Z-scheme heterojunction through interface engineering for broad spectrum photocatalytic water splitting

Journal Pre-proof Z-Scheme Heterojunction through Interface Engineering for Broad Spectrum Photocatalytic Water Splitting Shu Xu (Formal analysis) (Investigation) (Visualization), Shuaiqi Gong (Formal analysis) (Investigation) (Visualization) (Writing original draft), Hua Jiang (Formal analysis) (Investigation) (Visualization), Penghui Shi (Conceptualization) (Formal analysis) (Investigation) (Visualization) (Writing - review and editing) (Supervision) (Resources) (Funding acquisition), Jinchen Fan (Formal analysis) (Investigation) (Visualization), QunJie Xu (Conceptualization) (Formal analysis) (Investigation) (Visualization) (Writing - review and editing) (Supervision) (Resources) (Funding acquisition), YuLin Min (Conceptualization) (Formal analysis) (Investigation) (Visualization)Writing -original draft) (Writing - review and editing) (Supervision) (Resources) (Funding acquisition)

PII:

S0926-3373(20)30076-X

DOI:

https://doi.org/10.1016/j.apcatb.2020.118661

Reference:

APCATB 118661

To appear in:

Applied Catalysis B: Environmental

Received Date:

28 November 2019

Revised Date:

4 January 2020

Accepted Date:

19 January 2020

Please cite this article as: Xu S, Gong S, Jiang H, Shi P, Fan J, Xu Q, Min Y, Z-Scheme Heterojunction through Interface Engineering for Broad Spectrum Photocatalytic Water Splitting, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118661

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Z-Scheme Heterojunction through Interface Engineering for Broad Spectrum Photocatalytic Water Splitting

Shu Xua, Shuaiqi Gonga, Hua Jianga, Penghui Shia,*, Jinchen Fana, QunJie Xua,b*, YuLin Mina,b*

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Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric

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Power,Shanghai University of Electric Power, Shanghai200090, P.R. China b

Shanghai Institute of Pollution Control and Ecological Security, Shanghai200092, P.R. China Email: [email protected]; [email protected]; [email protected]

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Graphical Abstract

Highlights 

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The composite material with near-infrared light (NIR light) response was prepared. Modification of bulk BP can control the band gap of the composite material. The rate of photocatalytic hydrogen evolution reaches 10.77 μmolg-1h-1 under NIR light.

Abstract: In this paper, we have designed the composite material of black phosphorus /tungsten nitride (BP/WN) with near-infrared light (NIR light) response as a noble-metal free photocatalyst for the first time. Modification of bulk BP by ball

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milling can control the band gap of the composite material to achieve efficient

photocatalytic hydrogen evolution in the near-infrared region. At wider than 420 nm,

and with illumination wider than 700 nm, a certain amount of hydrogen can be produced without any hole quencher. The rate of photocatalytic hydrogen evolution

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from pure water reaches 10.77 μmolg-1h-1 under NIR light without any cocatalyst. The

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presence of a Z-type heterojunction effectively separates photogenerated carriers and prolongs the lifetime of photogenerated carriers, thereby enhancing the photocatalytic

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performance. The results indicate that BP/WN photocatalysts can provide a new approach to artificial photosynthesis and renewable energy conversion.

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Key words：photocatalyst, Z-type heterojunction, NIR light

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Introduction

It is well-known that hydrogen (H2) is a kind of the green and renewable energy

which can be produced from water using solar energy. In addition, solar energy is the most abundant source of all clean and sustainable energy.[1, 2]. To ease the growing energy crisis, H2 is becoming important gradually. Therefore, photocatalytic water splitting without hole quencher to produce H2 has attracted more attention during the past several decades. However, it is a challenging task to make full use of solar energy

resources. As we know, near-infrared (NIR) light occupying ca. 50% of the solar radiation is well worth developing and utilizing[3]. However, the photon energy beyond the 700 nm near-infrared region is small (less than 1.77 eV) and the high carriers recombination rate limit the efficiency of the photocatalytic reaction in the NIR range. Therefore, designing a new NIR responsive photocatalyst without any hole quencher is highly imperative, which can achieve boosted NIR light driven H2 evolution performance[4]. As we know, the performance of the photocatalyst is generally influenced by the

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following main factors: (1) the range of light absorption; (2) efficient separation and fast transfer of photogenerated electron-hole pairs[5, 6]; (3) excitonic eff ects mediated by coulomb interactions between photogenerated electrons and holes[7]. It should be noted that the minimum band gap for overall water splitting is 1.23 eV

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theoretically, and large overpotentials are always needed to overcome the activation

barriers for surface chemical reactions that produce H2 and O2 experimentally.

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Therefore, it is difficult for the semiconductor photocatalyst to achieve the effect of photocatalytic water splitting of H2 evolution by hydrogen under irradiation of NIR

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light beyond 700 nm [1, 8]. For example, TiO2[9, 10] and ZnO[11] have substantially no response in the visible (vis) to NIR region[12]. Therefore, it is very important to

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overcome the above shortcoming.

Meanwhile, compared with the conventional semiconductor photocatalyst, many conductors can act as photocatalysts with NIR light response due to their extremely

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high carrier density no band gaps and special partial occupation bands can be similarly taken as the conduction band[13, 14]. Besides, they also have a fully

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occupied band (B-1) and a lowest unoccupied band (B1) with some similarities to the semiconductor. As shown in Scheme 1, the H2 evolution process (I-IV) is divided into a one-step process of the electron-hole pairs creation or a two-step process of the electron-hole pairs creation. As long as the electron and hole potentials excited by the above process conform to the redox potential, it is possible for the conductor to achieve photocatalytic water splitting in the near-infrared region without sacrificial agents.

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Scheme 1. Possible Phototransition Processes (I−IV) in the Conductors, Holding Promise for Simultaneously Ensuring IR Light Harvesting and Appropriate Band Edge Positions

Matched with H2O Reduction Potentials (B-1, the highest fully occupied band; B1, the lowest

unoccupied band; CB, conduction band; h+, the photogenerated hole; e−, the photogenerated

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electron. The black line in the CB band represents the Fermi level.)

It is interesting that tungsten nitride (WN) is just a suitable conductor for

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photocatalysis. At first, WN is well known as a transition metal tungsten nitride similar to the catalytic properties of some metals[15]. WN is generally used as an

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inexpensive and easily synthesized material for electrocatalysis[16-18]. Recent studies have reported that WN shows indeed good performance in photocatalysis[19].

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Moreover, tungsten nitride can achieve effective photocatalytic hydrogen production performance without sacrificial agents compared to most photocatalysts. The light absorption range of WN is extremely wide to achieve photocatalytic water splitting at

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765 nm light. However, it is still limited by its poor H2 evolution performance. In order to adjust the performance, black phosphorus (BP) is chosen to modify them.

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As we know, phosphorus exists in three main forms in nature: white, red, and

black allotropes[20]. Unlike white phosphorus and red phosphorus (RP), black phosphorus (BP) attracts extreme attentions on account of its excellent properties in recent years. Two dimensional (2D) structure and excellent conductivity result in the large specific surface area and high-speed transmission of electrons [21]. At the same time, it possesses a bandgap from ∼0.3 eV of bulk BP to ∼2.0eV of monolayer phosphorene with the characteristics of the adjustable bandgap[22]. So it is important

to obtain a suitable bandgap BP to construct appropriate photocatalytic composites [23]. Taking into account that the adjustable bandgap of BP is influenced by its thickness, it is urgent to get an appropriate BP nanoflake. High energy ball-milling is a good method to get BP nanoflake (BP-BM) [24] which can be used as a noble metal-free photocatalyst for H2 evolution[8, 25]. In this contribution, the binary nanohybrid (BP-BM/WN) material composed of the BP and WN nanosheet by high energy ball-milling (BM) under the protection of Ar has been prepared which can be seen in Scheme 2[26]. Firstly, bulk BP can be

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synthesized by the previously reported method of low-pressure mineralized red phosphorus[27]. Meanwhile, inspired by the simple high-energy ball milling method in recent years to easily strip graphite powder into several layers of graphene sheets,

we managed to prepare BP nanosheets by the same method to achieve the purpose of

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changing the BP band gap by controlling the layers. According to the experiment and calculation, since the minimum VB value of BP is 1.16 eV and the maximum value of

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CB is -0.43 eV, which can form a Z-scheme heterojunction with the band strucrture of WN[28, 29]. The photocatalyst shows a high H2 evolution performance of ＞420 nm,

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which can produce 9.42 μmol of H2 for 5 hours and can still produce a certain amount of 0.54 μmol of H2 at a time of ＞700 nm for 5 hours. More interestingly,

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BP-BM/WN can get the high H2 evolution without any hole quencher. It can be considered that the high photocatalytic H2 evolution performance is due to the formation of a Z-scheme heterojunction by WN and BP, which can greatly

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enhance the effective electron transfer at the interface and inhibit the reorganization of photogenerated charges, thereby improving the photocatalytic performance[30].

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Therefore, BP-BM/WN, as a noble metal-free photocatalyst with wide-spectrum solar absorption, can provide a new paradigm for sustainable photocatalytic H2 evolution without hole quencher.

Scheme 2. Schematic illustration of the fabrication of the BP-BM/WN hybrids.

RESULTS AND DISCUSSION

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Characterization of BP/WN The morphology of the samples was characterized by scanning electron

microscope (SEM), transmission electron microscope (TEM), highresolution TEM (HRTEM) and element mapping. As shown from the SEM images in tabure S1, the

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thin sheet-like structures with the size of 100 nm to 1 μm of WN have been self-assembled to form the morphology of the flower. Moreover, from the TEM image

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it can be found that the width of sheet-like BP is about 3 to 4 μm (Figure 1a)[31]. The clear lattice stripes of BP crystal plane (040) with a d-spacing of 0.26 nm can be

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easily found from HRTEM (Figure 1b), and the lattice stripes of WN crystal plane (200), (111) can be found in Figure 1d simultaneously. From the BP/WN diagram, we

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observe the lattice fringes of the WN (200) crystal plane and the lattice fringes of the BP (040) crystal plane. To further demonstrate the formation of the heterojunction, it is found that the interlacing of WN and BP lattice fringes reflects the generation of

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heterojunction marked with the yellow lines from the TEM image of BP/WN (Figure 1f). Therefore it is concluded that the improvement of photocatalytic performance is

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due to the interface between WN (200) planes and BP (040) planes. The distribution of W, N, P elements from the BP/WN is also analyzed by Energy Dispersive Spectrometer (EDS) (Figure 1h-j), from which BP and WN are evenly distributed uniformly from the EDS element mapping and the red, blue and purple parts represent P, W and N, respectively. And the STEM image shows the area which we analyzed (Figure 1g). From them, it can infer that WN is attached to BP. Furthermore, there is no big change in TEM of photocatalysts with different weight ratios of BP-BM and

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WN (Figure S3a). And the WN becomes fragmented after ball milling (Figure S3b).

Figure 1. TEM (a, c, and e) and HRTEM (b and d) images of BP-BM (a and b), WN

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nanosheets (c and d), and BP-BM/WN (e and f). STEM image (g); EDX elemental mapping of P (h), W (i), and N (j).

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Atomic force microscopy (AFM) can be applied to demonstrate the morphology and thickness of prepared BP, BP-BM and composites (Figure S4 and S5). As

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reported in the literature[32, 33], the thickness of a single layer of BP is about 0.5-0.7 nm. Comparing Bulk BP with BP-BM, it can be clearly found that the thickness of black phosphorus has changed greatly. The AFM indicates that the thicknesses of

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Bulk-BP are between 50 and 70 nm (Figure S4), which is equivalent to 100 to 120 phosphorene layers. In contrast, the thicknesses of three isolated BP-BM flakes

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measured from AFM images are 0.65, 2.68 and 3.09 nm (Figure S5), which can correspond to the number of layers with 1, 4 and 7 layers of black phosphorus, respectively. Due to previously known studies, the photocatalytic H2 evolution performance of the bulk BP is much lower than that of the BP nanosheets. So the photocatalytic performance of BP can be improved by ball milling. As shown in Figure 2a, X-ray diffraction patterns of bulk BP and BP ball-milling(BM) for 6h show the characteristic peaks of BP[34]. However, it can also

be observed that the crystallinity of BP is greatly reduced after ball milling. Furthermore, XRD patterns show that all the diffraction peaks of BP and WN are well displayed without any impurity peak. WN belongs to the space group of Pm3m (221), and the diffraction peaks corresponding to 37.7°, 43.8°, 63.7° , 76.5°, and 80.6° can be indexed as (111), (200), (220), (311), and (222) diffraction planes respectively. It is obvious that there is no apparent influence on the XRD peaks of WN after the modification of WN by BP. The reason is that the relatively weak van der Waals (VDW)interaction between WN and BP cannot alter the crystal structure of WN.

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Moreover, the XRD peaks of the BP-BM become weaker and broader, suggesting that the crystal property is significantly weakened after ball-milling. As shown from the

typical diffraction peaks of both WN and BP in the Figure 2b, it is obvious that the BP-BM/WN peak shows a slight blue shift compared to that of the pure material,

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which can be ascribed to the successful synthesis of BP-BM/WN. In summary, it can

be considered that BP and WN have been successfully combined. Furthermore, from

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the Raman spectrum in Figure 2c, three typical characteristic peaks can all be observed from the BP and BP/WN at about 348, 421, and 447 cm-1, which are

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assigned to A1g , B2g , and A2g modes of BP respectively, indicate that the BP/WN still maintains the vibration structure of the BP. It can be clearly observed that

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compared with Bulk-BP, the slight red shift of the vibration mode of BP-BM proves reduction of the thickness of BP-BM to some extent compared with that of Bulk-BP.

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Since the thickness of BP is related to the number of layers, it confirms that ball

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milling can modify the number of layers of BP.

Figure 2 XRD patterns of Bulk BP and BP-BM (a); XRD patterns of BP-BM, WN, and BP-BM/WN (b); Raman spectra of WN (black line), BP-BM (red line), and BP-BM/WN

(blue line) (c).

Furthermore, to investigate the interfacial interactions between BP and WN and the chemical configurations in BP/WN, the XPS spectra have been used to confirm the W, N, and P elements in Figure 3. As shown in Figure 3a, the peaks at 129.5, 130.3, and 134.7 eV are respectively assigned to P 2p3/2, P 2p1/2 and oxidized phosphorus (PxOy) accordingly. A peak centered at about 133.9 ev can be ascribed to the typical for P-N coordination[35, 36], which is benefit for reducing photo-carrier recombination. In W 4f spectrum (Figure 3b), two strong peaks of W from W-N locate

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at 34.8 and 32.6 eV confirm the W element as the predominant component in WN[19]. In addition, the weak peak at 37.6 eV is attributed to W-O bond as a typical

phenomenon in nitride, which indicates the trace of WO3 in the complex[19]. Furthermore, three peaks at a binding energy of 398.4, 401.2, and 402.8 eV are

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observed in the N1s spectrum (Figure 3c). The main peak at 398.4 eV is derived from the N atoms combing with W atoms, while peaks at 401.2 eV and 402.8 eV are related

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to N-P and N-N bonds[19], respectively. Furthermore, the presence of N-P demonstrates the interaction between the WN and BP interfaces. Due to the presence

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performance[8].

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of N-P, the stronger binding of the substance is benefit to improve the catalytic

Figure 3. XPS spectra of P 2p (a); W 4f (b) and N 1s (c) of BP-BM/WN.

As displayed in Figure 4b, it is obvious that the bulk BP shows a very wide

absorption from the UV to the NIR region. Moreover, WN can also absorb light from UV to NIR. Therefore, BP-BM/WN composites can respond from UV to NIR as well. The BP-BM shows a significant absorption edge around 750 nm, which cause the BP/WN sample not only a significant red shift compared to the BP absorption band

but also promote more light absorption region. Compared with BP-BM, the increase of BP-BM/WN in light absorption is mainly due to the broad spectral absorption of WN, which can provide more energy for the photocatalytic process, thereby increasing the catalytic efficiency. The band gap of BP derived from the UV-vis-NIR diffuse reflection according to the KM function is about 1.59 eV (Figure S8). Moreover, from ultraviolet photoelectron spectroscopy (UPS) spectra (Figure 6)，it is concluded that the flat band potential of BP-BM is -0.44eV vs. NHE. Thus, it is confirmed that the conduction band (CB) potential of BP-BM is about -0.44 eV vs.

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NHE. According to the calculated band gap, the valence band (VB) of BP-BM can be calculated to be 1.15 eV vs. NHE. The result proves that the BP-BM can be used as a

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photocatalyst for H2 evolution.

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Figure 4. UV-vis-NIR DRS spectra of BP-BM, WN and BP-BM/WN (a); The AQE of BP-BM/WN (b)

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The band structure of WN and BP is further investigated by theoretical calculation. We perform DFT calculation on the (200) lattice plane of WN and the (040) lattice

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plane of BP[31]. Atomic conformations for WN (200) and BP (040) crystal planes were illustrated shown in Figure 5(a) and (b), respectively. According to the density functional theory (DFT) calculation, the corresponding calculated density of states (DOS) could be also divided into B1, CB, and B-1 bands. And the band structures of WN can be clearly illustrated from the Figure 5. The continuous bands above the Fermi level of both WN are indexed as the unoccupied band B1 because there is no electron above the Fermi level at the ground state. In detail, the region above 0 eV can

be attributed to the B1 band, while the region below -5.79 eV can be attributed to the B-1 band with strongly hybridized W 5d and N 2p states. The partially occupied conduction band CB can be divided into two regions: (a) From -5.79 eV to -1.4 eV, it is the first nonbonding state with the presence of a high peak of the DOS at -2.8 eV; (b) from -1.4 eV to the Fermi level, the second nonbonding state appears, resulting in another high peak of the DOS at -1.2 eV. Below the CB, two regions of DOS with the band gap of 5 eV are observed: one band B-2 from -20 to -16 eV with mainly N 2s states, and the other band B-1 between -9.7 and -5.79 eV with strongly hybridized W

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5d and N 2p states. Moreover, the DOS of 3.50 states eV-1 atom-1 at the Fermi level reveals the metallic nature of WN[19]. From this result, the conclusion that the WN

has properties close to the metallic materials can be obtained. Similarly, the band structure of BP was also calculated. For BP (040) surface, the top of the valence band

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(VB) has been observed at the Fermi level. And the bottom of the conduction band (CB) of BP can also be founded at 1.235 eV. Note that the band gap between VB and

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CB was calculated to be 1.235 eV, which is similar to the experimental result 1.59 eV obtained from UV-vis-NIR diffuse reflection. When irradiated under the simulated

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sunlight, the electron-hole pairs can be generated by transitions and the electrons are excited to the unoccupied CB while the holes remain VB, which can reach the

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condition for photocatalytic H2 evolution.

Figure 5. DFT model and the total density of states (TDOS) of BP (a); DFT

model and the total density of states (TDOS) of WN (b); schematic diagrams of BP/WN calculation band structure (c). Meanwhile, the ultraviolet photoelectron spectroscopy (UPS) is carried out to confirm the ionization potential of WN. The onset (Ei) and cutoff (Ecutoff) energy was 1.07 and 16.28 eV respectively, as shown in image (Figure 6a). And the ionization potential of WN can be easily gotten through subtracting the width of the UPS spectra from the excitation energy is about 21.21 eV. Thus, the ionization potential of WN is calculated to be -6 eV, which is similar to the energy -5.79 eV of

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the top of the B-1 obtained from density functional theory (DFT) calculations. Therefore, the result of DFT calculation of WN can be proved by UPS test. Both of the calculation and experimental results confirm the metal properties of WN. In the

same time, the VB edge of BP-BM is about 1.15 eV vs. NHE which can also be

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analyzed from its UPS spectrum. And the bottom of CB can be inferred about -0.44

eV vs. NHE. The CB potential of BP-BM is -0.42 eV vs. NHE which can also be

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analyzed by the Mott-Schottky (MS) plots is similar to the result of UPS spectrum. It indicates the reliability of the result and the theoretical possibility for photocatalytic evolution.

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H2

Figure 6. Ultraviolet photoelectron spectrum (UPS) of WN (a) and BP (b). Insets: UPS in the onset (right) and the cutoff (left) energy regions.

Electrochemical impedance spectroscopy can be effectively used to compare the internal charge transfer of different materials. The diameter of the semicircle represents the size of the internal charge transfer resistance. As shown in Figure 7a,

the charge transfer resistance of BP-BM/WN is the smallest under illumination, indicating that the BP-BM/WN composite possesses the excellent conductivity and can accelerate the charge separation compared with the raw material. As usual, the smaller arc radius indicates the lower charge transfer resistance, thus the faster interfacial charge transfer. These dramatically reduced barriers may benefit from the forming heterostructure between BP and WN, causing the quick charge transfer at the surface of BP-BM/WN. The photocurrent response tests on all-light, >420 nm, >700 nm conditions are also carried out to study the generation and separation of interfacial

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charge. We can see from Figure 5b that the corresponding magnitude of photocurrent follows the order of BP-BM/WN>BP-BM>WN, indicating that the material

composite with the heterojunction can speed up the process of charge generation and separation[37]. The obvious light current appears under NIR light irradiation of the

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λ>700 nm filter, which demonstrates that the BP-BM/WN material can respond in the NIR light region and indicates that the electron-hole pairs can be effectively separated

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under 700 nm illumination. For the analysis of near-infrared photocurrents under NIR light irradiation of the λ>700 nm, we can observe that the corresponding magnitude of

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photocurrent follows the order of BP-BM/WN>BP-BM>WN>Bulk BP. And the photocurrent intensity of all samples remained stable under long-term irradiation,

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which proves the stability of the sample. The separation of the ability of the charges under full light irradiation also can be examined over the photocurrent response in Figure S7. We can also observe from the LSV image (Figure 7d) that the BP / WN

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sample has a lower overpotential at the same current density than Bulk BP, BP-BM, and WN, and also indicates that the formation of a heterojunction between WN and

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BP contributes to the separation of photogenerated carriers[30].

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Figure 7. The PEC measurements: (a) EIS of BP-BM/WN, BP-BM; WN (b) photocurrent response of BP-BM/WN; (c) photocurrent response under NIR light (with the filter of λ>700

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nm) irradiation; (d) LSV of BP-BM/WN, BP-BM, WN.

Besides, the photoluminescence (PL) spectrum has been used to study the transfer of photogenerated electrons. When the excitation wavelength is fixed at 375 nm, the

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distinct broadband around 570 nm have been observed in all samples. As shown in the PL spectra (Figure 8a), BP-BM/WN shows the weakest peak intensity, which

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indicates the reduction of charge recombination. And the effective separation of electrons and holes at the interface between BP and WN can be accelerated so that

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more photogenerated electrons participate in the reaction of hydrogen reduction. Since the B1 band potential of WN is higher than the CB potential of BP, the photogenerated electrons of WN are transferred to the BP surface, preventing their recombination with holes. Therefore, it illustrates that BP-BM/WN possess a stronger charge separation capability than BP and WN. The lifetimes of three photocatalysts are shown in time-resolved PL spectra in Figure 8b. Results show that the intensity-average (τ) PL lifetimes for BP-BM/WN photocatalyst is the longest in

comparison with Bulk-BP and WN catalysts. As a result, the decay rate of electrons becomes slower, and the lifetime is prolonged. And it can be inferred that when the electrons of BP-BM are excited to its conduction band, the P-N trap sites[8] beneath the conduction band can serve as electron trap level to capture most of the electrons. This result suggesting that the formation of BP-BM/WN heterojunction is able to

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effectively suppress the charge carriers’ recombination and elongate their lifetimes.

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Figure 8. The steady-state PL spectra (a); Time-resolved PL spectra (b).

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Photocatalytic H2 evolution

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Figure 9. Photocatalytic H2 evolution from DI water on different catalysts under visible light (>420 nm) irradiation (a); Effect of BP-BM: WN ratio in BP-BM/WN on photocatalytic H2

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evolution rate under visible light irradiation for 5 h (b); Photocatalytic H2 evolution based on BP-BM/WN with >700 nm light irradiation (c); Cycle stability test on BP-BM/WN

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photocatalytic H2 evolution under >700nm light irradiation (d).

The performance of the samples have been tested as following: 10mg catalyst was dispersed in 10 mL of deionized water and added into a 35 mL cylinder reactor and

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sealed with a rubber septum. Then the suspension was placed in an ultrasound machine for 20 min. And the whole system was deaerated by Ar bubbling into the

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dispersion for 2 h to completely remove the dissolved oxygen. Afterward, the system was irradiated by using a xenon lamp (Asahi Spectra, HAL-320W, output wavelength:

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350−1800 nm) with a 420 and 700 nm cutoff filter. The current of the xenon lamp is set to 15A. The volume of the produced H2 was analyzed with a gas chromatograph (Techcomp, GC7900). For recycle experiments, the catalysts were collected by high-speed centrifugation after the last round of photoreaction and did the same steps as what the first recycle did. Figure 9a is a photocatalytic H2 evolution rate diagram of materials, BP-BM/WN, Bulk-BP, WN and BP-BM are tested under visible light (>420 nm) irradiation,

respectively. It is obvious that pure Bulk BP does not have photocatalytic H2 evolution properties. And only a small amount of H2 can be detected when we used pure BP-BM or WN materials individually. The low photocatalytic H2 evolution performance of the pure materials is mainly due to the fast recombination of photogenerated charges, resulting that a large number of photogenerated electrons fails to participate in the final H2 evolution process.[38] It can be observed that the photocatalytic H2 evolution rate of BP-BM/WN is 188.42 μmolg-1h-1, which is about 12 times that of BP-BM and about 108 times WN, indicating that BP-BM/WN can

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serve as an efficient noble metal-free photocatalyst for H2 evolution without any other cocatalysts and sacrificial reagents. The H2 production performance of BP-BM/WN is greatly improved compared with those of Bulk BP, BP-BM and WN, indicating that

the generation of heterojunction and P-N coordination bond has a significant effect on

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the improvement of photocatalytic performance. We also studied the effect of the

weight ratio of BP-BM and WN on H2 evolution performance under the same total

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mass conditions. The optimal ratio of BP: WN in the current photocatalytic system was 3:2, which can produce a H2 evolution rate of 188.42 μmol g-1 h-1 (Figure 9b).

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Beyond the optimal ratio, when the weight ratio of BP continues to increase, the WN is relatively decreased, resulting in a decrease of photocatalytic efficiency and vice

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versa.

Furthermore, the remarkable photocatalytic H2 evolution performance of BP-BM/WN under NIR light irradiation has been investigated. As is well known, bulk

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black phosphorus has no performance under near-infrared light, while WN and BP-BM shows only weak properties of H2 evolution (1.49 μmolg-1h-1 and 2.36

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μmolg-1h-1). Figure 9c shows continuous H2 evolution with approximately 0.54 μmol during BP-BM/WN photocatalytic reaction under >700 nm for 5 h in the presence of DI water (10.77 μmolg-1h-1). It also can be clearly seen that the photocatalytic water splitting ability of the composite is much improved, which is attributed to the formation of the heterojunction causes the decreased recombination rate of the photo-generated carrier. In order to investigate the stability of the BP-BM/WN photocatalyst, it was

circulated under >700 nm light irradiation for 15 h, and no significant decrease in the photocatalytic performance of the material was observed in Figure 9d. After three recycles, the catalysts are stored for a month protected under Ar and test the H2 evolution. The H2 evolution performance has slightly declined, but it remains stable, indicating that BP-BM/WN remained stable after the photocatalytic reaction. And part of the reason for the decrease in photocatalytic performance in the cyclic reaction is due to the loss of catalyst during centrifugation. Moreover, it shows stable and continuous hydrogen generation under long-term NIR light. And its stability remains

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in the visible range (Figure S10).

Figure 10. The H2 evolution of BP-BM/WN under >420 nm light irradiation and 60 degrees

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Celsius constant temperature without light irradiation (a); The H2 evolution of BP-BM/WN under >700 nm light irradiation and 40 degrees Celsius constant temperature without light

irradiation (b); The H2 evolution of BP-BM/WN under light irradiation or dark condition (c); The H2 evolution of WN in different condition under >420nm light irradiation (d).

In order to determine the role of the composite as a photocatalyst, the controlled experiment without light irradiation was carried out. When the reaction system under light irradiation, the temperature of the reaction system will rise. So we provide a

continuous constant temperature water bath corresponding to the temperature environment to eliminate the influence of temperature on the experiment. As we can see from the result (Figure 10), there is almost no H2 evolution without light irradiation. And the tiny H2 evolution may be due to the pyroelectric effect of BP[39]. At the same time, the control experiment of the ball milled WN was also carried out. Thus, we can conclude that the light irradiation plays a major role in our experiment. And we can get the conclusion that ball-milling does not improve the performance of H2 evolution. Besides, the different ball-milling time experments was also conducted

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to find out the most appropriate ball-milling time of BP (Figure S11). The apparent quantum efficiency (AQE) of H2 evolution for BP-BM/WN is showed

in Figure 4b at λ=420, 500, 600, 700, 800 ± 15 nm, respectively. When the 800nm bandpass filter is loaded on the xenon lamp, although the AQE is low (0.24%) in

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Figure 4b, we can still observe stable and continuous H2 generation. And the TEM of BP-BM/WN were measured after the photocatalytic reaction (Figure S2), it can be

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concluded that BP-BM/WN remained stable in the photocatalytic response without any significant change. In short, BP-BM/WN demonstrates its unique superiority over

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other photocatalysts recently.

Proposed Mechanism for Photocatalytic H2 Evolution.

It is well known that the separation and transfer of effective charges is the key to improve photocatalytic performance at present[40]. For BP nanosheets, the band gap is directly related to its number of layers. Therefore, the bulk BP needs to be ground by high-energy ball milling. In the process, the VB of BP is moved positively, and the CB is moved in the negative direction, resulting in the larger band gap of BP-BM compared to that of the Bulk BP. According to UPS spectra, it concludes that the CB

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and VB edges of BP-BM are -0.44 eV and 1.15 eV respectively. Similarly, the B1 and B-1 edges of WN are respectively -1.61eV and 1.50 eV. Based on the above analyses, we propose a possible Z-scheme mechanism accounting for the enhanced

photocatalytic overall water splitting of BP-BM/WN as illustrated in Figure 11c.

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From the viewpoint of the band structure of BP and WN, a heterojunction is expected at their interface when WN is attached on the surface of BP-BM. Due to the presence

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of this heterojunction, the partially occupied CB in WN generates electrons and holes under the excitation of light and the photogenerated electrons transition to the B1

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band[8, 41]. BP can act as an electron acceptor from adjacent WN due to the potential difference between the VB of BP and the B1 band of WN, which inhibits

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recombination of the photogenerated charges in WN. Hence, the BP layer will remain positive charges, and accordingly, the WN layer will accumulate negative charges (Figure 11a). This process is different from the traditional process in that the built-in

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electric field from WN to BP is formed at the phase junction interface between BP-BM and WN, so that photogenerated carriers can be continuously separated and

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transferred through the Z-scheme structure, greatly improving photocatalytic performance. However, a traditional type II charge transfer pathway requires more energy due to band bending and internal electric field, which is not conducive to continual charge transfer in thermodynamics (Figure S12). And the Z-scheme structure can also be supported by photoelectrochemical measurements (Figure 11b). Onset potentials of cathodic photocurrent over the BP working as a H2-evolving photocatalyst in the Z-scheme system possessed more positive than that of anodic

photocurrent over WN, which indicates that photogenerated electrons in WN will transfer to BP. The above results can be proved from the PL spectra that the recombination rate of photoregenerated carriers of BP-BM/WN is lower than that of pure BP-BM and WN. Photogenerated electrons in CB of BP are trapped by P-N coordination bonds at the interface for water splitting, thereby improving its photocatalytic H2 evolution performance. Surprisingly, there is no O2 evolution throughout the reaction. And the reason is that the holes possessing oxidizability on the CB and B-1 of WN can react with water to produce ·OH which can be detected by

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EPR spectra (Figure S15a)[42, 43]. And the decomposition of Rhodamine B dye solution[44, 45] (Figure S15b and c) can also prove production of hydroxyl radicals (·OH). It indicate that photocatalytic reaction we studied is part of the overall water

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splitting[46].

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Figure 11. Z-scheme charge transfer pathway (a); Current-potential curves of BP and WN

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under visible light irradiation (b);Proposed schematic diagram for the visible and NIR light activated photocatalytic H2 evolution using BP-BM/WN in the DI water (c).

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CONCLUSIONS

In conclusion, the Z-scheme noble metal-free photocatalyst with a 2D/2D

heterojunction by simple high energy ball milling have been prepared. The photocatalytic hydrogen production tests on catalysts under both >420 and >700 nm light irradiation without cocatalyst and sacrificial agent have been performed. The generation of the 2D heterojunction effectively suppresses the transfer of carriers, and the metal properties of WN and the superior conductivity of black phosphorus

facilitate the transfer of carriers. The results can provide new ways in researching noble metal-free photocatalysts with broad harvesting in wide UV-vis-NIR light region in developing the solar-energy conversion.

Experimental Section Materials:

Red

phosphorus

(99.99%,

bulk,

1–5

mm),

Sodium

tungstate(Na2WO4·2H2O) and hydrochloric acid (HCl) are purchased from Aladdin Regent Co., Ltd., Shanghai. Potassium sulfate (K2SO4) is obtained from Zhanyun

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Chemical Industry, Shanghai. All reagents were used as received without further purification.

Preparation of Black Phosphorus (BP): Bulk BP is synthesized by the previously reported method of low-pressure mineralized red phosphorus[27]. First, 2 g of red

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phosphorus was added into 18 mL of deionized water, and the resulting solution was

transferred into a 25 mL Teflon-lined stainless steel autoclave and subjected to

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hydrothermal reaction conditions at 180 °C for 18 h to remove the oxide layers. Then, 500 mg of red phosphorus, tin (20 mg), and SnI4 (10 mg) are sealed in an evacuated

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quartz glass tube (~1×10-3 Pa). We heat the quartz glass tube at a rate of 1.35 degrees Celsius per minute to 650 degrees for 5 hours, and then cool the tube to 500℃ with a

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rate of 0.33℃ min-1, followed by a natural cooling process. The obtained product needs to be washed several times with hot toluene and acetone to remove excess mineralizer, vacuum dried and stored in an argon-protected glove box.

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Preparation of ball milling Black Phosphorus (BP-BM): BP-BM is obtained by mechanically milling Bulk BP using planetary ball-mill apparatuses. First, 1 g of BP

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was added into a stainless-steel jar with a capacity of 100 cm3 at a ball-to-powder weight ratio of 30:1. Then, the ball-milling process was conducted under an argon (Ar) atmosphere for 16 h with a rotation speed of 500 r min-1. After ball-milling, the obtained BP was collected and protected under Ar for later use[20]. Preparation of Tungsten nitride nanosheet (WN): 7 mmol of Na2WO4·2H2O and 1.75 mmol of K2SO4 is dissolved in 40 mL of deionized water by magnetic stirring to form a transparent solution. Then the pH value is adjusted to 1.0 by the drops of 1 M

HCl solution until the appearance of yellow precipitates[47]. Successively, the mixture is transferred to a 100 mL Teflon-lined autoclave and held at 180 ℃ for 12 h. The yellow precipitates formed in the solution were then collected by filtration, washed with DI water and ethanol several times, and finally dried in a vacuum oven at 80 ℃ for overnight. The obtained tungsten oxide powder is heated at 450 ℃ in the air for 3 h and then annealed at 700 ℃ for 3 h under the NH3/Ar flow of 100 sccm to obtain WN. Preparation of BP-BM/WN: 0.2 g of WNS stainless-steel balls and 0.3 g of BP-BM

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are added into stainless-steel jar with a capacity of 100 cm-3 at a ball-to-powder weight ratio of 30:1. Then, the ball-milling process is conducted under an argon (Ar)

atmosphere for 48 h with a rotation speed of 700 r min-1. After ball-milling, the obtained BP is collected and protected under Ar for later use[26, 32].

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Apparent quantum efficiency(AQE) Measurements

The AQE was calculated according to the following equation:

number of evolved H2 molecules×2 ×100% number of incident photons

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AQE=

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The measurement of the number of incident photons is the same as that for photocatalytic H2 evolution.

Photoelectrochemical Characterizations

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Typically, a mixture of 7.5 mg of sample and 1 mg of ethylcellulose was added to a mixed solution of 1 mL of terpineol and 0.5 mL of ethanol, and ultrasonication was

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sufficiently performed for 12 hours to uniformly distribute the sample in the solvent. Finally, 8 μL of the suspension was evenly spread on the FTO electrode to a 1.0*1.0

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cm-2 area and dried in ambient condition. All the photoelectrochemical characterizations were carried out in a

three-electrode cell system. A catalyst electrode on FTO, carbon rod and Ag/AgCl electrode were served as working electrode, counter electrode, and reference electrode, respectively. 0.5 M Na2SO4 was selected as an electrolyte. A 300 W Xenon lamp with the filter of AM 1.5G was used as a simulated solar light source. Visible light and NIR light were achieved using 420 nm and 700 nm cut-off filters. The photocurrent

responses were recorded with 10 sec turn-on and turn-off interval times. The LSV curves were conducted at a scan rate of 50 mV s-1.

Characterization X-ray diffraction (XRD) pattern was recorded using a Burker-AXS D8 Advance in the angular range of 20° to 90° in 2θ angle. X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos Axis UltraDLD with Al Kα X-ray (1486.6 eV) radiation. The measurement of scanning electron microscopy (SEM) images was obtained on a field

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emission scanning electron microscope (FESEM, JEOL, FEG-XL30S). The morphologies and structure of the as-prepared sample were examined with

high-resolution transmission electron microscopy (TEM) using a JEOL JEM-2100 F with Cs correction. The atomic force microscopy (AFM) images were performed on

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Agilent 5500AFM with the sample powders dispersing into ethanol. UV-vis diffuse

reflectance spectroscopy (UV-vis-DRS) measurement was conducted on a

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spectrophotometer (Shimadzu, UV-2401PC), and BaSO4 was used as the reference. The Photoluminescence (PL) images were analyzed on RF-5301 PC. The H2 detection

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was collected by gas chromatography (Techcomp, GC7900). Time-resolved photoluminescence (TRPL) spectra were excited by a Coherent F900 flash lamp. The

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Raman spectra were obtained using a Horiba Jobin Yvon LabRAM. The hydroxyl radical was detected by electrochemical electron paramangnetic resonance spectrometer (EPR, EMX Xplus， Bruker Biospin，Germany).

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Calculation method

Density functional theory (DFT) calculations were carried out to study the molecular

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interactions between solvents and membranes. All the calculations were performed using the Materials Studio 7.0 CASTEP program from Accelrys[48]. We utilized the generalized gradient approximation (GGA) within Perdew, Burke, and Ernzerhof (PBE) for exchange-correlation energy[49]. The core electrons were treated by an ultrasoft

pseudopotential.

The

minimization

algorithm

used

was

the

Broyden-Fletcher-Goldfarb-Shanno (BFGS) scheme[50]. The valence electron functions were expanded into a set of numerical atomic orbitals using a

double-numerical basis with polarization functions (DNP). The reciprocal space was sampled with a (6×6×1) k-point grid that was automatically generated using the Monkhorst-Pack method. The cut-off energy is 517.0 eV. To obtain accurate results, we optimized the atomic coordinates, which were obtained by minimizing the total energy and atomic forces. The convergence criteria were as follows: the maximal force on the atoms was 0.03 eV Å-1, the stress on the atomic nuclei was less than 0.05 GPa, the maximal atomic displacement was 0.001 Å, and the maximal energy change

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per atom was 1.0e-5 eV.

Author Contributions

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Conceptualization, Penghui Shi; Qunjie Xu; Yulin Min Experiment, Shu Xu, Shuaiqi Gong, Hua Jiang; Formal Analysis, Investigation, and Visualization, Shu Xu, Shuaiqi Gong, Hua Jiang, Penghui Shi, Jinchen Fan, Qunjie Xu, YuLin Min, Writing-Original Draft, Shuaiqi Gong, YuLin Min; Writing- Review and Editing, Penghui Shi; Qunjie Xu; Yulin Min Supervision, Resources, and Funding Acquisition, Penghui Shi; Qunjie Xu; Yulin Min

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Declaration of interests

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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 work was supported by the National Natural Science Foundation of China (NSFC) (Grants no. 21671133, 21271010), the Shanghai Municipal Education Commission (No.15SG49),

Technology

Commission

of

Shanghai

Municipality

18JC1412900) and International Joint Laboratory on Resource Chemistry.

(18020500800;

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