Metal-Free Photocatalyst for H2 Evolution in Visible to

Aug 31, 2017 - efficient and stable metal-free photocatalysts with broadband solar absorption from the UV to ..... Cycle stability test on BP/CN photo...
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Metal-Free Photocatalyst for H2 Evolution in Visible to Near-Infrared Region: Black Phosphorus/Graphitic Carbon Nitride Mingshan Zhu,† Sooyeon Kim,† Liang Mao,‡ Mamoru Fujitsuka,*,† Junying Zhang,‡ Xinchen Wang,§ and Tetsuro Majima*,† †

The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan Department of Physics, Beihang University, Beijing 100191, People’s Republic of China § College of Chemistry, Fuzhou University, Fuzhou 350002, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: In the drive toward green and sustainable chemistry, exploring efficient and stable metal-free photocatalysts with broadband solar absorption from the UV to near-infrared region for the photoreduction of water to H2 remains a big challenge. To this end, a binary nanohybrid (BP/CN) of twodimensional (2D) black phosphorus (BP) and graphitic carbon nitride (CN) was designed and used as a metal-free photocatalyst for the first time. During irradiation of BP/CN in water with >420 and >780 nm light, solid H2 gas was generated, respectively. Owing to the interfacial interaction between BP and CN, efficient charge transfer occurred, thereby enhancing the photocatalytic performance. The efficient charge-trapping and transfer processes were thoroughly investigated with time-resolved diffuse reflectance spectroscopic measurement. The present results show that BP/CN is a metal-free photocatalyst for artificial photosynthesis and renewable energy conversion.

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INTRODUCTION Hydrogen (H2) evolution from water using solar energy is an ideal future energy source.1−3 From economic and environmentally friendly viewpoints, being sufficiently efficient, earthabundant, nontoxic, stable, and capable of harvesting light is required for the optimal photocatalysts.2−6 Although great efforts have been made for photocatalytic H2 evolution by using diversified semiconductor photocatalysts, noble metals (such as Pt) or some kind of non-noble metals must be used in most cases as extra cocatalysts to promote efficient charge separation from the semiconductor to the cocatalyst’s surface, where water is reduced to H2.2−4 Until now, in the drive toward green and sustainable chemistry, polymers such as poly(p-phenylene),7 graphene oxide,8 graphitic carbon nitride (g-C3N4, abbreviated as CN),5,9 boron carbides,10 and boron nitride11,12 have been studied as photocatalysts without any metal elements for the photoreduction of water to H2. As a typical metal-free photocatalyst, CN grabs extreme attention owing to its high thermal and chemical stability, low cost, and nontoxic nature, since the discovery of CN as a metal-free photocatalyst for H2 evolution in 2009.5,13−17 Unfortunately, CN achieves sufficient efficiency only when it is loaded with expensive noble metal cocatalysts, due to its fast recombination of photoinduced charges (holes and electrons).9,14,16 In view of the solar energy utilization, suitable materials as photocatalysts need not only to have high efficient solar-tohydrogen conversion but also to harvest the broad spectrum of solar light, from the UV to near-infrared (NIR) region.18 © 2017 American Chemical Society

Phosphorus (P) comprises a fraction of 0.1% of the earth’s crust, which is one of the most abundant elements preserved in the earth.19 Recently, one of allotropes of P, two-dimensional (2D) black phosphorus (BP), has received great attention with respect to optical and electronic applications, since the first report on field-effect transistors in 2014.19−22 This new 2D material has a different direct band gap in monolayer, few-layer, and bulk forms, from ∼2.1 eV for monolayer phosphorene to ∼0.3 eV for bulk BP. The direct-band-gap properties enable BP to work as an efficient photocatalyst with broadband solar absorption.23−33 For example, Yu’s group reported a black−red phosphorus heterostructure as a visible-light-driven photocatalyst for degradation of organic pollutants.29 Liu’s group also reported photocatalytic degradation of rhodamine B by using Ag/BP nanohybrids under visible-light irradiation.30 Li’s group23 and Ding’s group25 independently deduced the potential of BP as an efficient water-splitting photocatalyst by density functional theory (DFT) calculations. Herein, a binary nanohybrid (BP/CN) composed of a BP nanoflake and a CN nanosheet was facilely obtained and first used as a metal-free photocatalyst for H2 evolution in the presence of methanol as a hole quencher. Compared to a trace amount of H2 evolution by a single component, the optimum H2 evolution for BP/CN reached 1.93 μmol for 3 h under >420 nm light irradiation without any metal elements. Unexpectedly, Received: August 8, 2017 Published: August 31, 2017 13234

DOI: 10.1021/jacs.7b08416 J. Am. Chem. Soc. 2017, 139, 13234−13242

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Journal of the American Chemical Society

Figure 1. TEM (a, c, and e) and HRTEM (b and d) images of BP nanoflakes (a and b), CN nanosheets (c and d), and BP/CN (e). XRD patterns of BP, CN, and BP/CN (f).

BP/CN also exhibited solid generation of H2 (0.46 μmol) under >780 nm for 3 h. The great enhancement of photocatalytic performance for H2 evolution is due to the strong interfacial interaction between BP and CN and efficient charge transfer at the interface, which inhibits the recombination of photogenerated charges and thereby improves their photocatalytic performance. To get clear proofs of the photocatalytic H2 evolution mechanism, time-resolved diffuse reflectance (TDR) spectroscopic measurements were carried out. The results indicate that BP/CN is a metal-free photocatalyst with broadband solar absorption, contributing to a new paradigm for designing photosynthesis in renewable clean energy conversion.

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CN, two clear peaks at 13.0° and 27.3°, indexing as (100) and (002), respectively, were detected.5,35−37 For BP/CN, typical diffraction peaks of both BP and CN were observed. Interestingly, the ratio of (040)/(111) diffraction peaks for BP (0.4/1) is the same as the standard pattern for BP, suggesting that BP nanoflakes are attached randomly on the surface of CN nanosheets. To clarify the interfacial interaction between BP and CN in BP/CN, TEM and HRTEM images, a HAADF-STEM image, and corresponding EDX elemental mapping of N and P elements were investigated (Figures S6 and 2). First, the lowresolution TEM image shows the typical morphology of a larger area of pieces of BP/CN (Figure S6, left). Then, three different

RESULTS AND DISCUSSION

Characterization of BP/CN. Atomic force microscopy (AFM) shows the fine morphology of as-prepared BP and CN (Figures S1 and S2). The transmission electron microscopy (TEM) image of BP shows a thin sheet-like structure with a size of 100 nm to 1 μm (Figure 1a). From high-resolution TEM (HRTEM), clear lattice fringes with a d-spacing of 0.26 nm corresponding to the (040) plane were observed from BP layers (Figure 1b). The lattice fringes corresponding to other facets were also observed (Figure S3). The selected-area electron diffraction (SAED) and Raman spectra (Figure S4) show a high-quality crystalline state for exfoliated BP nanoflakes. Figure 1c shows a TEM image of as-synthesized CN nanosheets with a silk-like structure and size ranging from 100 to 800 nm, and the HRTEM image indicates amorphous structures (Figure 1d). Figure 1e clearly shows that BP nanoflakes were attached on the surface of CN nanosheets. The components of BP, CN, and BP/CN were also analyzed by EDX (Figure S5). XRD patterns (Figure 1f) show that all diffraction peaks of BP are well consistent with the standard pattern (JCPDS No. 73-1358),27 whereas the intensities of peaks are changed. For example, the intensity of (021) and the ratio of (040)/(111) in BP (5.1/1) are different from the standard pattern, which is due to the oriented {010} facets in the layer structures of BP.34 For

Figure 2. HAADF-STEM image (a), EDX elemental mapping of N (b) and P (c), and overlay of HAADF-STEM of N (green) and P (red) elements (d) of BP/CN. 13235

DOI: 10.1021/jacs.7b08416 J. Am. Chem. Soc. 2017, 139, 13234−13242

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Journal of the American Chemical Society

Figure 3. XPS spectra of C 1s (a), P 2p (b), and N 1s (c) of CN, BP, and BP/CN.

peak at 133.2 eV was observed in BP/CN, assigned to P−N of P3N5.13,36,39−41 For the N 1s spectrum of CN (Figure 3c), the three peaks at 399.2, 399.9, and 401.2 eV are attributed to the sp2-bonded N (C−NC), tertiary nitrogen N−C3 groups, and amino groups (C−N−H), respectively.35−37 Similar to C 1s, the binding energies of N 1s in BP/CN shift to higher binding energies by around 0.3 eV. The higher and lower binding energy shifts of N 1s and P 2p in BP/CN are ascribed to the decrease and increase of the electron density of CN and BP, respectively, owing to strong interfacial interactions between BP and CN. A similar phenomenon was reported by others.35 Furthermore, we tried to synthesize bulk BP with CN composites (bulk BP/CN) with a similar method. However, the XPS data of C 1s, N 1s, and P 2p for bulk BP/CN show the same data compared to pure CN and bulk BP (Figure S9), indicating that the bulk BP does not have activity to form the interaction between BP and CN. The most commonly used method to demonstrate the P−N bond in the reported literature for P/CN-based samples is FTIR.37,41,42 In general, a weak peak located at around 940−970 cm−1 can be observed, which is assigned to the P−N stretching mode. To confirm the formation of P−N coordinate bonds in the BP/CN composites, FT-IR spectra of CN and BP/CN were measured (Figure S10). For CN, the absorption band at 3190 cm−1 originates from the stretching vibration modes of the N− H and hydroxyl of the adsorbed H2O. The typical stretching modes of C−N heterocycles are assigned to the peaks ranging between 1200 and 1650 cm−1. The peak located at 810 cm−1 is attributed to the out-of-plane bending vibration of characteristics of triazine rings.37,41 When BP nanoflakes were hybridized with CN, all the characteristic vibrational peaks of CN were maintained, while a small new peak located at around 960 cm−1 was observed. This peak is assigned to a P−N stretching mode, confirming the generation of a P−N coordinate bond in the BP/CN composites.37,41,42 To further show the chemical interaction between BP and CN, the high-resolution solid-state 31P and 13C NMR spectra of pure BP, CN, and BP/CN nanocomposites were measured. Figure S11 shows the 31P NMR of pure BP with a peak around 17.5 ppm, which is attributed to a P−P bond in the crystal of BP. For BP/CN, a peak around 17.1 ppm was observed and assigned to free BP. Besides this peak, two additional peaks around −0.2 and 5.4 ppm were observed, which are assigned to P−N coordinate bonds in the BP/CN.42,43 Moreover, with increasing the weight ratio of CN, the intensities of these two peaks increased. For bulk BP/CN, besides the main free BP peak, no additional peak was observed. Owing to a similar component of BP in the BP/CN and pure BP, this big difference might be due to the interaction between BP

areas (blue dotted circle) were chosen from the pieces of BP/ CN. Two layers are easily seen from the green dotted circles, which are BP nanoflakes and CN nanosheets (Figure S6a−c). To analyze the different layers and interface interaction of BP and CN, HRTEM images for the interface of two components were observed in the green dotted circles (Figures S6a′−c′) to show clear lattice fringes with a d-spacing of 0.26 nm on one side of the images, corresponding to BP layers. Furthermore, dotted circles in Figure S6a′−c′ display distinct interface interactions in the BP/CN junctions. The HAADF-STEM image of these pieces of BP/CN hybrids and corresponding EDX elemental mapping of P and N elements evidently indicate the uniform hybridization of BP and CN (Figure 2). The green and red color areas represent CN and BP, respectively. Furthermore, these images also show the different components and interactions between BP and CN nanosheets in the above TEM images (Figure S6). XPS Analyses, FT-IR, and Solid-State NMR. To further investigate the interfacial interactions between BP and CN and the chemical configurations in BP/CN, XPS spectra were measured and clearly showed C 1s and N 1s in CN and BP/CN and P 2p in BP and BP/CN (Figures S7 and 3). Besides C, N, and P elements, some contaminate elements originating from environment atmospheres were observed. For the C 1s spectrum of CN, three main peaks were observed at the binding energies of 284.8, 285.9, and 287.9 eV (Figure 3a). The binding energy for the C 1s peak at 284.8 eV is attributed to the carbon-containing contaminants, which was used as the standard reference carbon. The binding energies at 285.9 and 287.9 eV were assigned to sp3-coordinated carbon bonds (C− N) and sp2-bonded carbon (N−CN), respectively.35,36 Interestingly, for BP/CN, the binding energies of C 1s shift to higher binding energies by ca. 0.2 eV in the XPS spectra. Usually, in comparison with a single component, the binding energy shifts in the binary hybrids are explained by a strong interaction between the two components.35 Similar to XPS, the onset around 18.14 eV of UPS for BP/CN also displayed a slight shift compared to pure CN (Figure S8). To clarify the interfacial interaction between BP and CN, XPS spectra of P 2p were measured (Figure 3b). Three bands at ca. 129.7, 130.4, and 134.1 eV were assigned to P 2p3/2, P 2p1/2, and oxidized phosphorus (PxOy) binding energies, respectively, in BP.38 Oppositely, these binding energies were observed to shift to lower binding energies by ca. 0.1 eV for P 2p and 0.2 eV for PxOy in BP/CN. The increase of binding energy indicates the weakened electron screening effect due to the decrease of the electron concentration, whereas the decrease of binding energy means an increase of electron concentration.35 In addition to these assigned peaks, a new 13236

DOI: 10.1021/jacs.7b08416 J. Am. Chem. Soc. 2017, 139, 13234−13242

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Journal of the American Chemical Society nanoflakes and CN, in which P−N coordinate bonds were formed. Some other reports showed similar position about the P−N bond signal in the spectrum of 31P solid-state NMR for the P-doped CN.42,43 To further confirm the interaction in BP/CN is either P and N or P and C, the 13C NMR spectra of pure CN and BP/CN nanocomposites were studied (Figure S12). Compared to pure CN, the chemical shifts of the BP/CN show little shift, suggesting the environment of C in the BP/CN is slightly changed. On the other hand, no additional peak was observed in the BP/CN. This result suggests that C elements do not have a strong interaction with P in the BP/CN composites. This phenomenon is matched with the above FT-IR result, in which the characteristic vibrational peaks of CN in the BP/CN were similar to CN, while a new peak of the P−N bond appeared. Thus, the small change is due to the P atoms being connected to pyridinic-N atoms by coordinate bonds. The relatively weak coordination bond did not have a strong effect to induce the big change in the chemical shift in the 13C solidstate NMR and FT-IR. Proposed Framework Structure of BP/CN. Generally, CN contains periodic heptazine units connected via tertiary amines. The high level of pyridine-like nitrogen in heptazine heterorings provides many electron lone pairs to capture metal and nonmetal ions in the ligands.13−16,40,44 For example, Qiao’s group recently reported molecular-level CN-coordinated transition metals as electrocatalysts.44 By using DFT theoretical computation, the most stable site of the fine structure of metal coordination in the CN ligand is the metal atom embedded within a void in CN. Within this structure, the metal atom is connected to two adjacent pyridinic-N atoms from two separate triazine units, forming a metal−N3C2 ring. Similar to this result, Zhu’s group calculated the substitutional and interstitial Pdoped models of CN systems with different doped sites.40 The results have identified that the P atom preferentially situates in the interstitial sites of in-planar positions of CN. Moreover, Hu’s group synthesized P-doped CN and concluded that P atoms could situate in the interstitial sites of CN by using FTIR and XPS spectra.41 To identify the possible P locations in the CN matrix, we used DFT theoretical computation to indentify the most stable sites of P in the CN systems. In the beginning, the P atoms were put in several different places, including in intralayer (positions 1−3) and interlayer (positions 4−6) positions of the CN matrix (Figure S13). After geometry optimization, the P atoms situate in the interstitial sites of CN, connecting to two adjacent pyridinic-N atoms from two separate triazine units to form a P−N3C2 ring with the lowest relative energy. These results suggest that the possible P locations are situated in the interstitial sites of CN, agreeing with previous results. Accordingly, it is reasonable to conclude that P atoms bind to the CN lattice to form the P−N coordinate bonds in the present case. The P atom preferentially situates in the interstitial sites of CN (Figure S14), in which P is connected to two adjacent pyridinic-N atoms from two separate triazine units to form a P−N3C2 ring.36,40,41 UV−Vis Diffuse Reflectance Spectra of BP/CN. The optical properties of the samples were analyzed by UV−vis diffuse reflectance spectra (Figure 4). BP nanoflakes exhibit a very broad absorption in the UV to NIR region with an absorption edge around 1740 nm. In contrast, CN nanosheets display a visible absorption edge around 463 nm, indicating a band gap of 2.7 eV. When BP was hybridized with CN, the

Figure 4. UV−vis diffuse reflectance spectra of CN, BP, and BP/CN.

edge of absorption for CN displays a slight red-shift (ca. 470 nm). Besides the main absorption of CN, a tail absorption edge was observed and assigned to the absorption of BP nanoflakes. The absorption of BP and CN was subtracted from the absorption of BP/CN to show additional absorption in the visible and NIR region (Figure S15), owing to interfacial interaction of BP and CN. Photocatalytic H2 Evolution Using BP/CN. The photocatalytic H2 evolution was performed over BP/CN in the presence of methanol as a sacrificial electron donor to quench holes generated from band-gap excitation. A trace amount of H2 was detected when only pure BP and CN were used as a photocatalyst under visible light (>420 nm) irradiation (Figure 5a), suggesting the fast recombination of photogenerated charges in pure BP and CN. On the other hand, when BP/CN was used as a photocatalyst, the amount of H2 increased with increasing the irradiation time and reached approximately 1.93 μmol under visible light irradiation for 3 h, indicating evidently that BP/CN can serve as an efficient metal-free photocatalyst for H2 evolution without any other cocatalysts such as Pt. Moreover, different sacrificial reagents such as ethylenediaminetetraacetic acid and triethanolamine were investigated and showed similar results (Figure S16). To keep the same total weight amounts of all samples, the effect of the weight ratio of BP:CN on H2 evolution was also studied. The optimal ratio of BP:CN in the current photocatalytic system was 1:4, giving a H2 evolution rate of 427 μmol g−1 h−1 (Figure 5b). In the BP/CN, CN is mainly excited to generate electrons and holes in CN under visible light irradiation. Beyond the optimal ratio, when the weight ratio of BP continues to increase, the CN is relatively decreased, resulting in a decrease of photocatalytic efficiency. Furthermore, the effect on photocatalyst dose was studied (Figure S17). When the photocatalyst dose continuously increased, the amount of H2 gas increased. However, such increased rate of H2 gas is not linear with the increase of photocatalyst dose, because the opaque dispersion in a highly concentrated dispersion can interfere with the utilization of incident light. The photocatalytic stability of BP/CN for H2 evolution was examined by repeating the experiment several times. Continuous H2 evolution was observed with no noticeable degradation in the subsequent runs (Figure 5c). After three recycles, the catalysts were stored for 2 weeks and still displayed a similar catalytic performance, indicating that BP/CN acts as a stable photocatalyst for H2 evolution. The slight decrease of the photocatalytic performance during the recycling reactions is attributed to loss of catalysts during the centrifugation process. The ICP, TEM, XPS, and XRD of BP/CN were measured after 13237

DOI: 10.1021/jacs.7b08416 J. Am. Chem. Soc. 2017, 139, 13234−13242

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Figure 5. Photocatalytic H2 evolution from water containing methanol (20 vol %) on different catalysts under visible light (>420 nm) irradiation (a). Effect of BP:CN ratio in BP/CN on photocatalytic H2 evolution rate under visible light irradiation for 3 h (b). Cycle stability test on BP/CN photocatalytic H2 evolution under visible light irradiation (c). Photocatalytic H2 evolution based on BP/CN with >780 nm light irradiation (d).

CN shows enhanced catalytic activity in the visible to NIR region compared to other systems. Proposed Mechanism for Photocatalytic H2 Evolution. It is well known that an efficient charge separation/transfer is crucial for the enhancement of the photocatalytic performance. For BP nanoflakes, the band gap depends on the number of layers. According to former reports,45,46 we concluded that the band-gap range is 0.76−0.45 eV for the present BP nanoflakes (Table S3). The valence band edge of CN is ca. 1.68 V vs NHE based on UPS data (Figure S8). Thus, from the viewpoint of band structure of BP and CN, a type I heterojunction is expected at their interface when BP is attached on the surface of CN (Figure 6). In this heterojunction, when BP/CN is irradiated with >420 nm light, CN is mainly excited to generate electrons and holes in CN. Then, BP acts as an electron acceptor from adjacent CN (Figure 6), inhibiting the recombination of the photogenerated charges in CN. The

the photocatalytic reaction and showed no obvious change (Figures S18−S21), indicating that BP/CN remained stable during the photocatalytic reaction. The absorption spectrum of BP/CN displays broad absorption from the UV to NIR region (Figure 4), indicating its possibility of NIR light-driven photocatalytic performance. To explore the photocatalytic performance of BP/CN under NIR light (>780 nm, viz. 780−1800 nm) irradiation, Figure 5d shows continuous H2 evolution with approximately 0.46 μmol during BP/CN photocatalytic reaction under >780 nm for 3 h in the presence of methanol (101 μmol g−1 h−1). On the other hand, no H2 was observed when bare BP or CN was used. Moreover, when we used a 808 nm laser as light source, ca. 0.59 μmol of H2 is obtained after 3 h of irradiation (Figure S22). The wavelength-dependent apparent quantum efficiency of H2 evolution over BP/CN was shown to prove that the activity was driven by the light-excited electrons (Figure S23). The optimum photocatalytic H2 evolution activity of BP/CN under simulated solar (∼100 mW cm−2) irradiation was studied to evaluate the efficiency for solar energy conversion. Figure S24 shows approximately 13.5 μmol of H2 evolution, and the solar-to-hydrogen efficiency was estimated to be around 1.51%. The turnover numbers of the above experiments are also summarized in Table S1. Notably, the bulk BP/CN only showed inferior H2-evolution activity under visible light irradiation, while no H2 evolution was observed under NIR irradiation (Figure S25). To compare traditional photocatalysts, Pt-decorated CN (3.0 wt %) was studied, as shown in Figure S26. The Pt/CN showed 3.04 μmol of H2 under >420 nm irradiation for 3 h, while no H2 was observed under >780 nm irradiation. We also summarized recent reports of the representative metal-free photocatalysts in Table S2. The BP/

Figure 6. Proposed schematic diagram for the visible and NIR light activated photocatalytic H2 evolution using BP/CN in the presence of methanol. 13238

DOI: 10.1021/jacs.7b08416 J. Am. Chem. Soc. 2017, 139, 13234−13242

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Table 1. Lifetimes of TDR Decays of CN, BP, and BP/CN under 400 and 780 nm Irradiation, Respectively, Calculated from the Time Profiles at 950 nm τ1 (ps)

a

sample

400 nm

CN BP BP/CN

11 (51%) 0.6 (90%) 8.4 (45%)

τ2 (ps) 780 nm

400 nm

0.8 (50%) 2.7 (27%)

195 (49%) 3.0 (10%) 127 (55%) i=n

τav (ps)a 780 nm

400 nm

780 nm

0.8 (50%) 60 (73%)

101 0.8 73

0.8 44

i=n

The average lifetime (τav) was then determined using the equation: τ = ∑i = 1 aiτi2/∑i = 1 aiτi .

Figure 7. TDR spectroscopic measurement for BP (a) and BP/CN (b) after irradiation with a 780 nm laser flash. Time profiles of normalized transient absorption at 950 nm (c).

955 nm, assigned to trapped electrons.47−50 The electrons decayed in the time range of 2.6−9.6 ps due to fast charge recombination of the electrons and holes generated in BP. Similarly, a broad absorption, assigned to the electrons in BP/ CN, was observed in the region of 850−1150 nm (Figure 7b). However, the decay lifetime in BP/CN was much longer than that in BP, suggesting that the P−N coordinate bond at the interface acts as the trap site of electrons to cause the H2 generation.52 The time profiles of the transient absorption at 950 nm for BP and BP/CN were fitted by two-exponential functions (Figure 7c), and their lifetimes are summarized in Table 1. In contrast to the single lifetime of normalized transient absorption decay for BP (0.8 ps), the time profiles of transient absorption at 950 nm for BP/CN indicate a short lifetime (2.7 ps, 27%) and long lifetime (60 ps, 73%), with an average lifetime of 44 ps. The surface trap states generally increase the lifetime of the excited semiconductors.52 Zhu’s group suggested a mechanism of enhancing the photocatalytic activity of Pdoped CN based on the first-principles DFT calculation, in which the P elements situated at the interstitial sites of in-planar positions of CN prolong the lifetime of photogenerated charges.40 This suggestion was experimentally shown in the present study from the measurement of a much longer lifetime

photoluminescence (PL), photoelectrochemical measurements, and electron reductive reaction revealed information on charge transfer (see Figures S27−S30 and Table S4 for details). Owing to the interfacial interaction between BP and CN, the P−N coordinate bond acts as the trap site of electrons, causing efficient H2 generation, while holes in BP are rapidly quenched by methanol (Figure 6). Owing to the fast recombination of electrons and holes (lifetime less than 1 ps) in BP under visible light irradiation (Table 1), BP does not work as a photocatalyst. Unexpectedly, BP/CN exhibited H2 generation under NIR light irradiation, while no photocatalytic activity of BP and CN occurred. When BP/CN is irradiated with >780 nm light, the excited electrons in the CB band of BP are trapped by interfacial P−N defects, thereby enhancing the photocatalytic performance. This proposition is discussed in detail based on the following results. Time-Resolved Diffuse Reflectance Spectroscopic Measurement. TDR spectroscopic measurement has obtained great attention because it is a powerful technique to analyze the kinetic mechanisms.47−51 In order to clarify the photocatalytic mechanism of BP/CN under visible and NIR light irradiation, the TDR measurements of BP and BP/CN were performed under 400 and 780 nm light irradiation, respectively. First, for NIR excitation, Figure 7a shows an absorption band centered at 13239

DOI: 10.1021/jacs.7b08416 J. Am. Chem. Soc. 2017, 139, 13234−13242

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Figure 8. TDR spectroscopic measurement for CN (a), BP/CN (b), and BP (c) after irradiation with a 400 nm laser flash. Time profiles of normalized transient absorption at 950 nm (d).

(44 ps) of electrons in BP/CN compared with 0.8 ps for BP and the much enhanced photocatalytic activity for hydrogen generation using BP/CN under NIR light irradiation. The P−N coordinate bond at the interface of BP/CN acts as the trap sites of electrons, having a longer average lifetime of electrons, causing a much enhanced photocatalytic activity. Since the electronegativity of P is smaller than that of C and N, protons (H+) can be adsorbed much easier on P and P−N coordinate bonding at the interface to generate the protonated states, where electrons are accelerated to be trapped, causing an efficient H+ reduction. On the other hand, for visible light excitation, similar TDR signals were observed (Figure 8), and their lifetimes are summarized in Table 1. The lifetimes of electrons for BP/CN nanohybrids were observed to be 8.4 ps (45%) and 127 ps (55%). Owing to the fast recombination of electrons and holes in pure BP under visible light irradiation, these electrons in BP/ CN mostly resulted from the exited CN. For CN, a short lifetime (11 ps, 51%) and a long lifetime (195 ps, 49%) were observed. The decay average lifetimes under the conditions are 101 and 73 ps for CN and BP/CN, respectively. Such decreased lifetimes are distinctly different from the lifetimes under NIR light excitation. Under visible light excitation, a photon is absorbed mainly by CN (ca. 80 wt %), which generates electrons and holes. Because of the type I heterojunction in BP/CN nanohybrids, the BP acts as an additional channel of electron transfer from excited CN to adjacent BP. Similar phenomena were observed in other hybrid systems.51−54 For example, Furube et al. measured TDR spectra of 0.2, 1.0, and 2.0 wt % Pt-loaded TiO2 and found that the lifetimes of TiO2 decreased with increasing the ratio of Pt nanoparticles, demonstrating an electron migration from photoexcited TiO2 to loaded Pt particles.51

To clarify the effect of BP in nanohybrids, TDR spectra were also measured for BP/CN with various, different weight ratios (Figure S31), showing that the intensity of the TDR signals decreased with increasing the BP amount. In the case of visible light excitation, owing to the low weight ratio of BP, the ratio of the trap states due to the interaction of BP and CN is much smaller than the main content of CN. Accordingly, the observed electron signals in the TDR measurements mainly came from CN, and such a pathway of the type I heterojunction for BP/CN is a major channel for electron transfer from excited CN to adjacent BP. When the BP/CN nanohybrids were under NIR excitation, CN can not be excited, and only BP is excited. After that, the interaction of the P−N coordinate bond acted as an efficient electron trap site, which prolongs the lifetime of excited BP. A different electron transfer pathway was also demonstrated by Wu’s group,52 who found a shorter lifetime for Pt nanoparticles decorated CN nanosheets, but a longer lifetime for the Pt atom doped CN system due to Pt−N/C bonds acting as trap states, promoting electron transfer and resulting in longer lifetimes of photogenerated electrons and higher photocatalytic H2 generation performance.

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CONCLUSIONS In conclusion, BP/CN was constructed and used as a metal-free photocatalyst for the first time. In the presence of methanol and without any metal elements, solid H2 evolution was observed via BP/CN under both >420 and >780 nm light irradiation. The interaction between P and N atoms at the interface in BP/ CN plays a crucial role in the improvement of catalytic performance. First, BP acts as the electron acceptor for excited CN under visible light irradiation, resulting in electrons in the conduction band (CB) of BP. Together with electrons in the CB of BP under NIR light irradiation, such electrons are 13240

DOI: 10.1021/jacs.7b08416 J. Am. Chem. Soc. 2017, 139, 13234−13242

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trapped in the P−N coordinate bond at the interfacial interface of BP and CN to cause the H2 generation. The present results show the advantages of BP/CN as a metal-free photocatalyst with broad absorption in the visible and NIR region in developing a new energy conversion system.

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Mamoru Fujitsuka: 0000-0002-2336-4355 Junying Zhang: 0000-0002-4860-8774 Tetsuro Majima: 0000-0003-1805-1677 Notes

The authors declare no competing financial interest.

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EXPERIMENTAL SECTION

ACKNOWLEDGMENTS This work has been partly supported by a Grant-in-Aid for Scientific Research (project 25220806 and others) from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government. M.Z. thanks the JSPS for a Postdoctoral Fellowship for Foreign Researchers (no. P15346).

Preparation of BP Nanoflakes. BP nanoflakes were synthesized by using a basic NMP solvent exfoliation.55 First, 20 mg of bulk BP was added into 20 mL of saturated NaOH/NMP. The dispersion was sonicated for 4 h by using a tip sonicator (Misonix XL-2000) under ice cooling at 10 W output power. After that, the dispersion was centrifuged at 2000 rpm for 20 min two times to remove nonexfoliated bulk BP, resulting in BP nanoflake NMP dispersions (0.2 mg mL−1). Preparation of BP/g-C3N4 Nanohybrids (BP/CN). First, CN nanosheets were synthesized by a thermal polymerization method.56 A 10 g amount of urea powders was dissolved into 15 mL of water under stirring. The pH of the solution was adjusted to 4−5 by using HCl (0.1 M). The solution was dried at 60 °C for 12 h and then transferred to a crucible with a lid. After that, the crucible was heated to 550 °C in a muffle furnace at a heating rate of 10 °C min−1. The precursor was maintained at 550 °C for 2 h and cooled to room temperature naturally, resulting in CN nanosheets. BP/CN were prepared by adding 12 mg of CN into 10 mL of NMP solvent and then treating with ultrasonication to form a homogeneous dispersion. Afterward, the 15 mL as-prepared BP NMP dispersion was added into a CN dispersion with sonication for 2 h. The mixtures were kept stirring overnight. The powders were collected by high-speed centrifugation, washed with ethanol thoroughly, and then dried in an oven at 40 °C overnight, resulting in BP/CN. The weight ratio of BP:CN was 1:4. BP/CN with various weight ratios was prepared by adding different amounts of BP nanoflakes to CN nanosheets. Photocatalytic H2 Evolution. Samples of 1.5 mg were dispersed in 5 mL of a methanol−H2O solution (Vmethaol:VH2O = 1:4) and then added into a 35 mL cylinder reactor and sealed with a rubber septum. The system was deaerated by Ar bubbling into the dispersion for 30 min. Before the photoreactions, the dispersion was sonicated for 5−10 min. Afterward, the system was stirred continuously and irradiated with visible light by using a xenon lamp (Asahi Spectra, HAL-320W, output wavelength: 350−1800 nm) with a 420 and 780 nm cutoff filter. The light intensity was approximately 0.3 W cm−2. The gases produced were analyzed with a gas chromatograph (Shimadzu GC-8A) equipped with an MS-5A column and a thermal conductivity detector. For recycle experiments, after the last round of photoreaction, the photocatalysts were collected by high-speed centrifugation and redispersed into 5 mL of a methanol−H2O solution (Vmethaol:VH2O = 1:4) solution. The catalytic reaction mixture was deaerated by Ar bubbling and treated by ultrasonication for 5−10 min for the next round. The details of the NMR, photoelectrochemical properties, PL, TDR measurements, and other characterizations are summarized in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08416. Experimental procedures, figures, and corresponding discussions of additional supporting experimental data (PDF)

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DOI: 10.1021/jacs.7b08416 J. Am. Chem. Soc. 2017, 139, 13234−13242

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DOI: 10.1021/jacs.7b08416 J. Am. Chem. Soc. 2017, 139, 13234−13242