O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution

O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution

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Available online at www.sciencedirect.com

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In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution Haoshan Wei a,b, Yong Zhang a,b,*, Guoao Zhang a,b, Jiewu Cui a,b, Yan Wang a,b, Yongqiang Qin a,b, Xueru Zhang c, Hark Hoe Tan d, Jiaqin Liu b,e, Yucheng Wu a,b,** a

School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, PR China Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei, 230009, PR China c Instrumental Analysis Center, Hefei University of Technology, Hefei, 230009, PR China d Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, 2601, ACT, Australia e Institute of Industry & Equipment Technology, Hefei University of Technology, No.193 Tunxi Road, Hefei, 230009, Anhui, PR China b

highlights  W/O

co-doped

hollow

graphical abstract g-C3N4

tubular structures were successfully obtained.  Porous structure with modulated specific surface area were generated by co-doping.  The doped g-C3N4 showed substantially enhanced photocatalytic hydrogen evolution.  W doping triggered formation of W eN chemical bonds with CN aromatic rings.  WeN bonds played an important role in the property enhancement.

article info

abstract

Article history:

Heteroatom co-doping has been considered as an effective strategy to simultaneously

Received 25 February 2020

overcome intrinsic shortcomings of g-C3N4 to achieve enhanced photocatalytic properties,

Received in revised form

in which the involved dopants could play its role in altering electronic structure, optical

26 September 2020

absorption and charge separation of the catalyst. Herein, W/O co-doped hollow g-C3N4

Accepted 29 September 2020

tubular structures are successfully obtained for the first time via a one-step thermal

Available online xxx

decomposition. By W/O co-doping, architecture of g-C3N4 is able to be modulated with

* Corresponding author. School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, PR China. ** Corresponding author. School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, PR China. E-mail addresses: [email protected], [email protected] (Y. Zhang), [email protected] (Y. Wu). https://doi.org/10.1016/j.ijhydene.2020.09.261 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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Keywords:

enhanced optical absorption towards visible region. In addition, narrowed band gap and

G-C3N4

restrained charge recombination are conducive for the excitation of electron-hole pairs

Tungsten

and transportation. Photocatalytic water splitting tests indicate that the co-doped hollow

Photocatalyst

tubular g-C3N4 structures enable superior activity for generating hydrogen up to

H2 evolution Heteroatom co-doping

403.57 mmol g1 h1 driven by visible light, nearly 2.5 times as high as that of pristine gC3N4. This work presents a rational strategy to design co-doped g-C3N4 as an efficient visible-light-driven photocatalyst. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Photocatalysis has been considered as a potential solution in dealing with growing energy crisis and environmental problems owing to its unique energy conversion and environmental purification capacity [1e3]. Based on the photocatalysis process, solar energy can be converted to chemical energy via water splitting into hydrogen and oxygen [4,5] or CO2 reduction [6,7], which can provide renewable energy resources. Hydrogen is an outstanding energy carrier and fuel featuring high-energy-density, pollution-free, also easily storable and transportable. Moreover, hydrogen is able to be used for recycling of carbon dioxide through the FischerTropsch reaction and methanol synthesis. Therefore, development of high efficiency photocatalysts for hydrogen evolution has attracted great interest. Among an abundant of investigated semiconductor photocatalysts [8], metal-free polymer organic semiconductor graphitized carbon nitride (g-C3N4) has received increasing consideration as a visible light responsive photocatalyst for hydrogen evolution [9e12] owing to its attractive abundant earth features, good stability, easy obtainment, low-priced, ecological friendliness and negligible toxicity [13,14]. However, low activity of pristine g-C3N4 impedes its practical applications for hydrogen evolution due to low layer-to-layer charge transfer efficiency, high rate of recombination, low electrical conductivity, insufficient visible light absorption and non-optimized electronic structure [15,16]. To deal with these challenges above, doping with heteroatom is considered as an effective approach to improve photocatalytic performance by modifying electronic structure of g-C3N4. So far, single element doping such as B [17], P [18], O [19,20], Mn [21], Fe [22,23], Ni [24] and co-doping e.g. SeP [25], OeK [26], FeeP [27], PeMo [28], CeCe [29] and CeO [30] of gC3N4 have been implemented which could promote hydrogen evolution by optimizing the positions of band edge, facilitating the generation and transportation of carriers, inhibiting the recombination of photo-generated carriers as well as enhancing light adsorption. Herein O-dopant is though a common impurity for non-oxide products, it was interestingly found that appropriate amount oxygen could facilitate the band gap narrowing and charge separation. In addition, codoping of O with other elements into the g-C3N4 could trigger the synergistic doping effect which substantially elevated its photocatalytic activity. In the photocatalyst system of g-C3N4 co-doped with O and K, for example, the doped

O atoms could strengthen the interlayer electron transfer channel, while the intercalation of K would enhance the interlayer transportation of carriers and promote the separation of photogenerated electron-hole pairs. Tungsten (W) is commonly employed as a transition metal dopant which could improve electron injection efficiency [31] and light absorption by introducing doping levels below the conduction band minimum [32]. By co-doping of O and W species, g-C3N4 with modulated electronic structure and architecture are expected to yield highly efficient photocatalytic hydrogen evolution properties. Moreover, DFT simulation results suggested that WeN bond formed in titanium oxide due to W/N co-doping considerably improved the photocatalytic efficiency, which is expected in our W doped g-C3N4 as well [33]. In the present work, W/O co-doped hollow g-C3N4 tubular structures were obtained for the first time via a facile one-step polycondensation method. The optical absorption properties, band structure, and photocatalytic performance of doped samples were systematically investigated. By W/O co-doping, the hollow g-C3N4 tubular structures shows significantly enhanced visible light photocatalysis with hydrogen production rate of 403.57 mmol g1 h1, nearly 2.5 times high that of pristine g-C3N4, which suggests a reasonable strategy for designing and producing highly efficient g-C3N4 photocatalysts.

Experimental Chemicals and materials Melamine (C3H6N6, 99%) was purchased from Aladdin Chemical Reagent Corp. Ammonium metatungstate hydrate ((NH4)6H2W12O40$xH2O, 99.5%) was purchased from Shanghai Macklin Biochemical Co., Ltd. H2PtCl6$6H2O (AR, Pt 37.5%), Triethanolamine (TEOA) and ethanol (AR, 99.5%) are acquired from Sinopharm Chemical Reagent Company. All chemicals used in the experiments are without any further purification.

Preparation of W, O/g-C3N4 photocatalyst W, O/g-C3N4 was prepared in a facile polycondensation process. The preparation procedure is described below: a certain amount of (NH4)6H2W12O40$xH2O and 10 g of C3H6N6 with the molar ratio of W/melamine of 0e3.0% were dissolved into 40 mL of ultra-pure water. Then stir the solution vigorously for

Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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4 h to obtain a uniform solution. Next, water was evaporated in petri dishes to obtain precursor powder at 60  C for 12 h. When the temperature dropped to room temperature, the obtained mixtures were placed into semi-enclosed porcelain boat with a lid to prohibit melamine from sublimating. The porcelain boat was heated in a tube furnace under nitrogen at 550  C for 4 h at a temperature increase rate of 10  C/min. Finally, grind the as-prepared powders in a mortar and sieve to fine powders. The samples were marked as x% W/g-C3N4 (x ¼ 0, 0.5, 1.0, 1.5, 2.0, 3.0).

Characterization The X-ray diffraction (XRD, X’Pert PRO MPD, PANalytical B.V., Netherlands) analyses were performed to characterize the crystal structure with Cu Ka (l ¼ 0.15418 nm) radiation operated at 40 kV/40 mA. The microstructure of the W/g-C3N4 was investigated by scanning electron microscopy (SEM, SU8020, HITACHI, Ltd., Japan). The chemical nature and XPS valence band spectrum of W/g-C3N4 were characterized by X-ray photoelectron spectroscopy (XPS) in ESCALAB250Xi spectrometer with Al Ka X-ray (hn ¼ 1486.6 eV) at 15 kV and 150 W calibrated with C 1s line at 284.8 eV. Transmission electron microscopy (TEM) analysis with energy-dispersive X-ray spectroscopy was performed to observe the W/g-C3N4 powders and the analyses were performed on a JEOL 2100F fieldemission transmission electron microscope operated at 200 kV. The UVeVis diffuse reflectance spectroscopy was recorded on a UV-3600 UVeViseNIR scanning spectrophotometer (Shimadzu) furnished with an integrating sphere. BaSO4 was used as a reference to measure all the samples. The N2 sorption isotherm and Brunauer-Emmett-Teller (BET) surface area were obtained by the nitrogen adsorption method (Quantachrome autosorb iQ). The FT-IR for the samples were measured using an FT-IR spectrophotometer (Nicolet, Thermo). The photoluminescence (PL) spectra were measured with a fluorescence spectrophotometer (F-4500, HITACHI, Japan). Time-resolved photoluminescence spectra (TRPS) of the samples were recorded by FLS920 fluorescence lifetime spectrophotometer (Edinburgh Instruments, UK) with 335 nm excitation.

Photocatalytic activity measurement Photocatalyst sample (50 mg, loaded with 1.0 wt% Pt) was put into an aqueous triethanolamine (TEOA) solution (100 mL, 20 vol%) in a sealed gas circulation system (Perfect Light Company Labsolar-III (AG)). After vacuuming, the reactor was irradiated from the top by a 300 W Xe lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd, China) with a cutoff filter (l > 400 nm). The generated gas was detected in situ by an online gas chromatograph (GC-7890B, Agilent, America) equipped with a thermal conductivity detector (TCD). A calibration curve has been obtained for the quantification of the hydrogen yield.

Photoelectrochemical performance test Photoelectrochemical (PEC) tests were carried on in a conventional three electrodes system on CHI670 electrochemical

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work station, with the sample films as working electrode, Ag/ AgCl (saturated KCl) as reference electrode, and Pt plate as counter electrode. The investigation was also conducted at the 300 W Xe lamp with a filter (>400 nm). The sample film electrodes were prepared by electrophoretic deposition (EPD) method on the FTO substrate according to the following modified protocol as previously reported [34]. W/g-C3N4 sample (50 mg) was suspended in 60 mL of acetone by ultrasound 45 min in an ultrasonic bath. Then, 25 mg of iodine was added into the supernatant, stirred for another 30 min in order to fully mix. The obtained suspensions were used for electrophoresis. In the procedure of EPD, FTO was parallel to the Pt plate electrode with a distance of 8 cm and the deposition area immersed into the solution is 1 cm2. A 50 V bias controlled by a potentiostat (ITECH IT6834) was applied for 15 min. Then, the obtained photoelectrode was kept in an oven at 70  C for 1 h to dry the electrode. Nyquist diagrams of electrochemical impedance spectroscopy (EIS) were obtained from 0.01 Hz to 100 kHz of the frequency. The Mott-Schottky plots were achieved in a three-electrode system at a preset frequency with a scan rate of 5 mV s1 and the three frequencies of 1, 5, and 10 kHz over a potential range from 0.8 to 1.0 V vs Ag/AgCl. Both of the two measurements were resolved in 0.5 mol L1 Na2SO4 at the open circuit potential (OCP) V vs Ag/AgCl under Xe lamp irradiation. The OCP decay of working electrodes was measured for 1200 s. The PEC performance of the obtained photoanodes were measured in 0.1 mol L1 KOH. The transient photocurrent response (TPC) was tested at 0 V vs Ag/AgCl under Xe lamp irradiation and obtained for five cycles of ONOFF.

Results and discussion Morphology and structure characterization Morphological information of g-C3N4 samples were characterized by FE-SEM, shown in Fig. 1 and Fig. S1. Fig. 1 (a) shows the pristine g-C3N4 sample SEM images, it shows significant block shape with smaller size particles around, in which the counted average block size is 3.05 mm (Fig. S1). As revealed in Fig. 1 (a and b), the g-C3N4 agglomerates seriously and freestanding lamellar structures are not obviously discerned. Doped by the tungsten precursor, the block size of 1.0% W/gC3N4 decreases significantly and hollow tubular structures with mesopores are observed, shown in Fig. 1 (c and d). Apparently, the doped elements could suppress condensation and break the long-range order of the g-C3N4 probably due to host-guest interactions. It is very beneficial for the material to increase specific surface area and catalytic active site, which are the important factors for improving photocatalytic performance. With higher doping concentration, as illustrated in Fig. 1 (e and f), 3.0% W/g-C3N4 exhibits thinner tube walls but larger tube diameters. The morphology dependence on the doping concentration can also be observed in the other three doped samples as shown in Fig. S2. Microstructure of the doped g-C3N4 was exposed by transmission electron microscopy (Fig. 2), in which the large block of the sample was split into nano-scale pieces with porous structure due to the element doping. High

Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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Fig. 1 e SEM images and high-magnification SEM image of (a, b) 0% W/g-C3N4; (c, d) 1.0% W/g-C3N4; (e, f) 3.0% W/g-C3N4.

magnification TEM images indicate cross-linked wrinkle like pattern of 1.0% W/g-C3N4 and 3.0% W/g-C3N4 as shown in Fig. 2 (b and c), respectively, while in-plane holes are not observed in the pristine g-C3N4 (0% W/g-C3N4) (Fig. S3). To determine the composition and elements distribution of the samples, EDS elemental mappings was carried out on the doped g-C3N4, as shown in Fig. 2 (e), indicating the presence of C, N, O and W. Besides C and N elements of g-C3N4, the distribution of W and O species suggests uniform doping of the two elements into the g-C3N4 sample. The EDS mappings of the sample 3.0% W/g-C3N4 (Fig. S4) also indicate that tungsten and oxygen have been successfully doped into the crystal. According to the previous computational studies about the transition metal embedded g-C3N4 [35], it is deduced that W6þ ions are coordinated into the large CeN rings upon the Nbridge linking of the triazine units. Therefore, W species are doped into the g-C3N4 lattice in the form of W6þ by forming WeN bonds. The formation of pore structure in the doped samples may originate from the formation of WeN. And O doping may trigger the formation of cyanide terminations

from the amino groups in melamine, which can prevent the creation of hydrogen bonds in g-C3N4. In this circumstance, the intermolecular force of g-C3N4 would be significantly weakened, which could affect the morphology of the material. XRD was conducted to confirm the phase structure of the doped carbon nitride as presented in Fig. 3. There are two evident diffraction peaks, situated at about 12.9 and 27.5 of gC3N4 (JCPD 87-1526), respectively, without additional diffraction peaks suggesting single phase of the sample, which indicates that tungsten and oxygen have been doped into the crystal successfully. This is in consistent with the TEM results. The smaller hump with the d value of 0.686 nm can be indexed to the diffraction plane of (100), which is the oriented melon unit that constitutes the basic building block of g-C3N4 [36]. The relatively sharper peak located at 27.5 with d value of 0.324 nm corresponds the distance between two graphitic stacked sheets of conjugated aromatic systems and can be indexed to the diffraction plane of (002) in g-C3N4 [37]. Nevertheless, the overall diffraction intensity of the W/g-C3N4 becomes much weakened with the increasing doping

Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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Fig. 2 e TEM images and high-magnification TEM image of (a, b) 1.0% W/g-C3N4; (c, d) 3.0% W/g-C3N4 and (e) the corresponding EDS elemental mappings in 1.0% W/g-C3N4. concentration that can be owing to the host-guest interactions between W, O and g-C3N4, in which a small amount of W species could suppress condensation of g-C3N4. The (100) peak gradually disappeared with the increase of W and O species content, manifesting that the order on the long range of gC3N4 was destroyed, similar with these case of other metaldoped g-C3N4 materials [27,38,39]. It is noteworthy that there are no obvious peak shifts in all of the diffraction patterns of the doped g-C3N4 samples, indicating the W atoms were not doped into the g-C3N4 lattice in the form of substitutional atoms. Considering bulk characterization characteristics of XRD, surface determination of the samples was depicted by FTIR testing showing in Fig. S5, in which the spectra for six samples are similar to each other, suggesting successfully doping of the elements into the g-C3N4. Based on above results, we assume that the W species were chemically coordinated to the crystal planes of g-C3N4 by forming WeN chemical bonds with CN aromatic rings, as in metalloporphyrins and metalophthalocyanines [38], giving rise to significantly decreasing structural correlation length of interlayer periodicity. To confirm our assumption, surface chemistry and electronic structure of x% W/g-C3N4 (x ¼ 0, 0.5, 1.0, 1.5, 2.0, 3.0) was investigated by XPS. The XPS survey spectra (Fig. S6) shows the presence of C, N and O in 0% W/g-C3N4, and C, N, W and O

in x% W/g-C3N4 (x ¼ 0.5, 1.0, 1.5, 2.0, 3.0). With the increasing amount of ammonium metatungstate, W and O signals gradually increases from the sample 0.5% W/g-C3N4 to 3.0% W/g-C3N4, suggesting W/O co-doping state of the g-C3N4, which accords with the XRD and the results of TEM mapping.

Fig. 3 e X-ray diffraction (XRD) patterns of the W/g-C3N4.

Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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The doping amount of W and O detected by XPS in each sample has been shown in Table S1. Fig. 4 shows the Highresolution X-ray photoelectron spectra of pristineg-C3N4, 1.0% W/g-C3N4 and 3.0% W/g-C3N4 samples together with fitting peaks. In the XPS spectra of C 1s, presented in Fig. 4 (a), C 1s of the pristine g-C3N4 are fitted with five peaks, located at 293.62, 288.5, 288.14, 286.41, and 284.79 eV, respectively. The peak at 288.14 eV is attributed to sp2 C atoms, which are bonded to N (NeC]N) in aromatic rings [40]. The peak located at the binding energy 288.5 eV can be ascribed to the oxidized carbon CeO and peaks at 286.41, and 284.79 eV correspond to sp2 CeNeC, and sp3 CeC bonds [41]. The p-p* excitations between the graphitic layers can be originated from the peak located at 293.62 eV [42]. The have de-convoluted XPS spectrum of N 1s in Fig. 4 (b) of the pristine g-C3N4 manifests the existence of four types of nitrogen species at 398.63, 399.91, 401.15 and 404.6 eV, respectively. It can belong to the sp2 N (C]NeC) in triazine rings, N-(C)3, the surface CeNH2 moiety in the CN framework, and p-p* excitations [43]. Compared with the doped samples, it is perceived the peak corresponding to N]CeN shifted to higher binding energy at 288.25 eV, but the

peak for CeO shifted to lower binding energy 288.48 eV for 3.0% W/g-C3N4 sample, displayed in Fig. 4 (a). These results can be attributed to the doping of O, whose electronegativity (O ¼ 3.5) is higher than that of N and C (N ¼ 3.0, C ¼ 2.5) in the aromatic CN heterocycles [44], preferentially bonding to C atoms by replacing the dico-ordinated N. In contrast with N 1s in Fig. 4 (b), the peak correlating with C]NeC and N-(C)3 shifted to higher binding energy at 398.72 eV and 400.04 eV, respectively. It signifies that the modification of W resulting in the chemical environment alteration around N 1s by forming WeN chemical bonds with CN aromatic rings. High-resolution X-ray photoelectron spectrum for O 1s of pristine g-C3N4 is obtained in Fig. 4 (c), which is split into three peaks located at binding energy 531.1, 532.18 and 533.5 eV, respectively. The peak at 532.18 eV stems from the surface hydroxyl groups (OeH). The peak located at binding energy 533.5 eV is attributed to the intermediates of melamine thermal-polymerization [41], which can greatly enhance the intermolecular force of g-C3N4. With the doping ratio increased, the proportion of surface hydroxyl groups decreased obviously, demonstrating lower intermolecular

Fig. 4 e High-resolution X-ray photoelectron for (a) C 1s, (b) N 1s, (c) O 1s and (d) W 4f together with the fitting peaks of the 0% W/g-C3N4, 1.0% W/g-C3N4 and 3.0% W/g-C3N4 samples. Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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force of g-C3N4. This result is beneficial for decreasing the size of g-C3N4, interpreting the morphology changes of the g-C3N4 from stacked lamellar structure to tubular one. The peak at 531.1 eV is derived from the O species of CeO and NeCeO in lattice [44]. Notably, as the doping content increased, the O species of CeO and NeCeO gradually increase which can be calculated from Fig. 4 (c). These results are reproduced in Fig. S7. It is fully confirmed that the bonds OeC have been formed in the basic aromatic CN heterocycle, which can echo the results from Fig. 4 (a). To clarify the valence of tungsten, XPS spectra for W 4f is fitted into three pairs of sub-peaks with the distance of each pair 2.15 eV (Fig. 4 (d)). W in all of the doped samples exhibits two chemical valence states, WeN and WeO. The peaks located at around binding energy 33.5 eV is ascribed to WeO 4f7/2 and the corresponding peak for WeO 4f5/2 located at 35.65 eV, corresponding to WO2 [45]. The peaks corresponding to W 4f5/2 and W 4f7/2 at 37.05 and 34.9 eV can be indexed as WN2 [46], which corresponds to the valence state of tungsten is WeN, indicating that W atoms have been doped into the crystal successfully. With W XPS spectra, shown in Fig. 4 (d) and Fig. S7 (d), the ratio of WeN/ WeO have been calculated as show in Table S2. Based on the TEM, EDS, XRD analysis and XPS results, it is further confirmed that W6þ ions are embedded into the big CeN rings upon the N-bridge linking the triazine units in g-C3N4.

Photocatalytic property evaluation Based on the above morphology and structure characterization results, it can be inferred that the W/O co-doping contributed greatly to the generation of the hollow g-C3N4 tubular structures. To evaluate H2 production performance of the tubular structures towards visible light, photocatalysis of the synthesized W/g-C3N4 samples were studied with Pt (1 wt %) as co-catalyst and TEOA (20 vol%) as cavity sacrificial agent in water. Fig. 5 (a) exhibits H2 production amount vs. time plots of different samples. Remarkably, the samples of x% W/gC3N4 (x ¼ 0.5, 1.0, 1.5, 2.0, 3.0) exhibit obviously higher activity than 0% W/g-C3N4. As unveiled by Fig. 5 (b), the sample 0% W/ g-C3N4 displays an average H2 production rate of about 171.21 mmol g1 h1, whereas the samples of x% W/g-C3N4 (x ¼ 0.5, 1.0, 1.5, 2.0, 3.0) show higher H2 generation rates of 358.52, 403.57, 268.86, 307.34 and 353.95 mmol g1 h1, respectively. The sample of 1.0% W/g-C3N4 exhibits the

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highest hydrogen generation rate, almost 2.5 times than that of the pristine g-C3N4 sample. However, a sudden decrease occurs in the hydrogen production for the sample 1.5% W/gC3N4, which may be related to the decrease in the specific surface area of the sample. This result indicates that WeN is of greater influence for improving the specific surface area. For heavily doped samples, the reduced performance of photocatalytic hydrogen evolution may be due to its worst crystallinity, which result in excessive lattice defects that increases the impedance of the material (Fig. S8). Some references have been listed for comparison with our experiments in Table S3. Transient photocurrent response (i-t) measurement has been considered as a reliable technique for valuing the effectiveness of charge carrier generation under light. As illustrated in Fig. 5 (c), under dark and bright conditions, all photocatalysts showed similar characteristics in current response, but the current density was significantly different. As indicated in the i-t profiles, electronic excitation bechanced on the surface of semiconductor under the light illumination, bringing out a remarkable enhancement of output current density, which return to its previous position clearly demonstrating the favorable photo reversibility of the synthesized photocatalysts. The 1.0% W/g-C3N4 presents the highest current density (300 nA cm2), 1.75 times than the current density of pristine g-C3N4 (171 nA cm2). Besides, the current density for 0.5% W/g-C3N4 is 282 nA cm2, 1.65-fold excess than the gC3N4. The other three doped samples also have better photocurrent response than the pristine g-C3N4, but lower than of 1.0% W/g-C3N4. Definitely, the results of transient photocurrent are in good agreement with the properties of hydrogen evolution, suggesting that the photocatalytic activity of the doped samples is higher.

Band structure analysis To further determine what effect W-doping have brought on the electronic structure of g-C3N4, optical absorption properties and band gaps of the prepared x% W/g-C3N4 (x ¼ 0, 0.5, 1.0, 1.5, 2.0, 3.0) samples have been studied by measuring UVeVis diffuse reflectance spectra. As described in Fig. 6 (a), it is easily acquired that the band edge of optical absorption shifts to the lower energy wave region gradually with the doping content of tungsten increased in the g-C3N4 host. After W and O doped,

Fig. 5 e Photocatalytic H2 evolution on the prepared samples under visible light irradiation (l ≥ 400 nm): (a) the typical time courses of H2 evolution; (b) the comparison of H2 evolution rate and (c) Transient photocurrent responses of the prepared samples. Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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Fig. 6 e (a) UVevis diffuse reflectance spectra for the x% W/g-C3N4 samples; (b) the corresponding plot of (ahn)1/2 vs. photon energy (hn) for the determination of optical band gap for x% W/g-C3N4. W/g-C3N4 samples exhibit a distinct enhancement of absorption throughout the visible region relative to that of the 0% W/ g-C3N4 sample. It should be beneficial to enhance visible-lightdriven photocatalytic performances with redshift and absorption intensity increased. Kubelka-Munk function has been employed to estimate the optical band gaps of the prepared samples from DRS, shown in Fig. 6 (b). According to Fig. 6 (b), the band gaps are 2.69, 2.60, 2.57, 2.52, 2.48 and 2.37 eV for x% W/g-C3N4 (x ¼ 0, 0.5, 1.0, 1.5, 2.0, 3.0), respectively. Markedly, the Eg (band gap) decreases with the doping content of W and O increased, demonstrating the tunable electronic structure with doping W and O into the g-C3N4 lattice. It further proves the strong interaction between the W and O species and g-C3N4 host. The influence of tungsten doping on the red shift of the absorption band for x% W/gC3N4 samples may be related to the charge-transfer transition between the orbitals of W 5d and N 2p [38]. XPS valence band spectra and Mott-Schottky plots have been analyzed for evaluating the band edge positions of the as-obtained W/g-C3N4 samples, which are demonstrated in Fig. 7 (a) and Fig. S9, respectively [47,48]. All the evident valence band edges of W/g-C3N4, extrapolated from the valence band spectra recorded by XPS, were near without apparent shift (all located at 2.18 eV). In order to fully understand the band alignments and the relative potentials of the

reactions at different reference standard, the VB vs. normal hydrogen electrode (NHE) could be calculated using the formula shown below [49,50], in which the work function F of the XPS analyzer and vacuum level are respectively estimated to be 4.37 eV and 4.44 eV (vs. NHE): VB ðvs:NHEÞ ¼ F þ Ebinding  4:44 As the result, the VB potentials (vs. NHE) of the as-prepared samples are all determined to be 2.11 eV. Combining the previously measured band gap values obtained in Fig. 6, the band structures of different samples could be successfully obtained, presented in Fig. 7 (b). The CB potential (vs. NHE), illustrated in Fig. 7 (b), of the different samples are located at 0.58, 0.49, 0.46, 0.41, 0.37 and 0.26 eV with the increase of tungsten source. The x-intercept of Mott-Schottky plots in Fig. S9 reveals the flat band potentials vs. Ag/AgCl (saturated KCl, pH ¼ 7). The results of VB value of XPS, shown in Fig. 7 (a), match well with the Mott-Schottky tests result. The semiconductors exhibit a typical characteristic of n-type for the positive slopes at miscellaneous frequencies. The conduction band (CB) potential of n-type semiconductors has been considered to be very close to the flat band potential and all of the band structures are satisfied for thermodynamic conditions of photocatalytic water splitting.

Fig. 7 e (a) XPS valence band (VB) of W/g-C3N4; (b) Schematic diagram of band structures of W/g-C3N4. Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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According to the above data, it can be concluded that the W, O-doping effect may play a dual role in affecting the hydrogen evolution in our case. From one hand, the doping causes positive shift of the CB potential which may make a lower reduction ability of electrons. On the other hand, elevated specific surface area, enhanced light absorption and narrower band gap are obtained and the microstructure of the doped samples is tune as well, which strengthens the photocatalytic hydrogen evolution.

Table 1 e Details of lifetime profile of the photocatalysts calculated by room temperature time-resolved PL spectra. samples 0% W/g-C3N4 1.0% W/g-C3N4 3.0% W/g-C3N4 a b

A1a

t1b/ns

A2a

t2b/ns

t/ns

2.166Eþ09 6.099Eþ07 5.526Eþ05

1.399 1.912 3.646

1127 935.3 1407

47.15 52.03 64.77

1.401 1.931 6.291

Pre-exponential factors. Excited-state luminescence decay time.

Photocatalytic mechanism PL spectra and TRPL spectra are investigated to confirm the separation characteristics of photo-generated carriers. As obtained in Fig. 8 (a), room temperature steady-state PL spectra illustrate that the intensity of PL emission peaks exhibit a gradual decrease with the increasing of the content of W, revealing that there is lower possibility of recombination of charge carriers occurs in the band to band [51]. Furthermore, TRPL spectra were employed to analyze the lifetime of charge carriers generated under illumination, presented in Fig. 8 (b). The following two-exponential equation has been used for fitting the decay curves to get results that are very close to the original data: It ¼ I0 þ

n¼2 X i¼1

  t Ai exp ti

in which I0 signals the baseline correction, Ai denotes the preexponential factors and ti correspond to excited-state luminescence decay time related to the ith part, respectively. t1 basically represents the lifetime corresponding to radiative energy decay process whereas t2 denotes nonradiative energy decay processes [52]. What’s more, the following equation has been used to evaluate the average decay time (tave.): tave: ¼

A1 t21 þ A2 t22 A1 t1 þ A1 t1

As shown in Table 1, the average lifetime of charge carriers for the 1.0% W/g-C3N4 and 3.0% W/g-C3N4 samples is 1.931 and 6.291 ns, respectively, which are evidently longer than that for 0% W/g-C3N4 (1.4 ns). It is noteworthy that both t1 and t2 have

improved when the doping content of W and O into the g-C3N4 host increased, especially for 3.0% W/g-C3N4 samples. The prolonged carrier lifetimes indicate the advantages of the introduced WeN bonds, enhancing the separation efficiency of photogenerated carriers. In contrast, the O-doping may decrease the lifetime of photogenerated carriers, which confirm the doped W plays a decisive role in prolonging the lifetime of charge carriers. For the N2 adsorption-desorption isotherms and the corresponding pore size distribution curves of W/g-C3N4 in Fig. 9, the representative N2 adsorption-desorption isotherms are of type IV with H3-type hysteresis loops (0.7 < P/P0 < 1.0), indicating the presence of mesopores and macropores in all samples [53]. Shown in Fig. 9 (a), the small hysteresis loop of W/g-C3N4 located at high P/P0 is caused by the aggregation of bulk g-C3N4 particles. Obviously All the doped samples exhibit the higher specific surface area (SSA) than the pristine g-C3N4, the SSA of 1.0% W/g-C3N4 is measured to be 35.63 m2/g, almost 2.5 times higher than that of the pristine g-C3N4 of 10.19 m2/g, indicating that a small amount of W and O species could favor the increase of SSA by suppressing condensation of g-C3N4. Interestingly, the change in specific surface area (Table 2) has a consistent trend with the ratio of WeN/WeO, calculated from in Table S2, implying a close correlation between the SSA and WeN bond formation. We speculate that the formation of WeN or WeO bond is a competitive process, which may affect the SSA as well as the chemical environment of the doped tungsten ions. It suggests that the formation of WeN bonds with the active nitrogen atoms in the aromatic ring may significantly break the structural correlation length of interlayer periodicity and increases the SSA of the doped g-C3N4.

Fig. 8 e (a) Room temperature steady-state photoluminesce spectrum (PL) of the x% W/g-C3N4 samples; (b) Room temperature time-resolved PL spectra of x% W/g-C3N4 samples and the relevant fitted data. Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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Fig. 9 e (a) Nitrogen adsorption-desorption isotherms and (b) BJH pore size distribution curves deduced from the desorption branch of the W/g-C3N4 samples. And the doped g-C3N4 with larger SSA provides more active sites for the photocatalytic reaction. The pore size distribution curves obtained from desorption branch chains using BJH model are presented Fig. 9 (b). All samples have two major pore distributions: a narrow center at approximately 4.0 nm and a wide one in the 10e100 nm range. The narrow center is ascribed to internal aggregate capillary pores, while the wide one should be originated from the released gas bubbles. The volume of BJH desorption cumulative pores have been calculated with diameters between 1.3 and 110 nm, which have been shown in Table 2.

Table 2 e The surface area and pore structure parameters of W/g-C3N4 photocatalyst. samples 0% W/g-C3N4 0.5% W/g-C3N4 1.0% W/g-C3N4 1.5% W/g-C3N4 2.0% W/g-C3N4 3.0% W/g-C3N4

SBET (m2/g)

Pore volume (cc/g)

10.19 30.18 35.63 27.16 33.65 38.39

0.035 0.1057 0.1341 0.1016 0.1104 0.1208

According to the results and discussions above, it can be concluded that the W/O co-doping conspicuously influences the photocatalytic hydrogen evolution of the g-C3N4. Ground on the above results and analysis, a possible mechanism is put forward to explain the photocatalytic water splitting into H2 over the W/O co-doped g-C3N4 catalysts irradiated with visible light, as presented in Fig. 10. Generally, each photocatalytic reaction involves three essential processes: absorption of light by semiconductor materials, generation, separation and transportation of charge carriers, and participating in chemical reactions on the catalyst surface [54]. The doping of W, O into the g-C3N4 framework evokes serial enhancements. In the first place, the ability to harvest light is improved with the absorption band edge and intensity boosted. In the second place, the narrowed band gap enhances the excitation of electron-hole pairs, thereby more electrons are provided for H2 evolution under the irradiation of visible light. In addition, separation and transportation of photogenerated carriers are boosted by the formation of WeN, the active sites of WeN generated in the covalent polymer skeleton can capture the electrons in the catalyst conduction band, which benefits the photocatalytic hydrogen evolution. Under visible light irradiation, the photogenerated electrons in the conduction band can be quickly captured by species W, leaving photogenerated holes in the valence band. Therefore, the recombination of the photogenerated charge carriers is effectively controlled, which can be confirmed by the results of PL and TRPL. In the photocatalytic H2-evolution reaction, which occurs on the surface of W/g-C3N4, the photogenerated electrons around the WeN active sites would reduce H2O to generate H2, while the photogenerated holes in the valence band can be captured by TEOA. Furthermore, the expanded specific surface area of the hollow g-C3N4 with tubular structure owing to co-doping effect provides abundant active sites for photocatalysis. Of course, it should be noted that, the W/O co-doping also causes positive shift of the CB potential which results in a lower reduction ability of electrons and affects the impedance of the sample, hence excessive doping would decrease the hydrogen evolution properties. Therefore, the photocatalytic performance of the W/O co-doped g-C3N4 for hydrogen evolution is supposed to be interpreted by a competitive mechanism based on the effect of electronic structure, band structure, optical absorption intensity as well as crystal structure of the doped g-C3N4, and the W/O doping concentration has to be carefully controlled. In this study, W/O co-doped g-C3N4 samples behave significant photocatalytic performance improvements. This may be due to the co-doping obviously improving the specific surface area, accelerating the carrier separation and transfer, enhancing the possibility of excitation of photogenerated carriers, reducing carrier recombination efficiency and lengthening the lifetime of photogenerated carriers. Therefore, W/O co-doping is very promising for enhancing the photocatalytic performance of g-C3N4. However, the relatively thicker tube walls are not conducive to further increase the specific surface area of the material, which can be seen in the SEM. In addition, the reduction capacity of photogenerated electrons has been dropped with the conduction band potential decreased. To address these shortcomings, some efforts should be paid to thin the wall thickness of tubular

Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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Fig. 10 e Schematic illustration of the proposed mechanism of photocatalytic H2 evolution under visible light irradiation with the as-prepared W, O-codoped g-C3N4 samples. structures, maintain the reduction ability of photogenerated electrons by modifying the electronic structure, and reduce material resistance to raise the carrier migration rate.

C3N4. This work may present a new rational design by metal and non-metal co-doping for in-depth analysis of photocatalytic hydrogen evolution of g-C3N4 to achieve an efficient visible light driven photocatalyst and can be extended other photocatalysis systems.

Conclusion In summary, tungsten-oxygen co-doped hollow g-C3N4 with tubular structure has been successfully prepared for the first time via a one-step pyrolysis. Photocatalytic water splitting tests reveal that the W/O co-doped g-C3N4 sample with the 1.0% W/g-C3N4 sample obtains a highest hydrogen production rate of 403.57 mmol g1 h1, nearly 2.5 times high than that of pristine g-C3N4. The structure and property characterization results suggest that the W/O doping significantly affects the band structure, specific surface area, electronic structure as well as crystallinity of the g-C3N4, thereby tuning the photocatalytic performance of the samples. By W/O co-doping, visible light absorption intensity has been enhanced, the absorption range has been expanded and the specific surface area expanded significantly. In addition, narrowed band gap and restrained charge recombination are conducive for the excitation of electron-hole pairs and transportation. However, the position of the conduction band potential shifts positively with the W/O doping. Therefore, excessive W/O doping would deteriorate the hydrogen evolution performance of the g-

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements This project is supported by the National Natural Science Foundation of China (Grant no.51772072,51402078), the 111 Project (B18018), the Foundation for Tianchang Intelligent Equipment and Instruments Research Institute (Grant No. JZ2017AHDS1147), Anhui Provincial Nature Science Foundation (No.1608085ME93), Fundamental Research Funds for the Central Universities (No.JZ2015HGCH0150, JZ2016HGTB0719), Young Scholar Enhancement Foundation (Plan (B) of HFUT, China (JZ2016HGTB0711). H.H.Tan acknowledges the award of the Overseas Distinguished

Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261

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Professorship (Haiwai Mingshi) by the Chinese Ministry of Education.

Appendix A. Supplementary Material The detailed information about Figs. S1eS9, and Tables S1eS3 were provided in the Supplementary material.

Appendix B. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.09.261.

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Please cite this article as: Wei H et al., In situ W/O Co-doped hollow carbon nitride tubular structures with enhanced visible-light-driven photocatalytic performance for hydrogen evolution, International Journal of Hydrogen Energy, https://doi.org/10.1016/ j.ijhydene.2020.09.261