Phase Engineering of 2D Violet/Black Phosphorus Heterostructure for Enhanced Photocatalytic Hydrogen Evolution

Rapid charge recombination has limited the application of black phosphorus (BP) as a visible light‐responsive photocatalyst. Violet phosphorus (VP), another 2D phosphorus allotrope, has drawn extensive attention for its excellent semiconductor property. However, its photocatalytic activity for hydrogen (H2) evolution is yet to be studied. Herein, a VP/BP heterostructure using a phase engineering strategy is constructed, wherein few‐layered VP is interlaced with BP to create a well‐matched heterophase interface by virtue of their identical chemical compositions and different crystal phases. Experimental and theoretical calculations reveal that VP and BP exhibit strong interaction at the heterophase interface, which is found effective in accelerating photogenerated electron transfer from BP to VP for improved charge separation efficiency. After being decorated with a Rh cocatalyst, the VP/BP heterostructure with utilization of visible light up to 700 nm shows ≈3‐ and 210‐times greater photocatalytic H2 evolution activity with respect to its counterparts. This study highlights feasibility of phase engineering and provides an alternative strategy in promoting charge separation as well as performance of photocatalyst.


Introduction
[7] As a new class of 2D material, ultrathin 2D-Xenes, [8][9][10] including phosphorene, [8] antimonene, [9] and borophene, [10] comprise a single layer of atoms that form a honeycomb-like lattice.Most 2D-Xenes are narrow-bandgap semiconductors and exhibit considerable potential for optoelectronic, catalytic, and energy conversion applications. [11,12]Among them, black phosphorus (BP) is a promising visible light-responsive photocatalyst for H 2 evolution owing to its wide spectral response and thickness-tunable direct bandgap ranging from 0.3 eV for bulk BP to 2.1 eV for single-layer BP. [13,14] However, its practical use is limited by its low photocatalytic activity and low stability owing to the rapid recombination of photogenerated carriers and sensitivity to water and O 2 .
Violet phosphorus (VP), as another crystal phase of layered phosphorus, was first reported by Hittorf in 1865. [15]owever, the synthesis and lattice structure of VP crystals remained unclear until recently. [16,17][20] VP has a layered structure and is regarded as the most stable allotrope of phosphorus, with unique electronic and optoelectronic properties. [18]oreover, theoretical calculations indicate that VP has an indirect bandgap of 1.68 eV, which translates to a direct bandgap of 2.02 eV after exfoliation. [19]This promising semiconducting property has prompted interest in the practical applications of VP in photocatalysis. [20]D/2D van der Waals heterostructures afford fabrication of highly efficient photocatalysts, while they always exhibited low charge transfer efficiency due to poor interfacial contact between the different semiconductor layers.[21,22] In this case, a phase DOI: 10.1002/sstr.202300123 Rapid charge recombination has limited the application of black phosphorus (BP) as a visible light-responsive photocatalyst.Violet phosphorus (VP), another 2D phosphorus allotrope, has drawn extensive attention for its excellent semiconductor property.However, its photocatalytic activity for hydrogen (H 2 ) evolution is yet to be studied.Herein, a VP/BP heterostructure using a phase engineering strategy is constructed, wherein few-layered VP is interlaced with BP to create a well-matched heterophase interface by virtue of their identical chemical compositions and different crystal phases.Experimental and theoretical calculations reveal that VP and BP exhibit strong interaction at the heterophase interface, which is found effective in accelerating photogenerated electron transfer from BP to VP for improved charge separation efficiency.After being decorated with a Rh cocatalyst, the VP/BP heterostructure with utilization of visible light up to 700 nm shows %3and 210-times greater photocatalytic H 2 evolution activity with respect to its counterparts.This study highlights feasibility of phase engineering and provides an alternative strategy in promoting charge separation as well as performance of photocatalyst.
[25][26] For example, the recently reported red phosphorus (RP)/BP heterostructure was proved to be an efficient photocatalyst for water splitting. [26]owever, because of the amorphous nature of RP, the atomically precise interface between RP and BP was hard formed, which is unfavorable for charge transfer.Accordingly, it is highly desirable to develop a novel phase engineering strategy to address the poor interface charge separation as well as low activity of VP.
Herein, we developed a phase engineering strategy by constructing a 2D/2D VP/BP heterostructure via liquid-phase exfoliation.[20] VP and BP were then mixed at mass ratios of 3:1, 1:1, and 1:3 and ground with N-methyl-2pyrrolidone (NMP) for %30 min using a pestle and mortar.Then, the VP/BP mixture was exfoliated by probe sonication in an ice-water bath (Figure S1, Supporting Information).Finally, the dispersion was centrifuged to collect the product for further use (Figure S2, Supporting Information).Crystal structure modeling revealed that both VP and BP exhibit layered structures, in which VP belongs to a monoclinic crystal system with a space group of P2/n and has a layered structure along the [001] direction, while BP belongs to an orthorhombic crystal system with a space group of Cmca and has a layered structure along the [010] direction.Notably, the [001] direction of VP aligns very well with the [010] direction of BP owing to the similar spatial arrangement of P atoms according to their unit cell parameters (Figure S3, Supporting Information).After exfoliation, the layered VP and BP surfaces exhibit high adsorption energies owing to the exposed active P atoms on the surface (Figure S4, Supporting Information); thus, a perfectly interlaced VP/BP interface is achieved (Figure 1A).The potential of the VP/BP heterostructures for photocatalytic H 2 evolution reaction was explored and the corresponding photocatalytic mechanism was proposed.This study provides a promising strategy for producing high-quality phosphorene photocatalysts and highlights how phase engineering can modulate charge transfer.

Material Characterizations
The X-ray diffraction (XRD) pattern of exfoliated VP (Figure 1B) contains sharp peaks of the {004}, {006}, and {008} planes at 16.48°, 24.68°, and 33.08°, respectively, further indicating that the VP crystals were layered along the [001] direction (Figure S5, Supporting Information). [18]High-resolution transmission electron microscopy (HRTEM; Figure S6, Supporting Information) reveals that bulk VP exposes {001} facets on its surface with highly crystalline structure.With the addition of BP at VP/BP mass ratios of 3:1, 1:1, and 1:3, the XRD patterns of the VP/BP heterostructures contain characteristic peaks of both BP and VP, as illustrated in Figure 1B, wherein the intensity of the BP peaks increased with increasing BP content.Consequently, the VP/BP heterostructure can be obtained by just varying the VP/BP mass ratio.TEM and HRTEM imaging revealed that the layered VP and BP had an interlaced structure with a clear heterophase interface (Figure S7 and S8, Supporting Information).The atomic arrangements corresponding to the crystal structures of VP and BP were clearly observed at the interlaced interface of the 2D/2D VP/BP heterostructure by high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM; Figure 1C).On one side of the interface, lattice fringes with a spacing of 0.45 nm and angle of %90°between them were observed, which were ascribed to the (200) and (020) facets of VP.On the other side, lattice fringes with spacings of 0.13 and 0.16 nm with an angle of %45°between them were observed, which were ascribed to the (202) and ( 200) facets of BP, respectively.Notably, an atomically precise interface is formed between the (001) facets of VP and the (010) facets of BP, demonstrating the success of phase engineering strategy.
Raman spectroscopy in Figure 1D reveals strong signals at 353, 373, 446, and 471 cm À1 for VP.There are no vibrational signals of BP in the Raman spectrum of the VP/BP (3:1) heterostructure, but the peaks representing VP are blueshifted.This blueshifting is enhanced for the VP/BP (1:1) heterostructure, which is representative of the strong interfacial interactions due to the high interfacial contact area.In contrast, the Raman spectrum of the VP/BP (1:3) heterostructure only contains peaks at 361.5, 443.4,and 470.2 cm À1 , similar to that of BP. [27,28] UV-visible diffuse reflectance spectroscopy (DRS; Figure 2A) demonstrates that the few-layer VP had an absorption edge at %677 nm, which is blueshifted compared to that of bulk VP crystals (%700 nm). [18]The blueshift of the absorption edge can be ascribed to the successful exfoliation of VP crystals, and this layer-dependent bandgap characteristic is consistent with previous reports on 2D-Xene materials. [29,30]Interestingly, the absorption edge shifted to longer wavelengths after the composite of BP.VP/BP samples with different mass ratios had absorption edges at wavelengths (λ) from 677 to 867 nm, corresponding to bandgaps (E = 1240/λ) from 1.83 to 1.43 eV, which is consistent with the results from the Tauc plots (Figure 2B). [31]In addition, UV photoelectron spectroscopy (UPS) was performed to determine the valence band maximum (VBM) by subtracting the width of the peak (Figure 2C and S9, Supporting Information) from the excitation energy.Thus, the relative band positions were estimated based on the DRS and UPS data (Figure 2D and S10, Supporting Information).Notably, the bandgap of VP straddles the H 2 and O 2 evolution potentials.With increasing BP content, the bandgap of the VP/BP heterostructure narrows, as shown in Table S1, Supporting Information.The calculated density of states (DOS) of the VP/ BP heterostructure reveals that the VB mainly comprises the P 2p orbital of VP, and its conduction band (CB) is constituted by the P 2p orbitals of VP and BP.The narrowing of the bandgap with increasing BP content is mainly due to the clear upshift of the VBM (Figure 2E).When the VP/BP mass ratio reaches 1:3, the VBM shifts to more negative values and is thus insufficient to drive O 2À oxidation.Notably, several bands mainly associated with the P 2p orbital of VP are observed between the VB and CB of the VP/BP heterostructure.This indicates that the localization of photoexcited charges of VP and BP and subsequent charge separation may be promoted via these bands. [32]oreover, as shown in Figure 2F, the p-band center of P active sites was À3.19 eV in the VP/BP heterostructure and was 1.70 and 0.31 eV lower than that of VP (À1.49eV) and BP (À2.88 eV), respectively.This demonstrates that the electronic structure of P active sites can be finely modulated in the VP/ BP heterostructure due to the strong interaction and the efficient charge transfer between VP and BP.
Background absorption at wavelengths of >700 nm became more evident with increasing BP content owing to the high density of crystal defects in BP. [33][34][35] This was further confirmed by high-resolution X-ray photoelectron spectroscopy (XPS), in which P defects can be characterized by the intensity ratio of the P 2p 3/2 and P 2p 1/2 peaks (at 129.6 and 130.5 eV, respectively).As shown in Figure 3A-C, the intensity ratio of the P 2p 3/2 and P 2p 1/2 peaks decreased with increasing BP content (Table S2, Supporting Information). [33]Moreover, the broad peaks centered at 131.6, 133.7, and 134.5 eV were indexed to P─O─P, O─P═O, and P 2 O 5 , respectively, which was also confirmed by Fouriertransform infrared spectroscopy (FTIR; Figure S11, Supporting Information).The oxidized phosphorus was more apparent for BP than VP, mainly resulting from the existence of O vacancies or surface suboxides on the BP surface. [34,35]hese defects (such as P defects and O vacancies) serve as charge-trapping sites and recombination centers, playing a detrimental effect on the photocatalytic performance. [36]he surface electronic states of the VP/BP heterostructure during the photocatalytic reaction were investigated by imaging the potential distribution on the surface with and without illumination by in situ Kevin probe force microscopy (KPFM). [37,38]is technique enables the location of accumulated photogenerated charges to be visualized directly by monitoring the contact potential difference (CPD) between the probe tip and sample surface with and without illumination.The CPD between the probe tip and sample is defined as CPD = (φ tip -φ sample )/e, where φ tip and φ sample are the work functions of the probe tip and sample surface, respectively.Figure 4A shows an atomic force microscopy (AFM) image of the topology of the VP/BP heterostructure without illumination, illustrating a clearly interlaced VP/BP interface.The corresponding 2D and 3D KPFM images (Figure 4B,C) show that the heterophase interface and boundary parts made a more negative charge distribution (red) owing to the accumulation of electrons on the opposite surface area (blue).No notable changes in the morphology or location occurred under external light illumination (Figure 4D and S12, Supporting Information); however, the surface potential became more negative (more red), as shown in Figure 4E,F.The CPD values were calculated and are shown in Figure 4G.Notably, the VP/BP heterostructure exhibited an average potential drop of %30 mV under illumination (À1.08 V AE 18.33 mV) compared to that without illumination (À1.05 V AE 19.03 mV), demonstrating that the accumulation and separation of photogenerated charges are effectively promoted at the heterophase interface.
Charge density difference calculations (Figure 4H) reveal that a large number of charges are accumulated at the VP/BP interface, leading to accelerated charge transfer between VP and BP owning to their strong interaction. [21,39]To confirm the charge transfer direction, we also calculated the plane-averaged differential charge density with results given in Figure 4I, where the negative charges are mainly accumulated on the BP surface, while the positive charges are mainly enriched on the VP surface.This means that the spatial separation of the photogenerated electrons and holes occurs on the heterostructure.This was also confirmed by the work functions (φ) of VP and BP, which were %5.04 and 4.71 eV, respectively (Table S1, Supporting Information).Bader charge analysis was performed to quantify the charge transfer amount (Figure S13, Supporting information), in which BP donated %0.07 |e| to VP, generating a VP/BP heterostructure with electron-rich VP and hole-rich BP.

Photocatalytic Performance and Mechanism Study
The photocatalytic H 2 evolution reaction over the VP/BP heterostructure was performed in the presence of methanol.First, the cocatalyst and loading method were optimized (Figure S14 and S15, Supporting Information), where a Rh cocatalyst was sequentially decorated on the VP/BP heterostructure surface by chemical reduction followed by site-selective photodeposition. [40]igure 5A shows the H 2 evolution as a function of irradiation time on VP, the VP/BP heterostructures, and BP.While layered BP showed low photocatalytic activity (0.04 μmol h À1 ), layered VP exhibited higher H 2 evolution activity (3.09 μmol h À1 ) compared to BP, probably due to the lack of crystal defects in VP.Although the number of crystal defects increased with increasing BP content, the VP/BP heterostructures had enhanced photocatalytic H 2 evolution activity.When the VP/BP mass ratio was 1:1, the H 2 evolution rate reached 9.19 μmol h À1 (almost 3 and 210 times higher than those of layered VP and BP, respectively) with an apparent quantum yield of %0.16% at λ = 420 AE 10 nm (Figure 5B).This high photocatalytic activity was ascribed to the effective electron transfer at the VP/BP interface.In contrast, the physical-mixed VP/BP showed low photocatalytic H 2 evolution activity (2.76 μmol h À1 ), which was ascribed to the lack of heterophase interface and the presence of BP reducing the number of incident photons absorbed by VP.With the VP/BP mass ratio decreasing to 1:3, the VP/BP heterostructure had a dramatically lower photocatalytic activity, which could be attributed to the low charge separation efficiency resulting from the high number of crystal defects. [41]he photocatalytic stability of the VP/BP heterostructure during H 2 evolution was investigated via cycling studies (Figure 5C).Continuous H 2 production is observed over four cycles with almost no decrease in performance.Over 80% of the initial photocatalytic activity is retained in the fifth cycle after storing the photocatalyst overnight under the reaction conditions, indicating the good stability of the VP/BP heterostructure (Figure S16 and Table S3, Supporting Information).Interestingly, the H 2 evolution rate trend of the VP/BP heterostructure is in good accordance with its UV-vis absorption curve (Figure 5D), indicating that the activity is photodriven.It is worth noting that obvious hydrogen evolution (0.28 μmol h À1 ) can be observed when it is irradiated by the wavelength of over 700 nm, demonstrating its wide visible light utilization.Moreover, both VP and the VP/BP (1:1) heterostructure exhibited a trace of O 2 evolution activity (Figure S17, Supporting Information), indicating that the photogenerated holes mainly existed on the VB of VP, since only photogenerated holes on the VB of VP could drive O 2À oxidation (Figure S18, Supporting Information).These results also demonstrate that the VP is promising for visible light-driven overall water splitting.
The photocatalytic mechanism of the VP/BP heterostructures in H 2 evolution is investigated using the time-resolved transient absorption spectroscopy (TAS) because it is a powerful technique to track the real-time photogenerated charge dynamics.As shown in Figure 6A-C, the TAS of VP, the VP/BP heterostructure (1:1), and BP display broad absorption bands in the range 500-700 nm under 400 nm excitation.[44] For a duration of 0-520 ps, the concentrations of photogenerated electrons in VP and the VP/BP heterostructures decrease with time, which is attributed to charge recombination within these materials.For BP, this decay trend is clear, due to the much faster recombination of photogenerated electrons and holes in BP than that in VP and the VP/BP heterostructures.The corresponding TAS decay curves probed at 550 nm are shown in Figure 6D.These curves were fitted to a biexponential function, and the lifetimes of photogenerated charges are listed in Table S4, Supporting Information.The average lifetime of photogenerated electrons in VP is much longer than that in BP (239.4 vs. 111.40ps).The introduction of BP into VP to form the VP/BP heterostructure shortened the average lifetime of photogenerated electrons to 142.10 ps, which is indicative of enhanced charge separation efficiency owing to the efficient electron transfer from BP to VP.This can also be confirmed by the transient photocurrent response and the electrochemical impedance spectra (EIS).As illustrated in Figure 6E, compared with the almost no photocurrent response of BP, both VP and the VP/BP heterostructure exhibited improved photocurrents during repeated on/off cycles.Importantly, the transient photocurrent response of the VP/BP heterostructure is almost three times higher than that of VP, indicating more photogenerated electron amount and higher charge separation efficiency under visible light.In the EIS spectra (Figure 6F), the VP/BP heterostructure exhibited a smaller arc radius with respect to VP and BP, demonstrating a smaller resistance for the charge transfer from BP to VP in the VP/BP heterostructure. [21,45]

Conclusion
In conclusion, we developed a phase engineering strategy to construct 2D/2D VP/BP heterostructures, in which few-layered VP interlaced with BP to create a well-matched heterophase interface owing to their identical chemical compositions and differing crystal phases.The experimental and theoretical data revealed that photogenerated electrons accumulated at the VP/BP heterophase interface, generating strong interactions between VP and BP, accelerating electron transfer from BP to VP, and effectively improving the charge separation efficiency.Together with decoration of a Rh cocatalyst, the VP/BP heterostructure shows %3and 210-times higher activity of water splitting than those of VP and BP, respectively, and utilizes visible light of up to 700 nm.Moreover, interlacing fewer-layered VP and BP can further enhance the photocatalytic activity owing to the shorter migration distance of charge carriers to the active sites.This study provides a novel strategy for producing high-quality phosphorene photocatalysts and highlights the potential of phase engineering for modulating the charge transfer process.

Experimental Section
[20] A mixture of 470 mg amorphous red phosphorus, 10 mg Sn, and 18 mg SnI 4 were sealed in a 14 cm-long quartz tube with an inner diameter of 10 mm and a thickness of 2 mm under a vacuum of 10 À4 pa.The quartz tube was placed horizontally in a three-zone muffle furnace.The empty side was kept at 450 °C for 6 h while the source region was kept in room temperature to realize the back transport process.The quartz tube was heated up where one end of the quartz tube with phosphorus source to 650 °C and the other empty end to 630 °C for 8 h.The sample was kept at these temperatures for 5 h and then cooled 100 °C down for 10 h.The end with phosphorus source was then kept at 550 °C with the other end at 530 °C for 30 h.The quartz tube was then slowly cooled to room temperature for 70 h to obtain VP.
Synthesis of Few-Layered VP: Few-layered VP was prepared using a liquid exfoliation method. [46,47]30 mg of VP crystals was immersed in a 50 mL of NMP and treated by a tip sonicator at a power output of 10 W under 5 °C water bath conditions for 4 h to exfoliate the VP.The resulting dispersion was centrifuged for 20 min at 3000 rpm to remove the bulk VP, and the supernatant containing few-layered VP was decanted gently.Then, the dispersion containing few-layered VP was centrifuged for 20 min at 11 000 rpm, and the precipitate product was resuspended in 10 mL NMP solution for further use.
Synthesis of VP/BP Heterostructures: The VP/BP heterostructure was synthesized by a novel phase engineering strategy, in which VP and BP crystals with a certain mass ratio (VP:BP = 3:1, 1:1, 1:3) were mixed and ground in the presence of NMP in a mortar for about 0.5 h.Subsequently, the mixture was sonicated in a 50 mL of NMP using a tip sonicator at a power output of 10 W under 5 °C water bath conditions for 4 h.Then, the dispersion was centrifuged for 20 min at 3000 rpm to remove the nonexfoliated VP and BP crystals.Finally, the dispersion containing VP/BP heterostructures was further centrifuged for 20 min at 11 000 rpm, and the precipitate product was resuspended in water for further use.For the physical-mixed VP/BP sample, 5 mg of few-layered VP and 5 mg of few-layered BP were dispersed in 50 mL H 2 O and stirred for 1 h.
Deposition of Cocatalysts: In this work, a sequential decoration of cocatalyst by chemical reduction followed by site-selective photodeposition was applied.In the NaBH 4 reduction, the VP/BP heterostructure powder (10 mg) was added to 20 mL of water and sonicated for several minutes.Subsequently, the required amount of RhCl 3 •3H 2 O or H 2 PtCl 6 •6H 2 O was added to the solution followed by the addition of NaBH 4 (1 mL, 0.1 M) with vigorous stirring.The resulting suspension was filtered to recover the product, which was then washed several times to completely remove any excess NaBH 4 .In the case of the photodeposition process, a specific amount of RhCl 3 •3H 2 O or H 2 PtCl 6 •6H 2 O was added to 150 mL of an aqueous 20 v% methanol solution containing the photocatalyst powder (10 mg).The suspension was evacuated to completely remove dissolved air and then exposed to visible light (λ ≥ 420 nm) with continuous stirring.For the synthesis of Co 3 O 4 -VP/BP heterostructures photocatalyst, the VP/BP heterostructures powder (10 mg) was added to 20 mL of water and sonicated for several minutes.Subsequently, a certain amount of Co 3 O 4 nanoparticles was added to the above solution with vigorous stirring.The mixture was continuously stirred for another 2 h, and the resulting suspension was filtered to recover the solid product.
Characterization: The powder samples were analyzed by XRD (MiniFlex 300, Rigaku; Cu Kα) over the 2θ range of 10°-60°.UV-visible DRS data were acquired using a V-670 JASCO spectrometer recorded over the range of 350-900 nm.Raman Microscopy was performed using a Thermo DXR 2xi Raman spectrometer equipped with excitation wavelengths of 633 nm.HRTEM images were obtained using a JEM-2100 microscope operated at an accelerating voltage of 300 kV.HAADF-TEM images were obtained with a Titan Cubed Themis G2 300 microscope operating at 300 kV equipped with a spherical aberration corrector.XPS data were acquired using a Thermo Scientific ESCALAB 250Xi spectrometer.UPS data were collected by using a Nexsa spectrometer with a He lamp source.Photocurrent measurements were performed on a CHI 770 electrochemical workstation using a three-electrode cell with the 2D materials on FTO as the working electrode, saturated calomel electrode, and platinum electrode as the reference electrode and the counter electrode, respectively.AFM and the in situ Kelvin probe force microscopy (KPFM) were conducted on an Asylum Research MFP-3D-BIO AFM instrument with a 50 W mercury lamp (equipped with filters to only produce 435-485 nm light) as the light source.
DFT Calculation Details: The Vienna ab initio simulation package (VASP) was used to perform density functional theory (DFT) calculations.The projector augmented wave (PAW) potentials were used to describe the interaction of ionic cores and electrons, and the PerdewÀ BurkeÀErnzerhof (PBE) exchangeÀcorrelation density functional was employed.The kinetic energy cutoff of 450.0 eV and MonkhorstÀPack k-point meshes of 3 Â 3 Â 3 in the Brillouin zone (BZ) were chosen.The dispersion interactions were considered within the dispersioncorrected DFT-D3 method of Grimme in the calculations.We constructed the VP(001) slab and BP(010) slab with vacuum gap of 25 Å, respectively.The force convergence criterion used for the geometry relaxation was 0.02 eV Å À1 .Band structures and DOS were calculated by the hybrid functional (HSE06) methods based on the atomic structure obtained from the full optimization by PBE functional, and the valence electrons were set as s 2 p 3 for P.
Photocatalytic Reactions: The photocatalytic reactions were examined in a Pyrex top-irradiation type reaction vessel connected to a closed-gas circulation system.For photocatalytic H 2 evolution of the VP/BP heterostructure, 10 mg of photocatalyst was dispersed in 150 mL of a 20 vol% aqueous methanol solution.For photocatalytic H 2 evolution of the physical-mixed VP/BP sample, 10 mg of the physical-mixed VP/BP sample was dispersed in 150 mL of a 20 vol% aqueous methanol solution.For photocatalytic O 2 evolution, each photocatalyst was dispersed in 150 mL of an aqueous 50 mM AgNO 3 solution.Prior to photoirradiation, the reaction system was evacuated to remove all air and then irradiated from overhead using a 300 W xenon lamp equipped with a dichroic mirror and a cut-off filter (λ ≥ 420 nm).The reactant solution was maintained at 283 K with a cooling water system during the reaction.The evolved gas products were analyzed using an online gas chromatography system consisting of a GC9790 chromatograph (FULI) equipped with a molecular sieve 5 Å column and a thermal conductivity detector, with high-purity N 2 as the carrier gas.
AQY Calculations: The AQY for the H 2 evolution reaction based on one-step photoexcitation is given by AQY ð%Þ ¼ ½2 Â nðH 2 Þ=nðphotonsÞ Â 100 (1)   where n(H 2 ) and n(photons) represent the number of H 2 molecules generated and the number of incident photons, respectively.

Figure 2 .
Figure 2. A) UV-vis DRS spectra of VP, VP/BP (3:1, 1:1, and 1:3) heterostructures, and BP.B) The corresponding Tauc plots.C) UPS profiles of VP and BP.D) Estimated band positions of VP and BP.E) DOS for the VP/BP heterostructure.F) The p-band center of VP, VP/BP heterostructures, and BP.

Figure 4 .
Figure 4. A) Topographic AFM image of VP/BP (1:1) heterostructure without illumination and the corresponding B) 2D and C) 3D KPFM images.D) Topographic AFM image of VP/BP (1:1) heterostructure with illumination and the corresponding E) 2D and F) 3D KPFM images.The green dotted lines represent the interlaced VP/BP interface.G) CPD distributions obtained from the KPFM images of the VP/BP heterostructure with and without illumination.H) Charge density difference of VP/BP heterostructure (isosurface value of 3.0 Â 10 À4 ).Yellow and light blue regions indicate charge accumulation and depletion, respectively.I) Plane-averaged differential charge density.