Oxidized impurity in transition metal nitride for improving the hydrogen evolution efficiency of transition metal nitride-based catalyst

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Oxidized impurity in transition metal nitride for improving the hydrogen evolution efficiency of transition metal nitride-based catalyst Weiliang Qi a,b,1 , Ying Zhou a,b,c,1 , Siqi Liu a,b,∗ , Honghong Liu a,b , Lok Shu Hui d , Ayse Turak d , Jun Wang c,∗∗ , Minghui Yang a,b,∗ a

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, PR China c College of Chemistry, Liaoning University, Shenyang, 110036, PR China d Department of Engineering Physics, McMaster University, Hamilton, L8S 4L7, Canada b

a r t i c l e

i n f o

Article history: Received 17 May 2019 Received in revised form 24 July 2019 Accepted 29 September 2019 Keywords: Transition metal nitride Reductive defects Photocatalytic Oxidized impurity Hydrogen evolution

a b s t r a c t Transition metal nitrides (TMNs)-based catalyst as promising alternative for precious metal-based catalyst in hydrogen evolution have gained ever increasing research attention. However, the metastable nature of TMNs often results in the inevitable formation of metallic, oxidized and hydroxide impurities during fabrication. The effects of the inevitable impurities over TMNs in hydrogen evolution reaction are still unclear. Herein, choosing Ni3 N as a typical example, the effect of oxidized impurity in transition metal nitride for hydrogen evolution has been systematically investigated. The metal oxdie-metal nitride heterojunction which has been in-situ created by calcinations process exhibits obviously improved performance in boosting hydrogen evolution efficiency of semiconductors as compared to metal nitride by forming Z-scheme pathway. Besides, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) measurements demonstrate the formation of reductive defects on the surface of semiconductors created by ammonia treatment which also boost the hydrogen evolution efficiency. Therefore, the optimal ternary sample exhibits the highest hydrogen evolution rate (151.5 ␮mol g−1 h−1 ) under solar light irradiation, which is about 54 times higher than that of pure sample. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction The global population explosion coupled with environmental pollutions from fossil fuel usage is considered major causes of global warming. This has triggered wide-spread interest in on developing alternative renewable energy source [1–3]. However, the effective utilization of the most abundant renewables (e.g., solar energy) is difficult because of their intermittent nature [4–7]. Solar hydrogen production using semiconductor-based photocatalysts provides a promising viable means for converting solar energy into storable fuels [8]. Due to its fast kinetics and small over-potentials in hydrogen evolution reaction (HER), precious metals such as Pt [9], Rh [10,11], Au [12,13], and oxides such as RuO2 [14] and

∗ Corresponding authors at: Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China. ∗∗ Corresponding author. E-mail addresses: [email protected] (S. Liu), [email protected] (J. Wang), [email protected] (M. Yang). 1 These authors contributed equally.

Rhx Cr2−x O3 [15] are usually required as co-catalysts for photocatalytic hydrogen evolution reaction (PHER). Nevertheless, the high cost, rarity, and poor stability of these precious-metal-based catalysts are major obstacles in their large-scale application [1]. Thus, huge amount of efforts has been dedicated to develop inexpensive, efficient, non-noble-metal co-catalysts for large-scale PHER [16,17]. Among various non-noble-metal-based co-catalysts, transition metal nitrides (TMNs), a class of metallic interstitial compounds, gains an increasing amount of attention because of their unique electronic structures. TMNs have an extremely small change in Gibbs free energy (GH* = 65 meV) for HER [18–21]. The combination between their low cost, high electrical conductivity, and good thermal stability, make them a viable alternative for high performance photocatalysis. Some typical TMNs, such as TiN [22], Ni3 N [16,23], Co3 N [24,25], Fe2 N [26] have been reported as efficient co-catalysts for improving the photocatalytic hydrogen (H2 ) production efficiency of semiconductors. However, the metastable nature of TMNs often results in the inevitable formation of metallic, oxidized and hydroxide impurities during fabrication. Although these inevitable impurities in TMNs are typically ignored or thought

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to lower the photocatalytic performance of TMNs-based photocatalysts [27], the relative experimental proofs are still lacking. In fact, it is known that the introduction of metal or semiconductor components into the photocatalytic systems always results in a more complex mechanism and might lead to an enhanced photocatalytic performance [28–35]. Therefore, a systematically investigation for the effects of the inevitable impurities over TMNs in photocatalysis is highly appealing yet still in its infancy. In this contribution, graphitic carbon nitride (g-C3 N4 ), a metalfree photocatalyst with desirable band-gap (Eg ≈ 2.7 eV), excellent chemical stability, non-toxicity and low cost [36–38] has been utilized as the scaffold for in-situ constructing a series of NiO/g-C3 N4 , Ni3 N/g-C3 N4 and NiO-Ni3 N/g-C3 N4 heterostructure photocatalysts. The samples are successfully prepared by controlling the calcination atmospheres and temperatures, and subsequently used for photocatalytic hydrogen production. Although Ni3 N shows small GH* in promoting H2 -generation kinetics and excellent conductivity in boosting photo-generated charge carriers separation over Ni3 N/g-C3 N4 , NiO-Ni3 N/g-C3 N4 has exhibited the highest H2 evolution rate (151.5 ␮mol g−1 h−1 ), which is about 54 times and 3 times higher than that of pure g-C3 N4 and Ni3 N/g-C3 N4 , respectively. It is due to the formation of metal oxdie-metal nitride-semiconductor heterostructures, which leads to a Z-scheme mechanism over NiO-Ni3 N/g-C3 N4 under solar light irradiation. Moreover, the heat treatment of g-C3 N4 nanosheets in the ammonia atmosphere can create reductive defects which also promote the improvement of H2 evolution rate over NiO-Ni3 N/g-C3 N4 heterostructures. This work provides new insight in rational design of novel TMN-based heterostructure photocatalysts in solar energy conversion applications. 2. Experimental section 2.1. Materials and reagents All chemicals were analytical grade and obtained from commercial suppliers and used without further purification. Nickel nitrate hexahydrate (Ni(NO3 )2 ·6H2 O, AR), Melamine (C3 H6 N6 , CP), Ammonium hydroxide (NH3 ·H2 O, GR) were purchased from the Sinopharm Chemical Reagent Co., Ltd. Sodium sulfate, anhydrous (Na2 SO4 , AR) was purchased from the Tianjin Kermel Chemical Reagent Co., Ltd. Chloroplatinic acid hexahydrate (H2 PtCl6 ·6H2 O, ACS, Pt 37.5% min), N,N-Dimethylformamide (DMF, AR) and Triethanolamine (TEOA, AR) were purchased from Aladdin Chemicals (China). 2.2. Synthesis of the photocatalyst Graphitic carbon nitride (g-C3 N4 ) was prepared by thermal poly-condensation, where melamine was used as a precursor as in Chen et al [39]. The Ni3 N/g-C3 N4 , NiO/g-C3 N4 and NiO-Ni3 N/g-C3 N4 were synthesized by a hydrothermal method. A series of Pt/g-C3 N4 (0.5%, 1%, 2%) were synthesized via the photo-deposition method in a Pyrex vessel. The detailed synthesis processes and the synthesis of the pure Ni(OH)2 , NiO, Ni3 N and NiO-Ni3 N samples are shown in the Supporting Information (SI). 2.3. Characterization of catalysts X-ray powder diffraction (XRD) measurements were performed using a powder X-ray diffractometer (RigakuMiniflex 600) with Cu-K␣ radiation (␭ = 1.54178 Å) in a 2␪ range from 10◦ to 80◦ at 1◦ min−1 . Fourier Transform Infrared (FT-IR) spectra of KBr powder-pressed pellets were recorded with a Thermo NICOLET 6700. The diffuse reflectance spectra of the samples were measured in the range of 200–800 nm using a UV–vis spectrophotometer

(UV–vis DRS, Hitachi U-3900, Japan) equipped with an integrating sphere attachment. The absorption spectra were obtained using the Kubelka-Munk relation [40], K/S = (1 − R)2 /2R, where R was the value of reflectance measurements (the relative value to the reference of BaSO4 ), K and S mean the absorption and scattering coefficients of the sample, respectively. Photoluminescence spectra for solid samples were investigated on a spectrophotometer (PL, Horiba JobinYvonFluoromax 4C-L, France) with an excitation wavelength of 365 nm. The morphological structure and microstructures of the samples were characterized by using scanning electron microscopy (SEM) (FE-SEM, Hitachi S4800, Japan) and transmission electron microscopy (TEM) (JEOL model JEM 2100 EX instrument) measurement, respectively. X-ray photoelectron spectroscopy (XPS) was collected using an AXIS Ultra DLD (Shimadzu, Japan) spectrometer with Mg K␣ excitation (1253.6 eV) and with Carbon as internal standard (C 1s = 284.6 eV). Specific surface area measurements were taken using the Brunauer-Emmett-Teller method (N2 adsorption-desorption, ASAP2020M, America). Electron spin resonance (ESR) studies were carried out on an ELEXSYS E500 spectrometer equipped with a nitrogen cryostat. The quality of the samples tested was 0.0470 g, which was then put into a quartz tube for testing. All the spectra were collected at room pressure and temperature. The electrochemical measurements were performed on a CHI760E electrochemical workstation. The detailed measurement process is shown in SI. 2.4. Photocatalytic activity test Photocatalytic H2 -generation experiment was performed in a Pyrex flask. A 300 W Xe lamp (PLS-SXE300, Beijing Perfect light Technology Co. Ltd. China) was employed as a light source. In a typical experiment, the as-prepared catalyst (20 mg) was dispersed in 50 mL of aqueous solution containing distilled water (42 mL) and triethanolamine (TEOA) (8 mL). The suspension was ultrasonicated for 20 min, re-suspended in 80 mL of aqueous solution containing TEOA (10 vol%). Before the irradiation step, a vacuum pump was used for 40 min to evacuate the dissolved air in the test system. After illuminating for 1 h, the hydrogen production was periodically analyzed using gas chromatography (Techcomp GC-7900) equipped with a thermal conductivity detector (TCD), using Ar gas as a carrier. 3. Results and discussion Fig. 1 illustrated the synthetic procedure for the NiO-Ni3 N/gC3 N4 heterostructures based on a hydrothermal method, followed by a nitridation process. First, the Ni2+ from the nickel salt are sample adsorbed by the ultrasonically treated g-C3 N4 under the long time vigorous stirring. Subsequently, the Ni2+ on the surface of the g-C3 N4 react with NH3 ·H2 O, the Ni(OH)2 nanoparticles will in-situ epitaxy on the surface of substrate g-C3 N4 under the hydrothermal condition [41]. The NiO-Ni3 N/g-C3 N4 heterostructures are prepared by nitriding the Ni(OH)2 /g-C3 N4 precursors under the ammonia atmosphere. By controlling nitriding temperature and atmospheres, the NiO/g-C3 N4 and Ni3 N/g-C3 N4 heterostructures are also successful synthesized from the Ni(OH)2 /g-C3 N4 precursors. The morphology of g-C3 N4 has been characterized by transmission electron microscopy (TEM) measurements. As shown in Fig. S2, two-dimensional (2D) g-C3 N4 nanosheets with the size over 1 ␮m have been observed. After in-situ growth of Ni(OH)2 onto the surface of g-C3 N4 nanosheets, Ni(OH)2 /g-C3 N4 exhibits a flower-like morphology. Small 2D Ni(OH)2 nanosheets with the thickness of around 20 nm vertically grow on the surface of g-C3 N4 nanosheets (Fig. 2A). Further heat treatment of Ni(OH)2 /g-C3 N4 forming NiO/g-

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Fig. 1. Schematic illustration of the synthesis process for NiO-Ni3 N/g-C3 N4 heterostructures.

Fig. 2. SEM images of (A) Ni(OH)2 /g-C3 N4 , (B) NiO/g-C3 N4 , (C) Ni3 N/g-C3 N4 , (D) NiO-Ni3 N/g-C3 N4 and energy dispersive x-ray element mapping images of (E) NiONi3 N-1.5/g-C3 N4 composite samples.

C3 N4 , Ni3 N/g-C3 N4 and NiO-Ni3 N/g-C3 N4 heterostructures, show similar morphology with Ni(OH)2 /g-C3 N4 as displayed in Fig. 2B–D. It is notable that the coverage of Ni-based nanosheets is getting tighter while increasing the amounts of Ni-based nanosheets (Fig. S3). The phase structures of a series of Ni(OH)2 /g-C3 N4 , NiO/g-C3 N4 , Ni3 N/g-C3 N4 and NiO-Ni3 N/g-C3 N4 samples are investigated by Xray powder diffraction (XRD) measurements. As shown in Fig. S4, all samples show two typical diffraction peaks at 13.1◦ and 27.5◦ which are indexed to the (001) and (002) planes of g-C3 N4 , respectively [36]. However, except for the obvious diffraction peaks at 19.3◦ , 38.6◦ and 52.2◦ attributed to the Ni(OH)2 for Ni(OH)2 /g-C3 N4 as shown in Fig. S4A; NiO/g-C3 N4 , Ni3 N/g-C3 N4 and NiO-Ni3 N/gC3 N4 exhibit almost no new diffraction peaks in XRD pattern (Fig. S4 B–D). This might results from the low loading amounts of Nibased materials (0.65 At% as displayed in Fig. S5). Although the energy-dispersive X-ray spectroscopy (EDX) spectrum and the cor-

Fig. 3. XRD patterns of (A) pure Ni(OH)2 , NiO, Ni3 N and NiO-Ni3 N samples. SEM images of (B) Ni(OH)2 , (C) NiO, (D) Ni3 N and (E) NiO-Ni3 N samples.

responding elemental mapping of NiO-Ni3 N/g-C3 N4 support the presence of Ni, C, N and O elements (Figs. S5 and 2E), the structures of Ni-based materials are hardly identified. Therefore, pure Ni(OH)2 , NiO, Ni3 N and NiO-Ni3 N have been synthesized for further investigation. As shown in Fig. 3B–E, pure Ni(OH)2 , NiO, Ni3 N and NiO-Ni3 N samples have exhibited 2D hexahedral nanosheets morphologies with the side length in the range of 200–300 nm which are similar with those in Ni-based compounds/g-C3 N4 nanocomposites. This result suggests that the heat treatment in the different atmospheres and temperatures hardly affects the morphology of Ni-based nanosheets. The XRD patterns of Ni(OH)2 , NiO, Ni3 N and Ni3 NNiO samples are displayed in Fig. 3A. The prominent diffraction

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Fig. 4. TEM image of (A) NiO-Ni3 N nanosheet. High resolution-TEM images of (B) NiO-Ni3 N, (C) Ni3 N, (D) NiO samples.

peaks of Ni(OH)2 , NiO and Ni3 N match well with the diffraction file JCPDS No: 01-078-0248, 03-065-2901 and 01-089-5144, respectively. NiO-Ni3 N exhibits pure phase without secondary impurities. The major diffraction peaks of NiO-Ni3 N are indexed to NiO and Ni3 N crystal phases. The morphology of as synthesized Ni-based samples are examined by Transmission electron microscopy (TEM) analysis. As shown in Fig. 4A, TEM image of NiO-Ni3 N indicates 2D hexahedral nanosheets morphology. In Fig. 4B, the HR-TEM image of the NiO-Ni3 N sample shows two distinct lattice fringes with d-spacing of 0.24 nm and 0.23 nm, which can be assigned to the (111) facet of NiO and the (110) lattice plane of Ni3 N, respectively [42,43]. The crossed lattice fringes highlighted in the white circle prove existence of NiO-Ni3 N heterostructure. The HR-TEM images of Ni3 N and NiO individually show the characteristic spacing of 0.23 nm and 0.24 nm for the (110) and (111) in Fig. 4C and D, respectively. It is notable that an amorphous oxide layer of about 3 nm thickness is also observed for the Ni3 N samples due the easily oxidized surface of TMNs materials. X-ray photoelectron spectroscopy (XPS) is used to characterize the state of the surface elements for the as-prepared NiO, NiO-Ni3 N and Ni3 N samples. The XPS survey spectra for the NiO, NiO-Ni3 N and Ni3 N samples are shown in Fig. 5A. It indicates that Ni and O elements exist in all samples while N element can only be observed in NiO-Ni3 N and Ni3 N samples. Fig. 5B shows the highresolution Ni 2p3/2 spectra for NiO, NiO-Ni3 N and Ni3 N samples. For the high-resolution Ni 2p3/2 spectrum of the Ni3 N, the peak at 852.5 eV corresponds to Ni 2p3/2 of metallic Ni (Ni-N) in nitride (Ni3 N) [14,44,45]. Meanwhile, an intense peak located at 855.3 eV is related to Ni2+ . Though this may be attributed to some surface oxides, as also observed previously by Xu et al. [46] and Jia et al. [47], the high intensity of this peak relative to the Ni3 N peak is not consistent with the relatively small amorphous layer observed in the TEM images of Fig. 4C. Another possible explanation is the existence of unreacted Ni(OH)2 , which also has peaks around 855.5 eV [48,49]. This is also supported by the intense hydroxyl peak evident in the O1s spectra in Fig. 5C as described below. For the highresolution Ni 2p3/2 spectrum of the NiO, the double peaks at 853.7

and 855.5 eV correspond to the expected Ni2+ state in NiO [44]. The high-resolution Ni 2p3/2 spectrum of the NiO-Ni3 N samples show a double peak shape similar to that of NiO, and the Ni-N peak cannot be observed. However, the lower binding energy Ni2+ 2p3/2 peak shows a slight negative shift of a 0.22 eV compared to that of NiO sample. The absence of metallic states and appearance of Ni+ states has previously been observed for pristine Ni3 N sample [45,50]. It is likely this peak is a convolution of NiO and Ni3 N states which are co-existing in this composite system. Fig. 5C shows the high-resolution O 1s spectra of as-synthetized NiO, NiO-Ni3 N and Ni3 N samples. For the high-resolution O 1s spectrum of the NiO, which can be assigned to adsorbed water at 532.8 eV, a hydroxyl peak at 530.8 eV from Ni(OH)2 . In addition, the peak at 529.2 eV is ascribed to lattice oxygen (Ni–O bond) from NiO [51]. The high-resolution spectrum O 1s of NiO-Ni3 N also show a peak shape similar to that of NiO, the peak of the lattice oxygen (529.1 eV) has a 0.11 eV negative shift compared to that of NiO, which might due to the effect of the existence of Ni3 N. For the high-resolution O 1s XPS spectrum of Ni3 N, the peaks located at 529.0, 530.8 and 532.0 eV are assigned to the lattice oxygen for NiO, hydroxyl oxygen from Ni(OH)2 and adsorbed oxygen species, respectively [16,44]. Though there is some evidence of surface oxide, the spectrum is dominated by the hydroxyl peak, supporting the possibility of unreacted Ni(OH)2 contaminants. Due to the high surface sensitivity of XPS, it is possible to have surface oxides or contaminants dominate the XPS spectrum, even if the XRD shows a single phase of bulk Ni3 N [46]. It is well known that a native dense NiO layer can form on the surface of Ni3 N [52], which explains the observation of lattice oxygen in the Ni3 N sample. Fig. 5D shows N 1s spectra for NiO, NiO-Ni3 N and Ni3 N samples. For Ni3 N and NiO-Ni3 N, the characteristic peak centered at 398.1 eV, which is attributed to N atoms bonded with Ni atoms (Ni-N) [44,45,53]. It is further confirmation that the Ni3 N really does exist in NiO-Ni3 N sample. The PHER performances of a series of Ni-species compounds/gC3 N4 heterostructures have been tested under solar light irradiation. As shown in Figs. S6 and S7, the optimal ratios of the Ni-species compounds over these samples are all 1.5 g Ni-species precursors (marked as Ni(OH)2 -1.5/g-C3 N4 , NiO-

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Fig. 5. (A) XPS survey spectra for pure NiO, NiO-Ni3 N and Ni3 N. High-resolution XPS spectra for (B) Ni 2p3/2 , (C) O 1s and (D) N 1s for NiO, NiO-Ni3 N and Ni3 N samples.

Fig. 6. (A) The comparison of the photocatalytic H2 evolution activity of different samples under light irradiation; (B) UV–vis diffuse reflectance spectra; (C) EIS Nyquist plots; (D) Photoluminescence spectra of g-C3 N4 base composite samples. (E) Schematic illustration of the photo-induced charge separation process in the NiO-Ni3 N/g-C3 N4 , NiO/g-C3 N4 and Ni3 N/g-C3 N4 composites.

1.5/g-C3 N4 , Ni3 N-1.5/g-C3 N4 and NiO-Ni3 N-1.5/g-C3 N4 ). The photoactivity comparison of the optimal Ni-species compounds/gC3 N4 heterostructures is displayed in Fig. 6A. It is found that all Ni-species compounds/g-C3 N4 heterostructures exhibit enhanced H2 generation rate as compared to pure g-C3 N4 nanosheets. The H2 generation rate of composite samples increase with the addition of Ni-species compounds in the sequence NiO-Ni3 N-1.5/g-C3 N4 (151.5 ␮mol g−1 h−1 ) > Ni3 N-

1.5/g-C3 N4 (53.2 ␮mol g−1 h−1 ) > NiO-1.5/g-C3 N4 (18.7 ␮mol g−1 h−1 ) > Ni(OH)2 -1.5/g-C3 N4 (11.1 ␮mol g−1 h−1 ). NiO-Ni3 N-1.5/g-C3 N4 heterostructures exhibit highest H2 generation rate, which is about 54 times and 3 times higher than those of pure g-C3 N4 nanosheets and Ni3 N-1.5/g-C3 N4 sample, respectively. This photocatalytic performance is even similar with that of the optimal precious metal-based photocatalyst 1 wt% Pt/g-C3 N4 (153.4 ␮mol g−1 h−1 , Fig. S8). Through a rough comparison to the

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other Ni based materials in Table S1, Ni3 N with oxidized impurity NiO have been proved to effectively improve the H2 -evolution rate. In order to study the photostability of the composite, the stability examination is shown in Fig. S9. An activity loss (about 27% of the original photocatalytic H2 -production activity) could be detected for the NiO-Ni3 N-1.5/g-C3 N4 sample after six consecutive reaction recycles, suggesting that the sample can’t be easily photocorroded and shows relatively stable photocatalytic activity [54,55]. To further study the effects of Ni-species compounds in improving the PHER performances of g-C3 N4 nanosheets, optical and electrochemical properties of the samples have been measured. Fig. 6B shows the UV–vis diffuse reflection spectra (UV–vis DRS) of the pure g-C3 N4 , Ni(OH)2 -1.5/g-C3 N4 , NiO-1.5/g-C3 N4 , Ni3 N-1.5/gC3 N4 and NiO-Ni3 N-1.5/g-C3 N4 samples. It is evident that loading different nickel species compounds onto the scaffold increases the optical absorption compared to pure g-C3 N4 , with a slight red shift in the absorption edge (from 2.63 eV to 2.36 eV, Fig. S10A). This result may due to the formation of chemical bonding between Ni-species compounds and g-C3 N4 nanosheets during in-situ fabricating process [54,56]. The improved optical absorption of the as-prepared samples are in agreement with the observed color change of the samples from light yellow to dark green (Fig. S11). As shown in Fig. S12, the absorption intensity gradually increases with the loading ratio of the Ni species compounds on the nanocomposite samples. By comparison, UV–vis DRS of pure Ni(OH)2 , NiO, Ni3 N and NiO-Ni3 N samples are characterized in Fig. S10B. The pure Ni3 N sample shows a typical metallic character which absorbs all photons while NiO and NiO-Ni3 N exhibit absorption edges at around 380 nm and 410 nm, respectively. It suggests the semiconductor properties of NiO and NiO-Ni3 N samples (Fig. S13). Controlled experiments of pure NiO, Ni3 N and NiO-Ni3 N samples for PHER have been performed as shown in Fig. S14. It is shown that both NiO and NiO-Ni3 N samples exhibit H2 evolution activities under solar light irradiation while the metallic Ni3 N has no PHER activity. Nyquist plots derived from electrochemical impedance spectra (EIS) measurements are useful in revealing the interfacial charge transfer process. As indicated in Fig. 6C, the Ni3 N-1.5/gC3 N4 composite has the smallest decreased diameter compared to other samples, suggesting that the smallest resistance and most efficient transfer of charge carriers between Ni3 N-1.5/gC3 N4 electrode and electrolyte solution are obtained [16,57]. This result is agreement with the electronic conductivities of the pure Ni-species compounds. As presented in Fig. S15, the semicircles of pure Ni-species compounds decrease in the sequence Ni3 N < NiO-Ni3 N < NiO < Ni(OH)2 . Generally, the PL spectra have been regularly utilized to study the charge separation performances in the excited semiconductors [56]. The photoluminescence (PL) spectra for composite samples under an excitation wavelength of 365 nm are shown in Fig. 6D. All the samples show a strong intrinsic emission band with a peak at 425–475 nm, which is attributed to the direct electron-hole recombination transition across the band gap of g-C3 N4 . The PL intensity of composite samples decrease with the addition of Ni-species compounds in the sequence Ni(OH)2 -1.5/g-C3 N4 > NiO-1.5/g-C3 N4 > Ni3 N-1.5/gC3 N4 > NiO-Ni3 N-1.5/g-C3 N4 , and also gradually decreases as the loading of Ni-species increases (Fig. S16) on the composite samples. Although Ni3 N-1.5/g-C3 N4 exhibits the best optical absorption property and most efficient charge carriers transfer, NiO-Ni3 N1.5/g-C3 N4 heterostructure show better charge carriers separation which might be the key role in improving PHER performance of the samples. In order to further determine the promoted charge separation mechanism of the NiO-Ni3 N/g-C3 N4 sample, the transient photocurrent-time (I-t) curves are measured for several visible light on-off cycles. It is generally recognized that photocurrent is produced due to the diffusion of photogenerated electrons to the back contact and, meanwhile, the capture of photogenerated

holes by electron-donor in the electrolyte. As such, the superior photocurrent density suggests a more efficient separation and longer lifetime of photoexcited electron-hole charge carriers. As shown in Fig. S17, the pure g-C3 N4 sample exhibits a lowest photocurrent density suggests the fastest charge recombination. It could be easily found that all the composite photocatalysts show higher photocurrent than the pure g-C3 N4 . The photocurrent density over NiO-Ni3 N-1.5/g-C3 N4 sample is the highest among the samples, suggesting the most efficient charge separation, thus most effectively improving the H2 -evolution rate [58,59]. The N2 adsorption/desorption isotherms and pore size distribution curves of the pure g-C3 N4 and NiO-Ni3 N-1.5/g-C3 N4 composite samples are shown in Fig. S18 and Table S2. The type III isotherms with type H3 hysteresis loops are observed, suggesting the existence of mesoporous and macropores structures in the pure g-C3 N4 and composite [60,61]. It is clear that the pore volume, pore diameter and surface area of NiO-Ni3 N-1.5/g-C3 N4 composite are higher than the pure g-C3 N4 sample. The presence of NiO-Ni3 N nanoparticles on the surface of g-C3 N4 might be the reason for the increased surface area. The larger surface area makes the composite own more active sites compare with the pure g-C3 N4 , thus shows higher photocatalytic H2 evolution activity. The relative band positions of different samples are identified by Mott-Schottky plots (Fig. S19) [62]. The flat-band potentials of the g-C3 N4 , Nitrided g-C3 N4 and NiO-Ni3 N-1.5/g-C3 N4 are estimated to be around −1.56 V vs. Ag/AgCl (i.e. −0.96 V vs. NHE), respectively. The flat-band potential of NiO is estimated to be around −0.58 V vs. Ag/AgCl (i.e. 0.02 V vs. NHE). Since both g-C3 N4 based materials and blank NiO show the characteristic behavior of n-type semiconductor, as observed from the positive slope of the linear plots [63], it is generally accepted that their conduction band (CB) potentials are considered to more negative than its flat-band potential by −0.2 V [64]. Thus, the CB potentials of g-C3 N4 based materials and NiO are speculated to be −1.16 and −0.18 V vs. NHE, respectively. Combining with the band-gap energy of g-C3 N4 and NiO, the valence band (VB) potentials of g-C3 N4 and NiO can be calculated to be 1.47 and 3.08 V vs. NHE, respectively. In view of the above discussion, a possible reaction mechanism for PHER over the NiO-Ni3 N/g-C3 N4 heterostructure is proposed (Fig. 6E). Under solar light illumination, both oxidized impurity NiO and active constituent g-C3 N4 are excited to produce photoinduced electrons and holes [65,66]. Because of the formation of semiconductor-metal nitride-semiconductor heterostructures, the NiO-Ni3 N/g-C3 N4 composite construct an all-solid-state Z-scheme photocatalytic system. The g-C3 N4 constituent is photosystem I (PS I) and the oxidized impurity NiO constituent is photosystem II (PS II). The conductor (C) Ni3 N as the electron mediator promotes the formation of the all-solid-state Z-scheme photocatalytic system. The insertion of the Ni3 N between g-C3 N4 and oxidized impurity NiO forms the known Ohmic contact with low contact resistance [67,68]. The photogenerated electrons from the CB of the NiO (PS II) can directly recombine with the photogenerated holes from the VB of g-C3 N4 (PS I) through the Ni3 N [69]. Subsequently, the photo-induced holes in the VB of NiO with high oxidation potential can be efficiently consumed by sacrificial agents TEOA and the photo-excited electrons in the CB of g-C3 N4 with the strong reductive capability can efficiently reduce the H2 O into H2 [70]. According to the previous analysis, as compared to the traditional heterostructure type-II system of NiO/g-C3 N4 [71] and metallic TMNs/semiconductor system of Ni3 N/g-C3 N4 , Z-scheme system of NiO-Ni3 N/g-C3 N4 provide more efficient photo-generated electrons and holes separation. Therefore, NiO-Ni3 N/g-C3 N4 sample exhibits the highest H2 evolution rate as compared to NiO/g-C3 N4 and Ni3 N/g-C3 N4 . As the composite samples are calcined in different environments, it is important to understand the impact of these heat

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Fig. 7. High-resolution XPS spectra of (A) C 1s and (B) N 1s for g-C3 N4 and g-C3 N4 -NH3 samples; (C) schematic procedures for the synthesis of g-C3 N4 with nitrogen defects during NH3 treatment. (D) Fourier transform-infrared (FT-IR) spectra, (E) electron paramagnetic resonance (EPR) spectra and (F) photoluminescence (PL) spectra (at 365 nm excitation) for g-C3 N4 catalysts exposed to different calcination environments.

treatments on the g-C3 N4 support structure, since the entire composite is required for adequate photocatalytic performance. With the H2 evolution activity of pure g-C3 N4 being so low, Pt cocatalyst are loaded on the g-C3 N4 as the reference substrates for the study of the effect of heat treatment environments. As shown in Fig. S20, while treating in air (1 wt% Pt/g-C3 N4 -air) decreases the evolution rate relative to the untreated catalyst (1 wt% Pt/g-C3 N4 ) from 153.45 ␮mol g−1 h−1 to 114.14 ␮mol g−1 h−1 , treating in NH3 (1 wt% Pt/g-C3 N4 -NH3 ) increases it to 220.13 ␮mol g−1 h−1 . XPS analysis has been utilized to study the surface chemical state of NH3 treatment g-C3 N4 in improving H2 evolution rate. In Fig. 7A, the C 1s spectrum of g-C3 N4 has distinct peaks at 288.1 eV, 286.1 eV and 284.8 eV, which correspond with N C N, C–NHX and C C coordination, respectively [65,72]. The existence of C C coordinated sites from graphitic carbon, with intensity as high as the N C = N feature, and the C–NHX feature related to the edges of heptazine units is already suggestive of defected g-C3 N4 [73,74], which is expected with thermal polymerization. With treatment in an NH3 atmosphere, no new peaks appear; however, there is a slight shift in both N C N and C–NHX peaks, potentially suggesting the formation of cyano groups (C N). As presented in Fig. 7B, the main N1 s peak at 398.5 eV corresponds to sp2 hybridized aromatic N bonded to carbon atoms to form C–N C framework. The peak at 399.5 eV is assigned to the tertiary N bonded to carbon atoms in the form of N-(C)3 . This is in agreement with previously reported N 1s XPS results [74]. The weaker peak with high binding energy at 401.0 eV is attributed to N H groups. In addition, after the NH3 treatment, the C–NC, N-(C)3 and N H groups [75] show 0.1 eV, 0.4 eV, and 0.3 eV negative shift when compared to those in pristine g-C3 N4 , respectively. The N-(C)3 peak exhibits a small shift to lower binding energy, which can be attributed to the generation of C N groups whose N 1s binding energy are intermediate between those of C–N C and N-(C)3 . Furthermore, the XPS data showed a progressive decrease in the N/C ratio (from 0.73 to 0.54) on the surface of g-C3 N4 -NH3 , suggesting the introduction of surface N defects [74]. FT-IR spectra have also been used to study chemical functional groups of unloaded g-C3 N4 , g-C3 N4 -air and g-C3 N4 -NH3 samples in Fig. 7D. The characteristic sharp absorption peak at 810 cm−1 is characteristic of the ring breathing mode of the triazine units

and the peaks at 1200-1650 cm−1 are attributed to the stretching vibration of the C–N heterocycles of the g-C3 N4 [72]. In addition, the peaks at 3269–3179 cm−1 are ascribed to the bending vibration modes of N H ( NH or –NH2 ) [76], which are in agreement with XPS results. A new peak located at approximately 2145 cm−1 appears for g-C3 N4 and g-C3 N4 -NH3 samples but is not observed in the g-C3 N4 -air sample. This peak can be attributed to the formation of C N bands with a defective structure [65,77]. According to the FT-IR and XPS results, it will appear that the C N groups may be located at one apex of the melon structure of g-C3 N4 -NH3 , where the C N groups originated from deprotonation of –C–NH2 (Fig. 7C) [65]. EPR analysis has been carried out to further prove the existence of defects over the heat treatment samples. As shown in Fig. 7E, all samples show a similar shape and a single feature (g = 2.00438) resulting from the generation of unpaired electrons on the carbon atoms of the aromatic ring within ␲-bonds and at defects. It is notable that both g-C3 N4 -air and g-C3 N4 -NH3 exhibit stronger signal as compared to untreated g-C3 N4 , suggesting the formation of more reductive nitrogen defects and oxidized oxygen defects in g-C3 N4 during heat treatment in the NH3 atmosphere and air atmosphere, respectively. PL spectra of the samples are displayed in Fig. 7F, NH3 -treatment significantly decreases the PL intensity compared with the emission peak of untreated g-C3 N4 . It indicates that the photo-generated charge carrier recombination is effectively suppressed by the formation of reductive nitrogen defects. Nevertheless, the oxygen defects acting as photo-generated charge carrier recombination centers promote the recombination of photo-induced electron-hole pairs and lead to higher emission peak intensity of g-C3 N4 -air than untreated g-C3 N4 [78]. 4. Conclusion In conclusion, NiO-Ni3 N/g-C3 N4 heterostructures composite sample has been successfully prepared through an in-situ hydrothermal method followed by calcination for efficient photocatalytic H2 production. The unavoidable formation of internal oxides, tuned by controlling the calcination conditions, can be

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taken advantage of to achieve higher H2 evolution rates than either pure material. The optimal NiO-Ni3 N-1.5/g-C3 N4 sample shows the highest H2 evolution rate which is about 3 times, 8 times and 54 times higher than that of the optimal Ni3 N-1.5/g-C3 N4 , NiO-1.5/g-C3 N4 and blank g-C3 N4 , respectively. The composite heterostructure exhibits a Z-scheme photocatalytic system for the H2 evolution reaction, where the NiO-Ni3 N heterostructure can effectively inhibit the recombination of photo-generated electron-hole pairs. Additionally, the exposure of g-C3 N4 to NH3 during the calcination process results in the formation of reductive nitrogen defects (N defects) in the sample which significantly enhances the H2 evolution rate for g-C3 N4 composite catalysts. For the NiO-Ni3 N/gC3 N4 photocatalyst both the existence of a Z-scheme mechanism from the NiO-Ni3 N heterostructure and the enhanced defects from the exposure of g-C3 N4 to NH3 leads to significantly higher photocatalytic performance.

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Declaration of Competing Interest [19]

There are no conflicts to declare. Acknowledgements This work is supported by National Key Research and Development Plan (Grant No. 2016YFB0101205) and Key Program of the Chinese Academy of Sciences (Grant No. KFZD-SW-320). M. Yang would like to thank for the Ningbo 3315 program. The authors greatly acknowledge the National Science Foundation of China (21371084, 21373005 and 31570154), Ontario Ministry of Research and Innovation Early Researcher Award (ER15-11-123) and Natural Sciences and Engineering Research Council of Canada (4361002013RGPIN) for financial support.

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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apmt.2019. 100476.

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Please cite this article as: W. Qi, Y. Zhou, S. Liu, et al., Oxidized impurity in transition metal nitride for improving the hydrogen evolution efficiency of transition metal nitride-based catalyst, Appl. Mater. Today, https://doi.org/10.1016/j.apmt.2019.100476