponding to MB, which can be decomposed to structure 2 at m/z 308.21 owing to the 7

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Fig. 7. Photocatalytic performance of pure g-C3N4 (a and b) and 15% FeSe2/CNNS (c and d) catalysts for removal of Cr(VI) (354 nm absorption peak) and methylene blue (MB, 654 nm absorption peak) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

potential. Moreover, the potentials of the CB of FeSe2 and g-C3N4 are approximately −0.51 and −1.28 V vs. SCE (namely, −0.27 and −1.04 V vs. NHE (normal hydrogen electrode)), respectively. Thus, the valance band (VB) edge potentials of FeSe2 and g-C3N4 are also calculated to be +0.82 and +1.67 eV, respectively, based on the analysis of band gaps (Eg) and the equation Eg = EVB - ECB [50]. More importantly, an obvious positive shift of Efb potential (∼ 0.53 eV vs. SCE) for the FeSe2/CNNS electrode can be observed, indicating a faster charge transfer rate at the interface between FeSe2 and g-C3N4. Because of good interface contact and rapid carrier migration, the FeSe2/CNNS composite also exhibits a much smaller arc radius, namely, a lower interfacial resistance for photocarrier transfer, than that of bare g-C3N4 under light irradiation (or without light) according to the results of EIS measurement (Fig. 9c) [49,51]. Moreover, the Bode image analysis from Fig. 9d shows two typical phase angle peaks, resulting from two interfacial migration kinetics and two time constants within the semiconductor-electrolyte interface and semiconductor interior. Meanwhile, the relating equivalent circuit is employed to fit EIS measurement data (Fig. S19), in which Ro, R1-DP, CPE1-DP, R2-DP and CPE2-DP represent the series impedance of this system, the impedance and the capacitance of space charge and double layer, respectively. As is known to all, the high frequency time constant is our major concern;

significantly improved photocatalytic performance for resulting composites could be attributed to the formed 2D/2D inter-plane heterojunctions between g-C3N4 and FeSe2 nanosheets. Fig. 9a depicts the optical properties of all samples by analyzing UV–vis absorption spectra. The absorption edge of pristine g-C3N4 nanosheets is around 460 nm [48]. With the introduction of FeSe2, the 15% FeSe2/CNNS composites show almost identical absorption edges and obviously enhanced photoharvesting within the 450 and 1200 nm wavelength range compared to bare g-C3N4, indicating no apparent effect on the band gap of g-C3N4. Moreover, FeSe2 nanosheets exhibits very broad absorption in the entire wavelength range (from 200 to 1200 nm). This increased absorption intensity in the visible light region would be advantageous to the hydrogen generation reaction. In addition, the bandgap energy (Eg) is 1.09 eV for FeSe2 and 2.71 eV for gC3N4, as calculated by the Kubelka-Munk function (Fig. S17) [49]. As displayed in Figs. 9b and S18, the Mott-Schottky slopes of both g-C3N4 and FeSe2 catalyst are positive, which is in accordance with the character of a typical n-type semiconductor. The estimated flat band potentials (Efb) derived from the intercept for g-C3N4 and FeSe2 are −1.08 and −0.31 V vs. SCE (Saturated calomel electrode), respectively. Note that the conduction band (CB) level of the n-type semiconductor is approximately 0.20 V more negative than that of the flat band

Fig. 8. The water contact angles of (a) pure g-C3N4 and (b) 15% FeSe2/CNNS composites. 8

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Fig. 9. (a) UV–vis DRS, (b) Mott-Schottky plots, (c) EIS Nyquist plots, (d) Bode spectra, (e) transient photocurrent density and (f) time-resolved PL decay spectra of pure g-C3N4 and 15% FeSe2/CNNS composites, respectively.

illustrates that the average emission lifetime of 15% FeSe2/CNNS composites obviously decreases to 4.42 ns compared to g-C3N4 (4.96 ns) (Eq. S2 and Fig. S21c). The results suggest that a new nonraditive decay pathway can be built owing to a faster and more efficient charge carrier migrate through the new channels and the photoexcited electron and hole pairs of hybrid are rapidly captured by reactive substrates [56]. Moreover, the electron transfer rate (kET) and efficiency (ηET) of g-C3N4 deposited with 15% FeSe2 nanosheets are 2.46 × 107 s−1 and 10.89%, respectively (Table S4 and Eq. S3,4), implying that the photoexcited electron-hole pairs are more likely to participate in the subsequent surface redox reaction, and then to enhance photocatalytic HER efficiency [57]. To elucidate the photocatalytic reactive mechanism and reveal how various active free radicals influence catalytic performance, a series of radical trapping experiments were first carried out. During the MB removal process, the isopropanol (IPA), ethylenediaminetetraacetate (EDTA) and benzoquinone (BQ) can serve as the scavenging agents of hydroxyl radicals (∙OH), holes (h+) and superoxide radicals (∙O2−), respectively [58,59]. In general, the ∙OH and ∙O2− radicals are the most powerful and nonselective oxidant, which can be generated via a photoexcited hole oxidative process on OH− and H2O as well as via a photoexcited electron reductive process on O2, respectively, and then they further can degrade all kinds of organic contaminant [60,61]. It is observed from Fig. 10 that the photocatalytic performance of all catalysts is dramatically decreased with IPA addition, implying that ∙OH radicals play a vital role in this reaction process. The performance is almost unchanged for MB degradation with the addition of EDTA and BQ, which indicates that h+ and ∙O2− radicals are not primary active

that is, the charge carrier kinetics lie in the space charge layer. As shown in Table S3, the value of electron lifetime and R1-DP are 4.15 ms and 3480.0 Ω for g-C3N4, and 5.04 ms and 2247.0 Ω for FeSe2/CNNS composites, respectively (Eq. S1), demonstrating that remarkably accelerated charge carrier migration kinetics are achieved between gC3N4 and FeSe2 nanosheets [52]. The HER polarization curves (Fig. S20) demonstrate that the FeSe2/CNNS photoanode has a much smaller overpotential and more pronounced cathodic current density than those of pristine g-C3N4 electrode, resulting in faster carriers transfer kinetics and significantly enhanced HER performance, which is in accordance with the forward shift of the flat potential for composite from M–S plot analysis (Fig. 9b) [34,53]. Figs. 9e and S21a exhibits transient photocurrent density curves of g-C3N4 and 15% FeSe2/CNNS photoanode under solar light irradiation in the presence of 0.5 mol/L Na2SO4 electrolyte [54]. It can be seen that the 15% FeSe2/CNNS photoanode displays an obvious improvement in photocurrent density of approximately 0.73 μA/cm2, which is 1.92 and 1.46 times that of g-C3N4 (0.38 μA/cm2) and (0.50 μA/cm2), demonstrating the effective separation and transfer of interfacial charge. Steady state and time-resolved photoluminescence (PL) spectra were used to clarify the photophysical characteristics of photocarriers. The PL intensity of FeSe2/CNNS decreases remarkably in comparison with that of pure g-C3N4 (Fig. S21b), proving the improvement of photocarrier separation rate. Moreover, a slight blue movement of approximately 15 nm after the incorporation of FeSe2 nanosheets is observed over FeSe2/CNNS, which originates from the decrease of layer number of g-C3N4 nanosheets and the interaction between FeSe2 and g-C3N4 nanosheets [55]. Time-resolved PL decay spectroscopy (Fig. 9f) 9

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Fig. 10. Photocatalytic removal of MB over (a) g-C3N4, (b) 15% FeSe2/CNNS and (c) FeSe2 samples with addition of various scavengers.

photocatalytic HER or organic pollutants removal process, i.e., the twoelectron reduction of O2 to H2O2 (Eθ = + 0.70 V vs. NHE) or oneelectron reduction of O2 to ∙O2− (Eθ = −0.046 V vs. NHE) (Eqs. (4)–(7) [66,67]. To confirm that H2O2 was generated in the FeSe2/CNNS system, a DPD-POD method was used to detect this intermediate product via UV–vis spectroscopy analyzer [68,69]. Fig. 13(a–c) illustrate that two major absorption peaks situated at 500 and 550 nm are apparently observed, evincing the formation of H2O2 over all catalyst surfaces (Fig. S22). The intensity of the H2O2 absorption peak at the same time follows the order of: g-C3N4 > 15% FeSe2/CNNS > FeSe2. Moreover, the concentration of H2O2 generation in the photocatalytic reaction for 120 min over g-C3N4, 15% FeSe2/CNNS and FeSe2 are 11.88 μmol/L, 4.46 μmol/L and 1.25 μmol/L, respectively (Fig. S23). This indicates that the two-electron reduction of O2 to H2O2 and ∙OH radicals from H2O2 decomposition have been triggered, which could be explained in terms of thermodynamics due to the higher CB potential of g-C3N4 (ECB = −1.04 V vs. NHE) or FeSe2 (ECB = −0.27 V vs. NHE) compared to those of O2/H2O2 or H2O2/∙OH (Eθ = + 0.70 V vs. NHE) [29]. Besides, the photocatalytic H2 evolution experiments of resulting samples were also conducted in a mixed solution containing amounts of H2O2 and Na2S/Na2SO3 (0.15/0.35 mol/L) in order to ascertain the effect of H2O2 in this system. Fig. 13d illustrates that the H2 generation rate of the pure g-C3N4 system remarkably decreases when different concentrations of H2O2 (8.82 mmol/L, 8.82 μmol/L and 8.82 nmol/L) are added, which demonstrates that the formed peroxides have a negative effect on the photocatalytic activity in the reaction process. A similar phenomenon was also found by Lu’s group [70]. Interestingly, along with the addition of H2O2, the H2 generation rate of the 15% FeSe2/CNNS system has hardly changed. As a result, such a negative effect of H2O2 on H2 evolution can be reduced in FeSe2/CNNS heterostructures, further indicating that the introduction of FeSe2 can accelerate H2O2 decomposition via a stepwise two-electron, two-step reduction pathway and protect reactive active sites, thereby leading to higher photocatalytic activity.

species. In addition, when N2 is used as a scavenger to exclude the reduction of O2 to produce ∙O2−, the removal efficiency of MB is also decreased to 46.9% for g-C3N4, 72.7% for 15% FeSe2/CNNS, and 78.5% for FeSe2, respectively. This observation significantly demonstrates that the O2 is necessary for boosting the photocatalytic reaction process. Second, the presence of ∙OH and ∙O2− radicals can be verified by terephthalic acid (TA) trapping (with a typical PL emission peak at 425 nm) [62] and NBT transformation methods (with a UV–vis absorption peak at 260 nm) [63], respectively. In a typical photocatalytic process, the ∙OH and ∙O2− radicals could be formed as important reactive oxygen species via the following pathways (Eqs. (1)–(3)): − + Photocatalyst + hυ → eCB + hVB

(1)

+ OH− + hVB → •OH

(2)

O2 + e−CB → •O2−

(3)

As illustrated in Fig. 11(a–c), the PL emission peak intensity of ∙OH over 15% FeSe2/CNNS increased remarkably as light irradiation time prolonged compared with those of bare g-C3N4 and FeSe2, indicating that the ∙OH radicals are formed. The ∙O2− trapping experiment also exhibited that there is little reduction inr UV–vis absorption intensity of NBT in the FeSe2/CNNS composites (Fig. 11(d–f)), which confirms that the ∙O2− radicals are already produced and could then be transformed into other active species. Furthermore, the electron spin resonance (ESR) technique was used to further probe the formation of ∙OH and ∙O2− radicals (Fig. 12). The characteristic signals of DMPO-∙OH and DMPO-∙O2− radicals are detected over all samples under light illumination [64]. Note that the redox potentials of H2O/∙OH and O2/∙O2− are + 2.70 V and −0.046 V vs. NHE, respectively [29,65]. The energy levels of VB and CB are +1.67 and −1.04 V for g-C3N4, as well as +0.82 and −0.27 V for FeSe2, respectively, according to UV–vis and M–S results (Fig. 9a and b). Apparently, in the case of all samples, the photoelectrons suited at the conduction band have enough energy to activate the oxygen, but not to reduce H2O to produce ∙OH by photoexcited holes. Questions arise concerning how the O2 and ∙O2− influence the photocatalytic reaction process, why ∙O2− is not a major active radical, and what will be transformed. It is generally recognized that the O2 will probably go through two reduction reactions during either the 10

e− + O2 + H+ → HO2•

(4)

e− + HO2• + H+ → H2 O2

(5)

2e− + O2 + 2H+ → H2 O2

(6)

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Fig. 11. PL spectra of TA−OH and NBT transformation for resulting catalyst (a, d) g-C3N4, (b, e) 15% FeSe2/CNNS and (c, f) FeSe2.

e− + H2 O2 + H+ → •OH + H2 O

(7)

inter-plane heterojunction interface, the difference between the transfer rates of e− and h+ will effectively restrain charge carrier recombination at their interface [71,72]. Furthermore, the unique configuration of 2D/2D FeSe2/CNNS inter-plane heterostructures allows both photocarriers to be exposed to the solution and to validly participate in the later HER, leading to improved photocatalytic performance. For path 1, the e− of CB of FeSe2 can reduce H+ into H2, while the corresponding h+ can oxidize the sacrificial electron donor Na2S/Na2SO3. Therefore, it can supply massive surface active sites for photogenerated electron transfer to produce H2. For path 2, the photogenerated e− will reduce O2 to generate H2O2, followed by H2O2 decomposing into ∙OH over the FeSe2/CNNS surface via a stepwise two-electron, two-step reduction procedure. Subsequently, they will couple with the generated h+ to oxidize MB into small molecules, H2O and CO2. In addition, other

Based on the above analytical results and corresponding band structures of g-C3N4 and FeSe2, a probable transfer mechanism of photoexcited electrons and holes over FeSe2/CNNS is proposed and exhibited in Fig. 14. The electrons can be excited and jump from the valance band (VB) to the conduction band (CB) of FeSe2 and g-C3N4, respectively, leaving holes on the VB of corresponding semiconductor materials under light irradiation. The photoexcited electrons on the CB of g-C3N4 (ECB= −1.04 eV vs. NHE) can transfer to that of FeSe2 (ECB= −0.27 eV vs. NHE), driven by the contact electric field. Conversely, the holes situated on the VB of g-C3N4 (EVB= +1.67 eV vs. NHE) will also be transferred to that of FeSe2 (EVB= +0.82 eV vs. NHE), thus establishing a type-I straddled band alignment. Although both the photoexcited e− and h+ migrate from g-C3N4 to FeSe2 across the 2D/2D

Fig. 12. ESR signals of (a) DMPO-∙OH and (b) DMPO-∙O2− radicals for the resulting catalysts under light irradiation in aqueous and DMSO solutions, respectively. 11

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Fig. 13. H2O2 generation in the photocatalytic reaction process over (a) g-C3N4, (b) 15% FeSe2/CNNS and (c) FeSe2. (d) Hydrogen evolution rates of pure g-C3N4 and 15% FeSe2/CNNS in H2O2 and Na2S/Na2SO3 mixed solution (the volume of H2O2 added is 10 u L; a = 0 mol/L, b = 8.82 mmol/L, c = 8.82 μmol/L, and d = 8.82 nmol/ L).

cocatalyst under light irradiation, being nearly 2.65, 1.73 and 1.19 times higher than those of pure g-C3N4, FeSe2 and 0D/2D FeSe2/CNNS nanocrystals. Besides, we also found that the FeSe2 nanosheets can accelerate the decomposition of H2O2 generated on the g-C3N4 nanosheet surface via a stepwise two-electron/two-step pathway. Such remarkably improved HER activity could be derived from efficient separation and transportation of photocarriers, acceleration of H2O2 decomposition and surface proton reduction with plenty of surface-activesites for HER as well as strong visible light harvesting. This work provides a promising strategy for developing and designing a highly efficient photocatalytic system for HER and for steering charge carrier transportation in a inter-plane heterostructures system.

remaining electrons will directly take part in the reduction process from Cr (VI) to Cr (III). In summary, the remarkably improved photocatalytic activity toward HER and pollutant removal by 2D/2D FeSe2/CNNS inter-plane heterojunctions nanosheets can stem from the following three factors: (i) the establishment of 2D/2D FeSe2/CNNS inter-plane heterojunction not only can broaden light absorption ability but can also promote rapid spatial migration of photocarriers; (ii) the 2D intimate contact endows a large reactive surface area and abundant catalytic active centers; (iii) a good hydrophilic interface, lower interfacial transfer resistance, effective and prolonged electron lifetime as well as improved H2O2 decomposition efficiency via a stepwise two-electron/ two-step reduction pathway of O2 can further achieve improved HER efficiency and the simultaneous removal of Cr(VI) and organic pollutants.

Declaration of Competing Interest The authors declare no competing financial interest.

4. Conclusion

Acknowledgment

In this work, the 2D/2D FeSe2/CNNS inter-plane heterojunction photocatalysts were synthesized by successive calcination, liquid exfoliation and in-situ hot injection routes. The resulting 2D/2D 15% FeSe2/CNNS composites showed the highest photocatalytic HER activity (1655.6 μmol∙h−1 ∙g−1) in Na2S/Na2SO3 aqueous solution without any

This work was supported by the National Natural Science Foundation of China (Nos. 21676213 and 21476183), the China Postdoctoral Science Foundation (No. 2016M600809), the Natural

Fig. 14. Schematic diagram of the charge carrier transfer of 2D/2D FeSe2/CNNS sample under solar light irradiation. 12

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Science Basic Research Plan in Shaanxi Province of China (Nos. 2017JM2026 and 2018JM5020), and the Graduate Student Innovation Funds of Northwest University (No. YYB17015).

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Appendix A. Supplementary data

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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118249.

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