Copper Phosphide-Enhanced Lower Charge Trapping Occurrence in

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Copper Phosphide Enhanced Lower Charge Trapping Occurrence in Graphitic-C3N4 for Efficient Noble-Metal-Free Photocatalytic H2 Evolution Wenchao Wang, Xiaolong Zhao, YIngnan Cao, Zhiping Yan, Ruixue zhu, Ying Tao, Xiaolang Chen, Dieqing Zhang, Guisheng Li, and David Phillips ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01421 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Materials & Interfaces

Copper

Phosphide

Enhanced

Lower

Charge

Trapping Occurrence in Graphitic-C3N4 for Efficient Noble-Metal-Free Photocatalytic H2 Evolution Wenchao Wang,†,‡,# Xiaolong Zhao,†,# Yingnan Cao,† Zhiping Yan,‡ Ruixue Zhu,*,‡,§ Ying Tao,† Xiaolang Chen,† Dieqing Zhang,† Guisheng Li,*,† David Lee Phillips*,‡ †

The Education Ministry Key and International Joint Lab of Resource Chemistry, Shanghai Key

Lab of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, China ‡

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong

§

School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210,

China KEYWORDS: g-C3N4, Cu3P, photocatalysis, H2 evolution, charge trapping, time-resolved spectroscopy

ABSTRACT: Graphitic carbon nitride (g-C3N4) fundamental photo-physical processes exhibit a high frequency of charge trapping due to physicochemical defects. In this study, a copper phosphide (Cu3P) and g-C3N4 hybrid was synthesized via a facile phosphorization method. Cu3P, as an electron acceptor, efficiently captures the photogenerated electrons and drastically improved the charge separation rate to cause a significantly enhanced photocatalytic performance. Moreover, the robust and intimate chemical interactions between Cu3P and g-C3N4

ACS Paragon Plus Environment

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offers a rectified charge transfer channel that can lead to a higher H2 evolution rate (HRE, 277.2 μmol h-1 g-1) for this hybrid that is up to 370 times greater than that achieved from using bare gC3N4 (HRE, 0.75 μmol h-1 g-1) with a quantum efficiency of 3.74 % under visible-light irradiation (λ = 420 nm). To better determine the photo-physical characteristics of the Cu3Pinduced charge anti-trapping behavior, ultrafast time-resolved spectroscopy measurements were used to investigate the charge carriers’ dynamics from femtosecond (fs) to nanosecond (ns) time domains. The experimental results clearly revealed that Cu3P can effectively enhance charge transfer and suppress photoelectron-hole recombination.

1. INTRODUCTION Solar-to-chemical conversion methods for renewable clean energy production have aroused increasing attention for their potential to help take the place of fossil fuels. H2 and O2 production from water using sun-light is regarded to be an ideal alternative for contributing to worldwide energy demand.1-8 Being eco-friendly, low cost and being easily available are three key advantages of semiconductor-based technology that can be utilized to photochemically produce H2 and also find utility in pollutants treatment.9-14 However, the practical application of photocatalysis has been limited because of the large rates for photo-induced charge recombination and hence a low electron utilization efficiency.15, 16 Although the use of a precious metal as a co-catalyst combined with a semiconductor can dramatically increase the photocatalytic performance, the natural rarity and greater cost of using precious metal cocatalysts restrict their use in industrial applications for H2 production.17, 18 Highly-efficient and sustainable photocatalysts are an essential research direction for solar-to-chemical conversion.1921

Therefore, we have chosen to investigate how to improve advanced functional materials by

increasing the rates of charge transfer while also suppressing the inactivation of the excited state


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electrons. A high-efficiency metal-free photocatalyst will be an encouraging alternative for water splitting photocatalysis. Recently, various approaches, including element doping or precious metal modification, have been employed to investigate the electronic and atomic models of photocatalysts to efficiently improve the photo-induced charge pair separation rate.22 Element doping, as a representative modification method, has been widely demonstrated to greatly promote charge transfer kinetics that can lead to efficient inhibited electron-hole pair recombination. However, the external element can also unavoidably cause energy loss since more recombination centers are present in the overall system.23 Graphitic carbon nitride (g-C3N4), due to a favorable optical gap (~2.7 eV) and a shortened charge carrier migration distance, has been widely used as a visible-light-driven photocatalyst for solar water splitting.24-28 To date, various nanostructured g-C3N4, including those with a helical morphology and hollow nanosphere structure, have been synthesized by hard/soft template methods.29, 30 However, the intrinsic high occurrence of charge trapping in gC3N4 cannot be modified very well. For maximizing photocatalytic performance, Wang et al reported results on metal-free g-C3N4 photocatalysts that show substantially greater H2 evolution activity

via

approaches

using

iodine/sulfur/Boron-mediation,

vacancy

engineering,

copolymerization and heterojunctions.31-36 Recently, in order to overcome the high-frequency of electron trapping in g-C3N4, Yu and coworkers reported that red phosphorus modified g-C3N4 produced via a chemical vapour deposition (CVD) method could also be used to increase the efficiency for photocatalysts to give greater H2 evolution activity. This work indicated the important role of red P atoms for active electron capture in the g-C3N4 plane.37 As one of the most abundant elements, phosphorus and phosphide have received considerable research attention. Transition-metal phosphides, such as Fe2P, Ni2P, and CoP, have been widely


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investigated in artificial photo-utilization systems as co-catalysts.38-40 Copper phosphide (Cu3P) as a narrow optical gap (1.3~1.4 eV) semiconductor with lower cost has been reported.41, 42 Li and coworkers independently deduced that Cu3P, as a co-catalyst, plays a significant role in photoactivity improvement due to the conduction/valence band shift at high loading content.43 Here, ultrafast time-resolved spectroscopies were employed to illustrate the high charge carrier anti-trapping occurrence in the g-C3N4 trap state with a low loading content of Cu3P. Cu3Pinduced excited electron capturing effectively inhibits photo-charge trapping by the g-C3N4 trap band and also increases the charge transfer rate in an artificial photo-utilization system. A prolonged photo-induced excited electron lifetime (209 ps) of the photocatalyst was observed via fs-TA and ns-TA spectroscopy measurements. The routes available and the robust chemical bonding interactions provide an efficient pathway for the charge transfer and anti-trapping between Cu3P and g-C3N4. Furthermore, the photoactivity of H2 evolution using pristine g-C3N4 can be enhanced substantially by the addition of a highly dispersed Cu3P co-catalyst into the reaction system. A superior H2 evolution rate (277.2 μmol h-1 g-1) can be obtained by using the optimum Cu3P/g-C3N4 hybrid under visible-light exposure, that is up to 370 times greater than that from using pristine g-C3N4 (0.75 μmol h-1 g-1). Our work thus helps explain how the transition-metal phosphides play a significant role for high-efficiency solar-energy utilization as a noble -metal-free optical material. 2. EXPERIMENTAL SECTION 2.1 Preparation of multi-layer & bulk g-C3N4. In this work, the chemicals are all analytical reagent (AR) grade. Graphitic carbon nitride was made using a molecule-self-assembly process as previously reported.44 Briefly, 20 g urea molecule was selected as the g-C3N4 precursor and calcinated in a ceramic crucible at 823 K and stabilized for 4 h under an air atmosphere. Thus,


4

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the light-yellow powder of the bulk and multi-layer g-C3N4 (marked as MCN) was acquired following natural heat dissipation to reach room temperature. 2.2 Preparation of few-layer & loose g-C3N4. As-prepared multi-layer g-C3N4 sample (210 mg) was dispersed in 210 mL of an isopropanol (IPA) and water mixed solution (v:v = 2:1), followed by ultrasonic exfoliation for 10 h. The suspension liquid was then kept in an autoclave at 453 K for another 10 h. The obtained mixture solution was then extracted from the autoclave and washed by ultrapure water and ethanol, respectively. The sample was put into an oven overnight under 353 K. Finally, a white powder containing mostly few-layer g-C3N4 was obtained (marked as FCN). 2.3 Synthesis of the Cu3P-FCN composite. Synthesis of the composite photocatalysts with 1.00 wt% of Cu loading was done as follows: mixing FCN (100 mg) and CuCl2•2H2O together in an aqueous ethanol solution and then evaporating the solvent at 343 K for 6 h under sonication. The light-green-yellow powder was kept at 673 K in air for 2 h. Finally, this hybrid compound and excess NaH2PO2 annealed in Ar flow at 573 K for 3 h at a ramp rate of 2 ℃ min-1 to make the Cu3P-FCN hybrid. For comparison, a series of photocatalysts, including various amounts of Cu3P on FCN, were prepared by the same procedures. The Cu/FCN mass ratio was adjusted from 0.25 wt% to 3.00 wt% in the samples used in this study. 2.4 Photocatalytic H2 evolution. The photocatalytic performance of water-splitting were measured by using a 100 mL Pyrex flask reaction cell under monochromatic light excitation (λ = 420 nm), as we reported previously.45 20 mg of the photocatalyst suspended in a mixed 80 mL solution composed of triethanolamine (TEOA, 10 vol%) and water under magnetic stirring. Four LEDs lamp (λ= 420 nm) were utilized as the visible-light source. In four different directions, all LEDs were positioned away from the reactor with a distance of ca. 1 cm. Bubbling with N2 gas


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for 15 min was used to remove air to ensure inert condition reaction conditions. The evolved gaseous product was detected via gas chromatography. The quantum efficiency (QE) was a significant piece of evidence to evaluate the utilization factor of the solar-light and this was calculated based on following equation:46 QE =

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐻2 𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 × 2 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑃ℎ𝑜𝑡𝑜𝑛𝑠

× 100

2.5 Photoelectrochemical measurements. Photoelectrochemical measurements were probed by a commercial electrochemical workstation using a neutral electrolyte (0.50 M Na2SO4). Platinum (Pt) foil (2.0 × 2.0 cm2, thickness 100 μm, 99.99%) was employed as a counter electrode candidate. The dip-coating method was used to prepare the working electrode by using a slurry (made using 20 mg photocatalyst powder that was ground with 2 mg of polyethylene glycol and 0.5 mL of ethanol) on a FTO substrate. This semi-manufactured working electrode was then annealed at 200 ℃ in the air. A commercial saturated calomel electrode (SCE) was employed as the reference electrode. The photo-response signal intensity and electrochemical impedance spectroscopy (EIS) signal were obtained under monochromatic light irradiation using a LED lamp. The photocurrent response was recorded using a 20 s on-off cycle. 2.6 Characterization Methods. To determine the crystal structure, X-ray diffraction measurements (Rigaku Dmax-3C with Cu-Ka irradiation) were made to gain information from 2θ degree. The morphology of the catalysts was examined using scanning electron microscopy and high-resolution transmission electron microscopy images that were collected by JEOL JSM6380LV and JEOL-2010F instruments, respectively. The ultraviolet-visible diffuse reflectance patterns were acquired with a UV-vis spectrophotometer (Shimadzu UV-2450). A PerkinElmer PHI 5000C ESCA system was employed to obtain X-ray photoelectron spectroscopy (XPS) data. Calibration of the binding energies used a contaminant carbon (C 1s = 284.6 eV) as a reference.


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Fourier transformation infrared (FT-IR) spectroscopy experiments were carried out with a PerkinElmer spectrum 100 FT-IR spectrometer. AFM experiments are done by using an Asylum Research MFP-3D instrument with a Si substrate. Brunauer-Emmett-Teller (BET) measurements were chosen to analyze the surface area from N2 adsorption and desorption isotherms. Steadystate

fluorescence

spectroscopic

measurements

were

done

using

a

fluorescence

spectrophotometer (Hitachi F-7000). The thermostability of the photocatalysts were studied with thermogravimetric analysis (Q50-TGA).


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3. RESULTS AND DISSCUSION

Figure 1. (a) Synthetic procedures for the Cu3P-FCN hybrids and digital photographs of the photocatalysts; (b, c, d) SEM images of MCN, FCN and (1.00 wt% Cu) Cu3P-FCN; (d, inset) EDX spectrum and (e) elemental mapping images of the (1.00 wt% Cu) Cu3P-FCN sample. Figure 1a presents the synthetic procedures for the Cu3P and g-C3N4 composites preparation, the precursor (urea molecule) was realigned into a yellow overlapping multi-layer g-C3N4 (MCN) assembled of nanosheets after high temperature calcination, corresponding to the SEM image given in Figure 1b. The complex network of multi-layer g-C3N4 was disassembled with a solvothermal technique combined with vigorous sonication. The morphological analysis of the SEM image (Figure 1c) indicates the exfoliated nanosheets of g-C3N4 appear as a loose few-layer structure (FCN). Additional morphological characterization of Cu3P-FCN is indicated in Figure


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1d and 1e. The SEM image does not show the presence of Cu3P nanoparticles in the (1.00 wt% Cu) Cu3P-FCN hybrid and this may be due to the low Cu3P loading content and small size that is outside of the resolution of the SEM measurement. However, energy-dispersive X-ray spectrum (EDX, inset of Figure 1d) and elements mapping (Figure 1e) further confirm the homogeneous distribution of the C, N, Cu and P elements. The digital photographs of the various photocatalysts in Figure 1a shows the colour change for prepared photocatalysts with various Cu3P loading contents. It was observed that the colours of the photocatalysts became deeper in intensity with an increased proportion of Cu3P. As shown in Figure 2, transmission electron microscopy (TEM) clearly exhibited the different structures of MCN (multi-layer) and FCN (few-layer). Furthermore, AFM measurements were employed to characterize the thickness of MCN and FCN in Figure 2e and f. FCN shows a thinner thickness (~ 13 nm) than that of MCN (~ 22 nm) due to the decreased number of g-C3N4 layers. Similarly, consistent with the Brunauer-Emmett-Teller (BET) data (Table S1), the exfoliated g-C3N4 exhibits a higher surface area (71.8 m2 g-1) than that of pristine g-C3N4 (46.4 m2 g-1). As displayed in Figure 2g, Cu3P nanoparticles with an average diameter of about 10−20 nm are well dispersed on the g-C3N4 plane. The high-resolution TEM (HRTEM) image obtained from the (1.00 wt% Cu) Cu3P-FCN indicates it has lattice fringes with 0.23 nm interplanar distances (inset of Figure 2h), correlating to the (112) plane of Cu3P.43 In this compound, the few-layer structure of g-C3N4 can be seen clearly after complex Cu3P loading procedures. SEM images and element mapping (Figure S1) of the pure Cu3P exhibited a stacking morphology which presented irregular shapes.


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Figure 2. TEM images of (a, c) multi-layer g-C3N4, (b, d) few-layer g-C3N4. AFM images and corresponding height cross-section profiles of (e) multi-layer g-C3N4, (f) few-layer g-C3N4; (g, h) TEM and (h, inset) high-resolution TEM images of (1.00 wt% Cu) Cu3P-FCN. The crystal structures of the pristine g-C3N4 and Cu3P-FCN samples were found using X-ray diffraction (XRD) spectra (Figure 3a). The XRD profiles of all of the materials displayed two main peaks at around 12.8°and 27.4° 2θ, which were assigned to the characteristic peaks (100) and (002) of the graphitic carbon nitride, respectively.47-49 This means that the graphitic loose stacking of the tri-s-triazine model is resilient and strong versus post-heating treatment. Additionally, a weak signal at around 17.6° 2θ is attributed to the (600) plane of g-C3N4.50 This latter feature can be explained by a distortion of the loose stacking structure in the melon sheet. As shown in Figure S2a, the pure Cu3P sample shows diffraction peaks located at 28.7, 36.2, 41.8, 45.1, 46.5, 47.3 and 56.8°, respectively, corresponding to the (111), (112), (211), (300), (113), (212) and (311) planes of Cu3P (PDF#02-1263). In addition, compared with FCN, no


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significant new peaks appear in XRD profile of Cu3P-FCN, indicating the characteristics of low loading content and high dispersion. The XRD pattern of the high mass ratio (20.0 wt% Cu) of Cu3P-FCN was recorded in Figure S2b, and these results suggest the presence of a Cu3P in gC3N4 framework. The 1200-1700 cm-1 stretching modes of CN heterocycles are all shown in bare g-C3N4 (MCN, FCN) and Cu3P-FCN according to the FT-IR experimental results (Figure 3c).51 The 805 cm-1 feature demonstrates the presence of the tri-s-triazine breathing mode. More importantly, a weak signal located near 945 cm−1 is associated with the P-N breathing motion in Cu3P-FCN.50

Figure 3. (a) XRD patterns and (b) UV-Vis absorption spectrum; (b, inset) the corresponding Tauc plots; (c) FT-IR spectra and (d) thermogravimetric analysis (TGA) profile of pure MCN, FCN and (1.00 wt% Cu) Cu3P-FCN composite. UV-visible diffuse reflectance spectra (UV-Vis DRS) measurements were carried out to define the photo-physical properties (i.e. light absorption range and band gap value) of the assynthesized catalysts (see Figure 3b). Compared to bare g-C3N4, the Cu3P-FCN hybrid exhibits significantly enhanced absorption from 350 nm to 600 nm. The absorption edges of MCN, FCN and Cu3P-FCN were observed at about 440 nm, 435 nm and 455 nm, respectively with a distinct


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𝑛

red-shift. In addition, based on the equation αhν = A(ℎ𝜈 ― 𝐸𝑔)

2

, where α, ν, Eg, and A are the

absorption coefficient, frequency of light, energy of the band gap and proportionality constant,52 the band gap values of MCN (2.82 eV), FCN (2.86 eV) and Cu3P-FCN (2.78 eV) can be obtained via the linear fitting given in the Figure 3b insert. This data clearly demonstrates the band gap of the Cu3P-FCN is smaller than that of both the bare g-C3N4 samples (MCN and FCN). The main cause is the strong chemical interactions between Cu3P and g-C3N4 that favour effective suppression of the recombination of the photo-induced charges which leads to a significant increase of the photoactivity using visible-light exposure. The TGA curves demonstrated that all of the catalysts displayed a slight weight loss from 30 to 400 ℃ likely due to the desorption of trace solvents, more importantly, a strong weight loss can be observed between 400 and 650 ℃, corresponding to the burning removal of organic ligands in the g-C3N4 network. The higher residual mass of Cu3P-FCN is assigned to produce CuO (about 10 %) at air atmosphere (Figure 3d).

Figure 4. High-resolution XPS spectra of (a) C 1s; (b) Cu 2p1/2 and Cu 2p3/2; (c) N 1s and (d) P 2p collected from (1.00 wt% Cu) Cu3P-FCN.


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For further proving the presence of P-N and P-C, X-ray photoelectron spectroscopy (XPS) probed the chemical composition and the electronic binding energy of the photocatalysts. The high resolution XPS profile in Figure 4a indicates the C 1s core level for Cu3P-FCN displayed two main features of 284.8 eV and 288.2 eV, assigned respectively to the characteristic peaks of C-C and N-C=N in the triazine rings.53-55 As shown in Figure 4a, compared to the high resolution XPS spectrum of the pure g-C3N4 (Figure S3), a new peak of the Cu3P-FCN hybrid arises at 286.0 eV (P-C, C 1s), in agreement with the strong chemical bonding interaction between Cu3P and g-C3N4 that is formed in the hybrids.37 Furthermore, the N 1s (Figure 4c) and P 2p (Figure 4d) spectra were also used to obtain deeper insight into the interfacial interaction and bonding type between the Cu3P and g-C3N4 systems. One new feature located at 401.4 eV (N 1s) is associated with the interaction between P and N. Additionally, as seen in Figure 4d, two bands are seen around 132.5 eV and 133.5 eV (P 2p1/2), also corresponding to the generation of P-N and P-C bonding interactions.56 Thus, a stronger interaction plays a significant role to effectively suppress the excited state electrons from being trapped by the trap state of g-C3N4 and also accelerate more active electrons to give a higher transfer rate from the g-C3N4 plane to the Cu3P parts of the system. Overall, robust interfacial interactions are present in Cu3P-FCN and this might be helpful for improving the photocatalytic performance of H2 evolution. In addition, an XPS analysis on the Cu3P-FCN surface (Figure 4b) exhibits two apparent features at 932.6 and 952.2 eV coming from the Cu 2p3/2 and Cu 2p1/2 species, which are typical of the Cu3P group.57


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Figure 5. (a) Effect of the Cu3P loading degree on photocatalytic H2 production from FCN and (b) recycle test for photocatalytic hydrogen evolution on (1.00 wt% Cu) Cu3P-FCN under LED light resource (λ= 420 nm). Figure 5a presents a comparison of the H2 evolution rates (HER) from water reduction on g-C3N4 nanosheets with different loading contents of Cu3P with (λ = 420 nm) visible-light excitation. The control experiment shows that no H2 evolution (only trace amounts were recorded) occurs on the multi-layer g-C3N4 (MCN) catalyst under analogous experimental conditions. Moreover, FCN causes a clear enhancement of the HER, ca. 0.75 μmol h−1 g−1 due to the higher specific surface area (Table S1) and wide light-absorption intensity (Figure 3b). A very tiny amount of Cu3P attached could cause a great increase in the HER. A mass ratio of Cu to g-C3N4 of 1.00 wt%, gave a maximum HER value of 277.2 μmol h−1 g−1 with a higher QE (3.74 %), which is up to 370 and 80 times with respect to FCN and Cu3P, respectively. However, a large excess of Cu3P loading content lead to a great decrease in the HER. These results could be caused by an emergent shielding effect and increased the degree of charge recombination sites in g-C3N4. It was observed that the particle size of Cu3P became larger upon increasing the concentration of Cu3P as shown in Figure S10. In the presence of a low concentration (< 1.00 wt%), the electron acceptor Cu3P with uniform distribution and ultrafine size on the surface of g-C3N4 may


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effectively promote active electron transfer and restrain electron trapping in the trap state of gC3N4. Nevertheless, excess loading Cu3P will cause unavoidable aggregation of Cu3P that will weaken the light absorption of g-C3N4. In addition, for comparison purposes, a bare FCN sample was also treated by the same phosphorization process. The photocatalytic activity of the phosphorization of FCN exhibits an insignificant increase in the HER (1.02 μmol h−1 g−1) using equivalent experimental conditions. For illustrating the important role of P and its potential application to replace a noble-metal co-catalyst, the photocatalytic performance of 1.0 wt% CuFCN, 0.1 wt% Pt-FCN and 0.2 wt% Pt-FCN are given in Figure S11, respectively. A significant enhancement of the photocatalytic activity is observed in the 1.0 wt% Cu-FCN composite (86.5 μmol h−1 g−1), about 115 times more than that of pristine g-C3N4 (FCN), due to the rapid electron transfer from g-C3N4 to Cu. Nevertheless, such a value is still lower than that of the (1.0 wt% Cu) Cu3P-FCN sample, suggesting the important role of phosphorization. It was also noted that the optimal Cu3P enhanced g-C3N4 sample exhibits a H2 evolution rate about 220 % higher than that (125.5 μmol h−1 g−1) of 0.1 wt% Pt-FCN, however, such a value was lower than that (390.0 μmol h−1 g−1) of 0.2 wt% Pt-FCN. Overall, as listed in Table S2, the loading of Cu3P on g-C3N4 can significantly boost the photocatalytic H2-evolution. The values observed here are significantly larger than those of Ni2P5 (162 μmol h−1 g−1), FexP (83.2 μmol h−1 g−1), and MoS2 (252 μmol h−1 g−1). The catalytic stability test of (1.00 wt% Cu) Cu3P-FCN was investigated in a long-term photocatalytic process, as depicted in Figure 5b. Only a slight decrease of the photocatalytic HER for Cu3P-FCN can be seen after 10 h of the cycling tests. More importantly, the XPS survey of Cu3P-FCN confirmed a slight binding energy shift of Cu 2p (from 952.2 to 952.5 eV) and P 2p (from 133.5 to 133.4 eV) before and after this 10 h experiment (Figure S4), suggesting there is little change and a superior stability of Cu3P is seen during the photocatalytic


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activity test. This result further confirms the excellent stability of the Cu3P-FCN photocatalyst. Interestingly, however, pure Cu3P shows a bad stability in the short-term photocatalytic process. No clear increase of H2 gas content can be seen after a 4 h continuous tests (Figure S5).

Figure 6. Photoelectrochemical investigation of MCN, FCN and (1.00 wt% Cu) Cu3P-FCN: (a) photocurrent response recorded with visible-light excitation (λ = 420 nm) at 0.50 V vs SCE; (b) Nyquist plot of electrochemical impedance; (c, d) Steady-state fluorescence spectra with the excited wavelength at 266 nm and 400 nm, respectively. To evaluate the photoelectrochemical properties of the photocatalysts, the photocurrent densities were measured with visible-light excitation (Figure 6a). The Cu3P-FCN composite exhibited a stronger photo-response and current density than that of pristine g-C3N4 at a bias voltage (0.50 V vs SCE) due to the improvement of the light capture capability after Cu3P loading as indicated by the UV-Vis DRS measurement. It is worth noting that the photo-response signals were highly renewable under the on-off cycles. As for the Cu3P-FCN composite, the Cu3P nanoparticles can substantially increase the photoelectron rate of transfer from the g-C3N4 conduction band while also effectively suppressing the excited state electrons from being trapped by the g-C3N4 trap band consistent with the photocurrent density, permitting more active photoelectrons to be


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captured by protons. The interfacial photo-physical properties of Cu3P-FCN and FCN were observed by using the EIS method (Figure 6b). The diameter of the semicircle in the Nyquist plot can be used to exhibit the charge transfer process and this corresponds to the charge transfer resistance (CTR) in the electrolyte solution. As shown in Figure 6b, it’s noteworthy that the CTR value for the (1.00 wt% Cu) Cu3P-FCN hybrid is noticeably lower than for the bare g-C3N4 samples. Thus, more active electrons were easily captured by protons to produce H2. Both MCN and FCN exhibited a fluorescence peak at around 430 nm under 400 nm excitation in Figure 6d. An obvious emission peak decrease can be observed after introducing Cu3P, which may be assigned to the low recombination rate of the photogenerated electrons and holes in the Cu3PFCN composite, as well as a robust contact among Cu3P and g-C3N4. Figure 6c shows a peak at around 360 nm that grows in under 266 nm excitation and this indicates the photogenerated electrons easily transfer from the ground state to the higher excited state and these may be trapped back by the trap state in g-C3N4.


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Figure 7. The fs-TA spectra at various delay times and the decay kinetics monitored at 736 nm of (a, b, c) FCN and (d, e, f) (1.00 wt% Cu) Cu3P-FCN, with the excitation of 400 nm laser are shown; Experimental decay kinetics (dotted lines) and two-exponential function fitted curves (solid lines) of FCN and Cu3P-FCN at long time scale and short time scale are shown separately. In order to find out why the addition of Cu3P can enhance the photocatalytic efficiency dramatically, femtosecond transient absorption (fs-TA) spectroscopy experiments were systematically carried out employing 400 nm laser light as the pump and a white light continuum (420-780 nm) as the probe (Helios, Ultrafast Systems). As shown in Figure 7a and d, the distinct negative absorption bands for FCN and (1.00 wt% Cu) Cu3P-FCN in the range of 450-650 nm have contributions from both the ground state absorption and the photo-induced emission, the positive bands at longer wavelengths, which are only observed for Cu3P-FCN (Figure 7d), represent the absorption of the photo-excited electrons in the conduction band. The fs-TA spectra for MCN (Figure S6) are quite consistent with the results of FCN. Overall, the pristine g-C3N4 samples (FCN and MCN) show similar spectroscopic behaviors upon visible-light excitation.


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The charge separation process occurs from the valence band (VB) to the conduction band (CB) and then with the generation of the photo-induced electrons and holes, part of the electrons and holes recombine (CB to VB) with the emission of fluorescence, meanwhile another pathway is suggested as an electron trapping process. The decay traces for the absorption of the photoinduced electrons of FCN was monitored at 736 nm and the initial and fully developed kinetics are shown separately in Figure 7b and 7c. The corresponding kinetic fitting parameters are provided in Table 1. The kinetics and the fitted constants for MCN are also given in Figure S6 and Table S3. The significant short decay component of 5.0 ps can be attributed to the quick recombination of the photo-induced electrons and holes and the component of 397 ps reveals the existence of an alternative pathway for the charge trapping process. The extremely fast electrons and holes recombination and electron trapping processes leads to the low photocatalytic hydrogen production efficiency observed for MCN. The generated trapped electrons are shown to be photocatalytically unreactive and experience a much longer time to go back to the VB. With regard to FCN, due to the electrons high-dispersion effect on exfoliated g-C3N4 nanosheets, the charge trapping process in FCN is effectively suppressed (698 ps, Table 1) and the prolonged charge recombination lifetime (49 ps, Table 1) indicates there will be a higher hydrogen production efficiency (QE, 0.01%) compared with MCN. A high charge trapping occurrence in the g-C3N4 trap band inhibits the utilization of the excited electrons that leads to an efficiency reduction of the surface photocatalytic reactions.58


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Figure 8. (a) The ns-TA spectra (top) and intensity decay kinetics (below) of FCN and Cu3PFCN under 650 nm; (b) Schematic diagram of the electron transfer process in FCN and (1.00 wt% Cu) Cu3P-FCN. With the introduction of Cu3P, the decreased fluorescence intensity of (1.00 wt% Cu) Cu3P-FCN under 400 nm irradiation (Figure 6d) indicates that the recombination rate of the excited electrons is effectively restrained. Besides, the absorption of the electrons on the excited state increases remarkably (Figure 7d). Owing to the more negative position of the conduction band of g-C3N4, more of the photo-induced electrons will transfer from the CB of FCN to Cu3P effectively. Figure 7e and f monitored the decay kinetics of the active electrons absorption in the Cu3P-FCN composite monitored at 736 nm. The increased lifetime of the active electrons indicates that the recombination rate of the photo-induced electrons and holes through the emission was suppressed notably with the addition of Cu3P. Hence, the photo-excited electron transfer process in the Cu3P-FCN heterostructure is confirmed. For convenience, a proposed schematic diagram of the charge trapping and recombination routes are described in Scheme 1. All of the above results illustrate that the presence of robust chemical bonding between Cu3P and g-C3N4 notably suppresses the charge trapping process (T1′, Scheme 1b). As the excited electron “guide”, the Cu3P-induced charge redistribution can promote more excited electrons to transfer


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from g-C3N4 to Cu3P with a decay component of 10 ps (Table 1) and a lower frequency charge trapping occurrence in the g-C3N4 trap band. The photo-induced electron lifetime (209 ps, τ2, Table 1) on Cu3P-FCN shows a slower charge recombination (R0′) rate in Cu3P, which is about 43.5 and 5.3 times longer than that of MCN (5.0 ps) and FCN (40 ps), respectively. These data agree well with the conclusion that Cu3P can inhibit the process of electron trapping (T1′and R2′). Thus, the photo-catalytic efficiency of H2 evolution from water reduction is extremely enhanced. Table 1. Exponential fitted parameters for the fs-TA (pump 400 nm) and ns-TA decay (pump 355 nm) of FCN and (1.00 wt% Cu) Cu3P-FCN hybrid and calculated QE data under 420 nm irradiation. Sample

Method

τ1

τ2

FCN

fs-TA

49 ps (65.2 %)

698 ps (34.8 %)

ns-TA

160 ns

N/A

fs-TA

10 ps (52.2 %)

209 ps (47.8 %)

ns-TA

120 ns

N/A

Cu3P-FCN

To further monitor the electron transfer process of FCN and Cu3P-FCN at longer time scales, nsTA measurements were carried out and these results are given in Figure 8. With the excitation of the 355 nm laser, both FCN and Cu3P-FCN showed a positive broad absorption from 400 nm to 750 nm that can be attributed to the absorption of the charge transfer states (trap states). Similar positive absorption features with lifetimes up to the microsecond time scale have been reported previously.59, 60 This type of long-lived species was shown to account for the high photocatalytic efficiency observed for semiconductors.61 As the transient absorption intensity implies the charge trapping efficiency, one can conclude that the charge trapping process is efficiently inhibited due to the charge transfer process between FCN and Cu3P. Figure 8a shows a comparison of the dynamics monitored at 650 nm and the parameters obtained from two exponential function fitting are given in Table 1. This data shows the lifetime of Cu3P-FCN (ca. 120 ns) is a little


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shorter compared with FCN (ca. 160 ns), indicating that the de-trapping process of the unreactive electrons are accelerated with the introduction of Cu3P. Therefore, the Cu3P-FCN hybrid shows up to a 370-fold enhanced photocatalytic rate of hydrogen evolution for the conditions we examined in this study. Figures S7 and S8 show a distinct bleaching signal due to the stimulated emission that was observed upon 266 nm excitation. This attribution is supported by the steady-state fluorescence spectra observed with the same excitation conditions (Figure 6c). A new peak around 360 nm was observed due to a photo-induced electron recombination from a higher energy level. The strong photoluminescence signals of FCN (Figure S7a) under the higher energy laser excitation reveal the existence of an emissive state at a higher energy level. For Cu3P-FCN (Figure S7b), the continuous long-lived absorption at around 450-650 nm is very evident, which may due to the high energy level charge separation state formed by the trapping process. Figure S9 shows the normalized PL decay traces monitored at the 354 nm wavelength and this PL signal can apparently be assigned to recombination of the trapped electrons from TB to VB. The parameters obtained from a two-exponential function fitting are given in Table S4. It has been known that carbon nitrides materials possess more than one trap state (such as shallow and deep trapped states) with different trap densities.58 Thus, the two components observed likely come from the different emissions from different kinds of trap states. Compared with FCN (27, 364 ns), the decreased lifetime of Cu3P-FCN (15, 255 ns) suggesting that the de-trapping process of the unreactive electrons are accelerated with the introduction of Cu3P. These results further consolidate our conclusions that the electron trapping process of FCN is suppressed dramatically in the presence of Cu3P.


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Scheme 1. Mechanism of photo-induced electrons transfer and charge recombination routes: (a) g-C3N4; (b) Cu3P-FCN following visible-light excitation. 4. CONCLUSIONS To summarize, the Cu3P and g-C3N4 hybrid examined here reveals a higher photocatalytic performance for H2 generation. A great enhancement of H2 evolution rate (HER, 277.2 μmol h−1 g−1) can be observed in this new Cu3P-FCN hybrid system, which is up to 370 times that of gC3N4 sample under visible-light excitation. Cu3P effectively suppresses excited electrons from being trapped by the g-C3N4 trap band and also promotes the electron transfer from g-C3N4 to Cu3P. Furthermore, the Cu3P/g-C3N4 hybrids show an ultralow recombination process of the photoelectron-hole. A low concentration of trapped unreactive electrons would greatly improve the excited electrons necessary for water reduction, which play a significant role for the photoactivity improvement. Our results here show the advantages of utilizing transition-metal phosphides in developing a new energy conversion and environmental purification system.

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Additional information including ultrafast time-resolved measurement set up, SEM and TEM images, selected area EDX analysis, XRD profile of Cu3P, XPS spectra, BET surface area, photostability test, and fs/ns-TA spectrum.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (D. L. P.). * E-mail: [email protected] (G. L.). * E-mail: [email protected] (R. Z.). Author Contributions # These

authors contributed equally.

Notes The authors declare no competing financial interest ACKNOWLEDGMENT This research was sponsored in part by grants from the Research Grants Council of Hong Kong (HKU17301815) to D.L.P. and partial support from the Areas of Excellence Scheme (Grant AoE/P-03/08), National Natural Science Foundation of China (21677099, 21876112, and 21876113), Shanghai (18DZ2254200, 18SG41), the UGC Special Equipment Grant (SEG-HKU-


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07) and the University of Hong Kong Development Fund 2013-2014 project “New Ultrafast Spectroscopy Experiments for Shared Facilities”.

REFERENCES (1) Kibria, M. G.; Qiao, R.; Yang, W.; Boukahil, I.; Kong, X.; Chowdhury, F. A.; Trudeau, M. L.; Ji, W.; Guo, H.; Himpsel, F. J.; Vayssieres, L.; Mi, Z. Atomic-Scale Origin of Long-Term Stability and High Performance of p-GaN Nanowire Arrays for Photocatalytic Overall Pure Water Splitting. Adv. Mater. 2016, 28, 8388-8397. (2) Li, G.; Lian, Z.; Wang, W.; Zhang, D.; Li, H. Nanotube-Confinement Induced SizeControllable g-C3N4 Quantum Dots Modified Single-Crystalline TiO2 Nanotube Arrays for Stable Synergetic Photoelectrocatalysis. Nano Energy 2016, 19, 446-454. (3) Zhang, G.; Lan, Z.-A.; Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chem. Int. Ed. 2016, 55, 15712-15727. (4) Pan, Y.; Sun, K.; Liu, S.; Cao, X.; Wu, K.; Cheong, W.-C.; Chen, Z.; Wang, Y.; Li, Y.; Liu, Y.; Wang, D.; Peng, Q.; Chen, C.; Li, Y. Core–Shell ZIF-8@ZIF-67-Derived CoP Nanoparticle-Embedded N-Doped Carbon Nanotube Hollow Polyhedron for Efficient Overall Water Splitting. J. Am. Chem. Soc. 2018, 140, 2610-2618. (5) Guo, F.; Shi, W.; Zhu, C.; Li, H.; Kang, Z. CoO and g-C3N4 Complement Each Other for Highly Efficient Overall Water Splitting under Visible Light. Appl. Catal. B-Environ. 2018, 226, 412-420. (6) Yang, X.; Tian, L.; Zhao, X.; Tang, H.; Liu, Q.; Li, G. Interfacial Optimization of g-C3N4Based

Z-Scheme

Heterojunction

Toward

Synergistic

Enhancement

of

Solar-driven

Photocatalytic Oxygen Evolution. Appl. Catal. B-Environ. 2019, 244, 240-249.


25


Page 26 of 35

(7) Li, H.; Xiao, S.; Zhou, J.; Zhao, J.; Liu, F.; Li, G.; Zhang, D. A Flexible CdS NanorodsCarbon Nanotubes/Stainless Steel Mesh Photoanode for Boosted Photoelectrocatalytic Hydrogen Evolution. Chem. Commun. 2019, 55, 2741-2744. (8) Wang, X.; Zhao, X.; Zhang, D.; Li, G.; Li, H. Microwave Irradiation Induced UIO-66-NH2 Anchored on Graphene with High Activity for Photocatalytic Reduction of CO2. Appl. Catal. BEnviron. 2018, 228, 47-53. (9) Ning, F.; Shao, M.; Xu, S.; Fu, Y.; Zhang, R.; Wei, M.; Evans, D. G.; Duan, X. TiO2/Graphene/NiFe-Layered Double Hydroxide Nanorod Array Photoanodes for Efficient Photoelectrochemical Water Splitting. Energy Environ. Sci. 2016, 9, 2633-2643. (10) Liu, J.; Xie, S.; Geng, Z.; Huang, K.; Fan, L.; Zhou, W.; Qiu, L.; Gao, D.; Ji, L.; Duan, L.; Lu, L.; Li, W.; Bai, S.; Liu, Z.; Chen, W.; Feng, S.; Zhang, Y. Carbon Nitride Supramolecular Hybrid Material Enabled High-Efficiency Photocatalytic Water Treatments. Nano Lett. 2016, 16, 6568-6575. (11) Xiao, S.; Zhang, D.; Pan, D.; Zhu, W.; Liu, P.; Cai, Y.; Li, G.; Li, H. A Chloroplast Structured Photocatalyst Enabled by Microwave Synthesis, Nat. Commun. 2019, 10, 1570. (12) Wang, W.; Li, F.; Zhang, D.; Leung, D. Y. C.; Li, G. Photoelectrocatalytic Hydrogen Generation and Simultaneous Degradation of Organic Pollutant via CdSe/TiO2 Nanotube Arrays. Appl. Surf. Sci. 2016, 362, 490-497. (13) Pan, D.; Xiao, S.; Chen, X.; Li, R.; Cao, Y.; Zhang, D.; Pu, S.; Li, Z.; Li, G.; Li, H. Efficient Photocatalytic Fuel Cell via Simultaneous Visible-Photoelectrocatalytic Degradation and Electricity Generation on a Porous Coral-Like WO3/W Photoelectrode. Environ. Sci. Technol. 2019, 53, 7, 3697-3706.


26

Page 27 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Chen, X.; Zhang, H.; Zhang, D.; Miao, Y.; Li, G. Controllable Synthesis of Mesoporous Multi-Shelled ZnO Microspheres as Efficient Photocatalysts for NO Oxidation. Appl. Surf. Sci. 2018, 435, 468-475. (15) Wenderich, K.; Mul, G. Methods, Mechanism, and Applications of Photodeposition in Photocatalysis: A Review. Chem. Rev. 2016, 116, 14587-14619. (16) Kapilashrami, M.; Zhang, Y.; Liu, Y.-S.; Hagfeldt, A.; Guo, J. Probing the Optical Property and Electronic Structure of TiO2 Nanomaterials for Renewable Energy Applications. Chem. Rev. 2014, 114, 9662-9707. (17) Raza, F.; Yim, D.; Park, J. H.; Kim, H.-I.; Jeon, S.-J.; Kim, J.-H. Structuring Pd Nanoparticles on 2H-WS2 Nanosheets Induces Excellent Photocatalytic Activity for CrossCoupling Reactions under Visible Light. J. Am. Chem. Soc. 2017, 139, 14767-14774. (18) Tian, L.; Yang, X.; Liu, Q.; Qu, F.; Tang, H. Anchoring Metal-Organic Framework Nanoparticles on Graphitic Carbon Nitrides for Solar-Driven Photocatalytic Hydrogen Evolution. Appl. Surf. Sci. 2018, 455: 403-409. (19) Han, S.-Y.; Pan, D.-L.; Chen, H.; Bu, X.-B.; Gao, Y.-X.; Gao, H.; Tian, Y.; Li, G.-S.; Wang, G.; Cao, S.-L.; Wan, C.-Q.; Guo, G.-C. A Methylthio-Functionalized-MOF Photocatalyst with High Performance for Visible-Light-Driven H2 Evolution. Angew. Chem. Int. Ed. 2018, 57, 9864-9869. (20) Pan, D.; Han, Z.; Miao, Y.; Zhang, D.; Li, G. Thermally Stable TiO2 Quantum Dots Embedded in SiO2 Foams: Characterization and Photocatalytic H2 Evolution Activity. Appl. Catal. B-Environ. 2018, 229, 130-138.


27


Page 28 of 35

(21) Zhang, M.; Shang, Q.; Wan, Y.; Cheng, Q.; Liao, G.; Pan, Z. Self-Template Synthesis of Double-Shell TiO2@ZIF-8 Hollow Nanospheres via Sonocrystallization with Enhanced Photocatalytic Activities in Hydrogen Generation. Appl. Catal. B-Environ. 2019, 241, 149-158. (22) Han, F.; Zhou, Z.; Zhang, X.; Huang, Z.; Li, M.; Guo, L. First-Principles Study on Stability and HER Activity of Noble Metal Single Atoms on TiO2: The Effect of Loading Density. J. Phys. Chem. C 2018, 122, 2546-2553. (23) Sivula, K.; Le Formal, F.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (αFe2O3) Photoelectrodes. ChemSusChem 2011, 4, 432-449. (24) Tan, Y.; Shu, Z.; Zhou, J.; Li, T.; Wang, W.; Zhao, Z. One-Step Synthesis of Nanostructured g-C3N4/TiO2 Composite for Highly Enhanced Visible-Light Photocatalytic H2 Evolution. Appl. Catal. B-Environ. 2018, 230, 260-268. (25) Zhang, G.; Li, G.; Lan, Z.-A.; Lin, L.; Savateev, A.; Heil, T.; Zafeiratos, S.; Wang, X.; Antonietti, M. Optimizing Optical Absorption, Exciton Dissociation, and Charge Transfer of a Polymeric Carbon Nitride with Ultrahigh Solar Hydrogen Production Activity. Angew. Chem. Int. Ed. 2017, 56, 13445-13449. (26) Zhu, M.; Kim, S.; Mao, L.; Fujitsuka, M.; Zhang, J.; Wang, X.; Majima, T. Metal-Free Photocatalyst for H2 Evolution in Visible to Near-Infrared Region: Black Phosphorus/Graphitic Carbon Nitride. J. Am. Chem. Soc. 2017, 139, 13234-13242. (27) Zhao, X.; Pan, D.; Chen, X.; Li, R.; Jiang, T.; Wang, W.; Li, G.; Leung, D. Y. C. g-C3N4 Photoanode for Photoelectrocatalytic Synergistic Pollutant Degradation and Hydrogen Evolution. Appl. Surf. Sci. 2019, 467-468, 658-665.


28

Page 29 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Han, C.; Wang, Y.; Lei, Y.; Wang, B.; Wu, N.; Shi, Q.; Li, Q. In Situ Synthesis of Graphitic-C3N4 Nanosheet Hybridized N-Doped TiO2 Nanofibers for Efficient Photocatalytic H2 Production and Degradation. Nano Res. 2015, 8, 1199-1209. (29) Zheng, Y.; Lin, L.; Ye, X.; Guo, F.; Wang, X. Helical Graphitic Carbon Nitrides with Photocatalytic and Optical Activities. Angew. Chem. Int. Ed. 2014, 53, 11926-11930. (30) Sun, J.; Zhang, J.; Zhang, M.; Antonietti, M.; Fu, X.; Wang, X. Bioinspired Hollow Semiconductor Nanospheres as Photosynthetic Nanoparticles. Nat. Commun. 2012, 3, 1139. (31) Zhang, G.; Zhang, M.; Ye, X.; Qiu, X.; Lin, S.; Wang, X. Iodine Modified Carbon Nitride Semiconductors as Visible Light Photocatalysts for Hydrogen Evolution. Adv. Mater. 2014, 26, 805-809. (32) Zhang, J.; Zhang, M.; Sun, R.-Q.; Wang, X. A Facile Band Alignment of Polymeric Carbon Nitride Semiconductors to Construct Isotype Heterojunctions. Angew. Chem. Int. Ed. 2012, 51, 10145-10149. (33) Lin, Z.; Wang, X. Nanostructure Engineering and Doping of Conjugated Carbon Nitride Semiconductors for Hydrogen Photosynthesis. Angew. Chem. Int. Ed. 2013, 52, 1735-1738. (34) Zhang, J.; Zhang, G.; Chen, X.; Lin, S.; Möhlmann, L.; Dołęga, G.; Lipner, G.; Antonietti, M.; Blechert, S.; Wang, X. Co-Monomer Control of Carbon Nitride Semiconductors to Optimize Hydrogen Evolution with Visible Light. Angew. Chem. Int. Ed. 2012, 51, 31833187. (35) Yang, P.; Zhuzhang, H.; Wang, R.; Lin, W.; Wang, X. Carbon Vacancies in a Melon Polymeric Matrix Promote Photocatalytic Carbon Dioxide Conversion. Angew. Chem. Int. Ed. 2019, 58, 1134-1137.


29


Page 30 of 35

(36) Fang, Y.; Xu, Y.; Li, X.; Ma, Y.; Wang, X. Coating Polymeric Carbon Nitride Photoanodes on Conductive Y:ZnO Nanorod Arrays for Overall Water Splitting. Angew. Chem. Int. Ed. 2018, 57, 9749-9753. (37) Jing, L.; Zhu, R.; Phillips, D. L.; Yu, J. C. Effective Prevention of Charge Trapping in Graphitic Carbon Nitride with Nanosized Red Phosphorus Modification for Superior Photo(electro)catalysis. Adv. Funct. Mater. 2017, 27, 1703484. (38) Hu, Z.; Shen, Z.; Yu, J. C. Phosphorus Containing Materials for Photocatalytic Hydrogen Evolution. Green Chem. 2017, 19, 588-613. (39) Li, S.; Wang, L.; Liu, S.; Xu, B.; Xiao, N.; Gao, Y.; Song, W.; Ge, L.; Liu, J. In Situ Synthesis of Strongly Coupled Co2P-CdS Nanohybrids: An Effective Strategy To Regulate Photocatalytic Hydrogen Evolution Activity. ACS Sustainable Chem. Eng. 2018, 6, 9940-9950. (40) Wang, X.-J.; Tian, X.; Sun, Y.-J.; Zhu, J.-Y.; Li, F.-T.; Mu, H.-Y.; Zhao, J. Enhanced Schottky Effect of a 2D–2D CoP/g-C3N4 Interface for Boosting Photocatalytic H2 Evolution. Nanoscale 2018, 10, 12315-12321. (41) Rauf, A.; Ma, M.; Kim, S.; Sher Shah, M. S. A.; Chung, C.-H.; Park, J. H.; Yoo, P. J. Mediator- and Co-Catalyst-Free Direct Z-Scheme Composites of Bi2WO6–Cu3P for Solar-Water Splitting. Nanoscale 2018, 10, 3026-3036. (42) Dutta, A.; Dutta, S. K.; Mehetor, S. K.; Mondal, I.; Pal, U.; Pradhan, N. Oriented Attachments and Formation of Ring-on-Disk Heterostructure Au–Cu3P Photocatalysts. Chem. Mater. 2016, 28, 1872-1878. (43) Shen, R.; Xie, J.; Lu, X.; Chen, X.; Li, X. Bifunctional Cu3P Decorated g-C3N4 Nanosheets as a Highly Active and Robust Visible-Light Photocatalyst for H2 Production. ACS Sustainable Chem. Eng. 2018, 6, 4026-4036.


30

Page 31 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44) Zhang, J.; Chen, Y.; Wang, X. Two-Dimensional Covalent Carbon Nitride Nanosheets: Synthesis, Functionalization, and Applications. Energy Environ. Sci. 2015, 8, 3092-3108. (45) Lian, Z.; Wang, W.; Li, G.; Tian, F.; Schanze, K. S.; Li, H. Pt-Enhanced Mesoporous Ti3+/TiO2 with Rapid Bulk to Surface Electron Transfer for Photocatalytic Hydrogen Evolution. ACS Appl. Mater. Interfaces 2017, 9, 16959-16966. (46) Xu, X.; Lai, L.; Zeng, T.; Yu, Y.; He, Z.; Chen, J.; Song, S. In Situ Formation of Pyridine-Type Carbonitrides-Modified Disorder-Engineered C-TiO2 Used for Enhanced VisibleLight-Driven Photocatalytic Hydrogen Evolution. J. Phys. Chem. C 2018, 122, 18870-18879. (47) Guo, Y.; Chen, T.; Liu, Q.; Zhang, Z.; Fang, X. Insight into the Enhanced Photocatalytic Activity of Potassium and Iodine Codoped Graphitic Carbon Nitride Photocatalysts. J. Phys. Chem. C 2016, 120, 25328-25337. (48) Jourshabani, M.; Shariatinia, Z.; Badiei, A. Sulfur-Doped Mesoporous Carbon Nitride Decorated with Cu Particles for Efficient Photocatalytic Degradation under Visible-Light Irradiation. J. Phys. Chem. C 2017, 121, 19239-19253. (49) Xia, P.; Zhu, B.; Cheng, B.; Yu, J.; Xu, J. 2D/2D g-C3N4/MnO2 Nanocomposite as a Direct Z-Scheme Photocatalyst for Enhanced Photocatalytic Activity. ACS Sustainable Chem. Eng. 2018, 6, 965-973. (50) Zhu, Y.-P.; Ren, T.-Z.; Yuan, Z.-Y. Mesoporous Phosphorus-Doped g-C3N4 Nanostructured Flowers with Superior Photocatalytic Hydrogen Evolution Performance. ACS Appl. Mater. Interfaces 2015, 7, 16850-16856. (51) Di, T.; Zhu, B.; Cheng, B.; Yu, J.; Xu, J. A Direct Z-Scheme g-C3N4/SnS2 Photocatalyst with Superior Visible-Light CO2 Reduction Performance. J. Catal. 2017, 352, 532-541.


31


Page 32 of 35

(52) Deng, W.; Zhao, H.; Pan, F.; Feng, X.; Jung, B.; Abdel-Wahab, A.; Batchelor, B.; Li, Y. Visible-Light-Driven Photocatalytic Degradation of Organic Water Pollutants Promoted by Sulfite Addition. Environ. Sci. Technol. 2017, 51, 13372-13379. (53) Dong, F.; Zhao, Z.; Xiong, T.; Ni, Z.; Zhang, W.; Sun, Y.; Ho, W.-K. In Situ Construction of g-C3N4/g-C3N4 Metal-Free Heterojunction for Enhanced Visible-Light Photocatalysis. ACS Appl. Mater. Interfaces 2013, 5, 11392-11401. (54) Qin, Z.; Wang, M.; Li, R.; Chen, Y. Novel Cu3P/g-C3N4 p-n Heterojunction Photocatalysts for Solar Hydrogen Generation. Sci. China Mater. 2018, 61, 861-868. (55) Li, K.; Zeng, X.; Gao, S.; Ma, L.; Wang, Q.; Xu, H.; Wang, Z.; Huang, B.; Dai, Y.; Lu, J. Ultrasonic-Assisted Pyrolyzation Fabrication of Reduced SnO2–x/g-C3N4 Heterojunctions: Enhance Photoelectrochemical and Photocatalytic Activity under Visible LED Light Irradiation. Nano Res. 2016, 9, 1969-1982. (56) Zhang, Y.; Mori, T.; Ye, J.; Antonietti, M. Phosphorus-Doped Carbon Nitride Solid: Enhanced Electrical Conductivity and Photocurrent Generation. J. Am. Chem. Soc. 2010, 132, 6294-6295. (57) Hou, C.-C.; Chen, Q.-Q.; Wang, C.-J.; Liang, F.; Lin, Z.; Fu, W.-F.; Chen, Y. SelfSupported Cedarlike Semimetallic Cu3P Nanoarrays as a 3D High-Performance Janus Electrode for Both Oxygen and Hydrogen Evolution under Basic Conditions. ACS Appl. Mater. Interfaces 2016, 8, 23037-23048. (58) Godin, R.; Wang, Y.; Zwijnenburg, M. A.; Tang, J.; Durrant, J. R. Time-Resolved Spectroscopic Investigation of Charge Trapping in Carbon Nitrides Photocatalysts for Hydrogen Generation. J. Am. Chem. Soc. 2017, 139, 5216-5224.


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(59) Walsh, J. J.; Jiang, C.; Tang, J.; Cowan, A. J. Photochemical CO2 Reduction Using Structurally Controlled g-C3N4. Phys. Chem. Chem. Phys. 2016, 18, 24825-24829. (60) Ye, C.; Li, J.-X.; Li, Z.-J.; Li, X.-B.; Fan, X.-B.; Zhang, L.-P.; Chen, B.; Tung, C.-H.; Wu, L.-Z. Enhanced Driving Force and Charge Separation Efficiency of Protonated g-C3N4 for Photocatalytic O2 Evolution. ACS Catal. 2015, 5, 6973-6979. (61) Kuriki, R.; Matsunaga, H.; Nakashima, T.; Wada, K.; Yamakata, A.; Ishitani, O.; Maeda, K. Nature-Inspired, Highly Durable CO2 Reduction System Consisting of a Binuclear Ruthenium(II) Complex and an Organic Semiconductor Using Visible Light. J. Am. Chem. Soc. 2016, 138, 5159-5170.


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Copper Phosphide-Enhanced Lower Charge Trapping Occurrence in

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