K+-induced crystallization of polymeric carbon nitride to boost its photocatalytic activity for H2 evolution and hydrogenation of alkenes

Journal Pre-proof K+ -induced crystallization of polymeric carbon nitride to boost its photocatalytic activity for H2 evolution and hydrogenation of alkenes Yangsen Xu, Chuntian Qiu, Xin Fan, Yonghao Xiao, Guoqiang Zhang, Kunyi Yu, Huanxin Ju, Xiang Ling, Yongfa Zhu, Chenliang Su

PII:

S0926-3373(19)31203-2

DOI:

https://doi.org/10.1016/j.apcatb.2019.118457

Reference:

APCATB 118457

To appear in:

Applied Catalysis B: Environmental

Received Date:

27 September 2019

Revised Date:

11 November 2019

Accepted Date:

23 November 2019

Please cite this article as: Xu Y, Qiu C, Fan X, Xiao Y, Zhang G, Yu K, Ju H, Ling X, Zhu Y, Su C, K+ -induced crystallization of polymeric carbon nitride to boost its photocatalytic activity for H2 evolution and hydrogenation of alkenes, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118457

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K+-induced crystallization of polymeric carbon nitride to boost its photocatalytic activity for H2 evolution and hydrogenation of alkenes

Yangsen Xu,a† Chuntian Qiu,a† Xin Fan,a Yonghao Xiao,a Guoqiang Zhang,a Kunyi

a

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Yu,b Huanxin Ju,c Xiang Ling,a Yongfa Zhub and Chenliang Su*a

International Collaborative Laboratory of 2D Materials for Optoelectronics Science

and Technology of Ministry of Education, Engineering Technology Research Center

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for 2D Materials Information Functional Devices and Systems of Guangdong

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Province, Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, PR China

National Synchrotron Radiation Laboratory, University of Science and Technology

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c

Department of Chemistry,Tsinghua University, Beijing 100084, PR China

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b

of China, Hefei 230026, PR China

Those authors contributed equally to this work.

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†

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E-mail: [email protected]

Graphical Abstract

Salt-induced structure remodeling polymer carbon nitride with improved crystalline structure and charge separation ability displayed remarkably enhanced photocatalytic activity in hydrogen evolution and water-splitting-based hydrogenation of alkenes

HIGHLIGHTS

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under visible light irritation.

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1, K+-induced remodeling of amorphous PCN to crystalline KPCN was realized.

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2, The charge separation and migration were significantly improved in the ordered KPCN.

3, The water-splitting-based selectively hydrogenation of alkenes strategy was achieved.

ABSTRACT: Crystalline semiconductors with ordered long-range structure and minimized phase defect are capable of efficient separation and diffusion of photoexcited charge carriers, which is crucial for achieving high photocatalytic performances. Here, we present a new strategy using KCl as structure inducer, where

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potassium ions (K+) act as a smart “binder” for re-ordering the structure of amorphous

polymer carbon nitride (PCN) to furnish K+ implanted crystalline PCN (KPCN). The

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X-ray photoelectron spectroscopy depth profiling with Ar+ cluster ion sputtering illustrated that the element K is uniformly distributed in bulk of KPCN. The

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microstructure evolution of KPCN under elevated temperature was identified using in

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situ Fourier-transform infrared spectroscopy. This crystalline structure endows the ordered electronic transmission channels in KPCN, thus enhanced the efficiency of

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hot charge carriers separation and migration, as well as visible light capture. Therefore, the re-ordered KPCN displays nearly 20 times enhancement toward

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photocatalytic hydrogen evolution, and high activity in water-splitting-based alkenes

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hydrogenation using the in-situ photo-generated H-species from water as sustainable H-source. The present work highlights a green and reliable strategy to remodel the structure of PCN by K+ thus dramatically boosting the photocatalytic activity for hydrogen evolution as well as water-splitting-based photosynthesis of high value-added fine chemicals.

Keywords: Structure remodeling, crystalline polymeric carbon nitride, photocatalysis, water splitting, hydrogenation

1. Introduction Photocatalyst has been deemed to be a ‘miraculous stuff’, which can bubble hydrogen from water over semiconductors under sunlight, to supply a clean and sustainable hydrogen energy replacing fossil fuels, thus solve the looming

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environmental and energy problems.[1] Heptazine based polymeric carbon nitride (PCN), a low cost and metal free semiconductor with high efficiency in solar energy

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utilization, is one of the most promising candidates that has elicited ripples of

excitement in the research field.[2-11] Nevertheless, amorphous structure and rapid

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recombination of photoexcited carriers resulted in PCN with moderate photocatalytic

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activity[12]. To address these issues, structure modification strategies, such as surface engineering,[13] acid or base etch for hierarchical porous structure[14, 15], thermal for

heterostructure

nanosheet

morphology[16],

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exfoliation

construction[18-21],

and

molecular

construction

of

modification[17], precious-metal-free

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cocatalysts for ameliorated photocatalytic H2 evolution over PCN[22, 23] have been

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systematically conducted to modify the textural and electronic properties of pristine PCN. Correspondingly, the photocatalytic performances are evidently enhanced. However, those direct treatments under high temperature or acid/base environment usually seriously destroyed the planar atomic structure of carbon nitride, and the cyrstallinity as well as the production yield obviously decreased.

The disordered structure and phase defects in amorphous PCN usually function as electron trap sites to extremely limit the efficient separation of photoexcited carriers, which is the key rate-determining step in photocatalysis.[24-27] Under light irradiation, the amorphous PCN structure in a thermodynamic metastable state runs counter to the structural stabilization. In contrast, crystalline semiconductors with lowest free energy, well-defined crystal structure, long-range atomic order, and lower

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phase defects generally show superior photocatlytic performance.[28, 29] Without obstacle caused by trap sites, the photoexcited charge carriers can transfer easily in crystalline structure to surface to participate the catalytic reactions. Therefore, the

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photocatalytic properties and activities are sensitively affected by the ordered

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structure/crystallinity of the semiconducters. To date, mass-production of a crystalline carbon nitride material with well-defined structure and good photocatalytic activity is

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still of great challenge.[30, 31] Conventional thermal polycondensation are available

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to construct a low polymerized structure thus difficult to achieve ordered polycondensed melone chains and layers.[32] Although high temperature could

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promote the polycondensation, it will decompose C-N motif, thus significantly decrease the product yield. To obtain a high crystalline PCN, critical synthetic process

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and/or specific equipment with high cost such as ultrahigh pressure (GPa)[33], chemical vapor deposition are always indispensible but unable for large scale preparation. Direct treatment PCN under high temperature is one of the effective pathways to modify the textural and electronic properties of PCN as well as to enhance its photoactivity.[34] However, the planar atomic structure of carbon nitride

was usually seriously destroyed. In recent, Antonietti group developed molten salt method (also called ionothermal process), where a molten inorganic salt was employed as the reaction medium to synthesize a broad range of inorganic crystalline materials and carbons.[35-37] With the liquid state salt assistant thermal polycondensation, the obtained carbon nitride exhibited crystalline configurations, and new geometrical shapes as well as higher efficient for H2 generation in compared

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with PCN.[30, 38-41] Nevertheless, high loading of salt is necessary (msalt/mprecursor＞ 10:1) and much low yield involved (less than 20%, see Table S1) in this process. And

indispensible water-sensitive lithium salt (LiCl, LiBr) made it fall into dispute in the

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widespread use. Most rescently, single salt as the solid template and/or structure

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directing agent has been demonstrated to be a promosing way to produce highly crystalline PCN-based materials, especially under an atmospheric environment [42,

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43].

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Herein, we report a sustainable salt-induced structure remodeling method for combing amorphous PCN to crystinalline KPCN. Accordingly, the efficiency of

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photoexcited charge carriers separation and migration from bulk to surface increased impressively, thus remarkably boosts its reactivity for multifunctional photocatalysis.

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The resulted KPCN exhibited almost 20-fold enhancement photocatalytic activity for hydrogen evolution compared to the amorphous one. Furthermore, the in-situ generated

H-species

from

water

has

been

water-splitting-based hydrogenation (WSH) of alkenes.

2. Experimental Section

successfully

utilized

for

2.1 Preparation of KPCN: Firstly, melamine (10.0 g) was warmed up to 540 °C for 4 h in a tube furnace in an air atmosphere. The pristine faint yellow PCN was washed, followed by drying at 80 °C for 12 h. Secondly, a designed dosage (i.e., ∼400 mg) amorphous PCN precursor was ground with KCl (300 mg) and 2 mL ethanol in a mortar for 10 min. After drying at 80 °C, the mixture was annealed at 550 °C for 3 h under N2 (99.99%, 0.02 L min-1) in a tube furnace (inner diameter is 5 cm,

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GSL-1700X, Hefei, China). After being fully washed with boiling deionized water, the product was collected followed by drying at 70 °C under vacuum.

2.2 Preparation of Pd/KPCN(PCN) catalysts: ~1.0 wt% Pd supported KPCN (or

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PCN) were prepared by photodeposition process. Briefly, KPCN (or PCN) (300 mg)

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was dispersed in a mix solution with 80 mL deionized water and 20 mL glycol. After untrasonication treatment for 2 h, 28 μL of 1.0 M H2PdCl4 was added into the mixture,

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and then the mixture was treated under 500W Xe lamp illumination for 1 h to reduce

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Pd2+. The brownish slurry was centrifuged and washed with deionized water and Ethanol. After dried in an oven at 80 oC overnight, as-prepared catalysts were denoted

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as Pd/KPCN (or Pd/PCN).

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2.3 Photoelectrochemical measurements: The photocurrent response of the prepared carbon nitrides was performed in a three-electrode cell made of quartz with an electrochemical workstation (CHI660E, Chenhua Instruments Co., China). The electrolyte solution was 0.1 mol/L Na2SO4 solution. The platinum wire and Ag/AgCl were used as the counter and reference electrodes, respectively, and the prepared samples coated on indiumtin oxide (ITO) sheet glass were employed as the working

electrode with an area of 0.5 ×0.5 cm2. A 300W Xe lamp with a 420 nm cutoff filter was utilized as the visible light source. 2.4 Photocatalytic H2 evolution: Photocatalytic water splitting were carried out in a 150 mL Pyrex flask reactor (Labsolar VIAG, Perfectlight Technology Co., Ltd., Beijing, China) via top-irradiation with a 300W xenon lamp (XE300C) under visible light (420 nm≤λ≤780 nm) at 5 °C. For each experiment, the photocatalyst (50 mg)

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was suspended in an aqueous solution containing 10 vol % TEOA solution. Pt cocatalyst (~1.0 wt %) was in situ loaded on the photocatalyst by photodeposition of H2PtCl6. The evolved gas was quantified online using a gas chromatograph (Fuli

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9890II, Zhejiang) equipped with a thermal conductivity detector (TCD) detector with

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argon as the carrier gas.

2.5 Hydrongenation of alkenes: Typically, 20 mg of 1wt% Pd/KPCN and 0.2 mmol

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of substrate and additive (NaHSO4 or AlCl3) were dispersed in a mixture solution

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with H2O:CH3OH = 1:1. The reaction mixtures were irradiated with a LED lamp (20 W, λ= 420 nm, Suncat instruments Co., Ltd., Beijing, China) for 4 h under Argon at

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25 oC by using a flow of cooling water during the reaction. After reaction, the mixtures were centrifuged and the supernatant was extracted by adding 5 mL of

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CH2Cl2, the isolated product yield was calculated by dividing the amount of the obtained desired product.

3. Results and Discussion

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Fig. 1 Schematic illustration of the preparation of carbon nitride for (a) State-of-the-art process

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and (b) K+-induced structure remodeling of PCN; (c) XRD patterns of the obtained carbon nitrides, KPCN (urea), KPCN (melem), KPCN (melamine) were synthesized by using urea, melem,

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melamine as precursor respectively, and then remodeled by KCl; (d) schematic showing the structure remodeling process from amorphous PCN to crystalline KPCN; (e) the side views of

heptazine rings, respectively.

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KPCN, (f) and (g) are the charge distribution of PCN and KPCN in single layer with four

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The K+-induced structural remodeling procedure for the preparation of crystalline KPCN from amorphous PCN is schematically described in Fig. 1. Firstly, the

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tri-s-triazine-based pristine PCN (bright yellow) was prepared by the traditional

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thermal polycondensation of white melamine (Fig. 1a) at 540 °C. Then, the synthesized PCN with amorphous structure was ground with KCl crystals, and annealed at 550 °C under N2 atmosphere. During this K+-induced structure remodeling process, KCl acts as the template and binder. First, the amorphous structure of PCN was reorganized in the confined spaces provided by the neighboring KCl crystals (Fig. 1b). Second, the potassium ions (K+) also fused in the interval

between melon chains and could be served as a “binder” for re-ordering the structure of amorphous PCN to furnish crystalline KPCN (Fig. 1d). After removing the KCl templates in boiling water, the light green yellow KPCN with layered feature and increased crystallinity was obtained. The XRD pattern evidenced structural changes among original PCN, post-PCN (post thermal treated PCN without KCl) and crystalline KPCN (Fig. 1c). Two typical

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diffraction peaks at 27.51o and 12.98o presented both in pristine PCN and post-PCN. Those two peaks can be attributed to the interplanar stacking of conjugated aromatic

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system and in-plane repeating unites of the continuous heptazine framework.[2, 5] Obviously, KPCN shows different XRD features with one main peak centered at

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28.11o, which is about 0.6o shift to higher angle compared to PCN, suggesting a

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decreased interlayer distances from 0.324 to 0.317 nm (Fig. 1e). The denser layer of KPCN could be ascribed to the ordered arrangement of melon chains in KPCN (Fig.

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1d). Accompanying with the increase of KCl loading, the crystallinity of KPCN gradually promoted further demonstrated the structure remodeling process (Fig. S1).

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In comparison with PCN and post-PCN, the low-angle reflection peak at 13o

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disappeared in KPCN, while two newborn peaks at 8.14o (d=1.085 nm) and 10.01o (d=0.883 nm) presented. Those two peaks could be assigned to the intrinsic (100) and (110) crystal facets of KPCN.[42] In addition, the crystallinity of KPCN was improved according to a semi-quantitative index based on the half-maximum (FWHM) of the (002) diffraction peak. The FWHM values of KPCN is 1.0 which much smaller than that of PCN (1.5) and post-PCN (1.3). Notably, when pristine PCN

synthesized from other precursors for example urea and melem, the obtained KPCN show the same XRD features (Fig. 1c). This vividly revealed that the salt-assisted thermal remodeling process is a facile and universal method to obtain crystalline KPCN by reforming amorphous PCN. In addition, the DFT theoretical calculation reveals that the charge distribution changed greatly after the structure remodeling process. The charge density in PCN is uniformly distributed throughout the C-N

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framework (Fig. 1f), while in KPCN it is accumulated in the local region closer to K+ (Fig. 1g). This suggested that the K+ could act as a linker to anchor the neighboring polycondensed tri-s-triazine chains and layers, thus lead to the remodeling of PCN

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is about 5.8% according to the ICP results.

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from amorphous structure to KPCN crystals. Additionally, the content of K in KPCN

Fig. 2 The TEM and HRTEM images of PCN (a), post-PCN (b) and KPCN(c, d); (e) the FFT pattern of KPCN; (f) Dark-field TEM images and the corresponding elemental mappings of C, N, K, Cl distribution and the overlap of all elements.

The tremendous structural changes were further approved by the high-resolution transmission electron microscopy (HRTEM). As shown in Fig. 2, the typical microstructure of PCN and post-PCN are composed of aggregated several-layer amorphous structure (Fig. 2a and 2b). In contrast, the TEM images of KPCN reveal a crystalline feature (Fig. S2) which is fully different with PCN and post-PCN. The overview of KPCN displays a well-aligned feature seemingly combed by a restructure

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force (Fig. 2c). Different from the amorphous carbon nitride layers that are randomly piled together, the crystalline KPCN was assembled by a large number of regularly

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arranged nanocrystals (Fig. 2d).

Fig. 3 (a) A schemetic show of the in situ FTIR. (b) FTIR spectra of PCN and KCl composites

under different temperature from 100 to 650℃.

Close-up view demonstrated the clear lattice fingers, further indicating that the assembled nanocrystals are with good crystalline. FFT patterns in Fig. 2e and Fig. S2e display the bright diffraction spots further reveal the good crystal nature of KPCN.

The element mapping (Fig. 2f) illustrates the uniform distribution of C, N, and K throughout the whole selected area. Only trace Cl was detected in the KPCN, as confirmed by XPS (Fig. S4). This result demonstrated that KCl (K+)-assisted process forced the rearrangement of melon chains and layers in PCN, and induced the structure remodeling from amorphous to crystalline state. In addition to the improved crystalline structure by K+-induced remodeling

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process, the chemical structures of PCN were also changed. In situ FTIR, a powerful

non-destructive technique to investigate molecular structure during reactions, was firstly used to probe the structure evolution of KPCN under the evaluated temperature

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(Fig. 3a). As shown in Fig. S3a, the FTIR result of KPCN is almost reminiscent of the

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bulk PCN or post-PCN. The similar distinct out of plane bending of heptazine units ring located at 809 cm−1, and the stretching and bending modes of conjugated CN

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heterocycles between 1000 and 1700 cm−1 indicate the formation of the extended

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conjugated CN-networks (Fig.3b).[44, 45] Upon heating, two bands (2368 and 2317 cm-1) corresponding to the asymmetric stretching vibrations of CO2 appeared (starts

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form 300°C), pointing out the release of absorbed CO2 gas or/and the decomposition of the melon chains from partial oligomerization of PCN. Two new vibration bands at

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around 2180 and 2148 cm-1 corresponding to the stretching vibration of cyano groups (C≡N) (Fig. S3b) presented in KPCN. These two weak bands probably stem from the decomposition of C-N rings during the remodeling process in the temperature range up to 400°C, and have been usually reported via salt assisted method [36, 38, 39, 42, 46]. Consecutive heating to 580°C leads to further decomposition of heptazine ring in

KPCN skeleton. These observations support the conclusion that 550°C is the best

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condition for remolding PCN.

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Fig. 4 The solid-state 13C NMR spectra (a) of bulk PCN, post-PCN and KPCN. Carbon (b) and Nitrogen (c) K-edge NEXAFS spectra of PCN and KPCN. Depth profiling of XPS spectra for

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KPCN exposed to Ar+ for 0 s, 20 s and 40 s. The XPS survey spectra between 220 and 620 eV (d), High-resolution K 2p and C 1s (e) and N 1s (f) spectra of KPCN.

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The changed chemical structures of PCN were also probed by

13

C magic angle

spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The

13

C CP-MAS

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solid state NMR experiments (Fig. 4a) clearly reveals the changes of 13C signal from

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around 164.4, 163.7 to 162.0 ppm for bulk PCN, post-PCN and KPCN, respectively, which probably ascribed to the increased charge density for the insertion of K+ in KPCN (Fig. 1g).[47] The signal at low field in KPCN located at 166.7 ppm maybe attributed

to

the

N

neighboring

the

electro-attracting

group

(-C≡N).

Synchrotron-based near edge X-ray absorption fine structure (NEXAFS) was further used to exploring the microstructure of PCN and KPCN. As displayed in Fig. 4b, the

carbon K-edge NEXAFS spectra of KPCN show characteristic resonances of PCN structure including π* resonance of π*C=N at 285.8 eV and π*C-N=C at 288.7 eV. Compared with PCN, a sharper shoulder at 288.0 eV (assigned to the π*C-N species) appears in KPCN. It can be ascribed to the out-of plane orientation in KPCN basal for the increased interlayer interaction (increased electron density).[48] The improved interlayer interaction could provide a high interconnectivity between parallel layers in

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KPCN, thus would boost the transfer of electron and the photocatalytic activity. In

nitrogen K-edge region (Fig. 4c), KPCN shows two typical π* resonances at 399.9

and 402.8 eV corresponding to aromatic C-N=C coordination in one hetpazine

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heteroring (N1, see the inset of Fig. 4c) and -NH- bridging unit (N2).[48] The π*

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resonance at 401.8 eV can be assigned to N-(C)3 in hetpazine heteroring (N3) and/or cyanic structure (-C≡N, N4)[49]. The X-ray photoelectron spectroscopy (XPS) depth

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profiling of KPCN by argon ion sputtering enables high-resolution chemical analysis

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of the layer composition/structure (Fig. 4d). As shown in Fig.4e, Two K 2p peaks located at 292.7 and 295.5 eV, as well as the doublet separation energy (2.8 eV)

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indicated the oxidation states of K+ ions in KPCN.[42] Importantly, the atomic percentage of potassium increased after wiping off the surface layer (2-5 nm) and then

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almost kept constant as increased the time of exposure to Ar+, signifying that K+ in bulk of KPCN is homogenous distribution. Due to the existence of positive charged K+ in KPCN, the charge compensation may occur similarly to that in potassium melonate featuring C≡N/C=O as a surface terminal group[45] (Fig. S4c), which is in good accordance with the FTIR and NMR results. The high-resolution N 1s spectra

disclose that the proportion of -NHx [50] in KPCN decreased compared to pristine PCN as time of exposure to Ar+, which could be ascribed to the existence of C≡N groups since cyano groups bind the similar C with -NHx or the interaction between -NH- and K+ (insert in Fig.4c).The major aromatic carbon and nitride species (N-C=N) did not show any changes between KPCN and PCN, indicating the same

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heptazine motif (Fig.4f).

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Fig. 5 (a) UV-vis diffuse reflectance spectra, the inset is the Kubelka-Munk function plot of carbon nitrides. (b) time-resolved fluorescence decay spectra of the bulk PCN and KPCN excited

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by 370 nm nanosecond laser. (c) SPV spectra of PCN and KPCN; the inset is the magnified view of PCN. (d) Photocurrent intensity of PCN and KPCN.

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Accompanying with the structure remodeling of PCN, the physicochemical and

photoelectric properties were also reformed. As shown in Fig.S5, the increased crystallinity of KPCN resulted in the decrease of BET surface area from 22.8 m2g-1 (PCN) to 14.7 m2g-1. The diffuse reflectance spectra (DRS) displayed in Fig. 5a shows KPCN significantly improved light harvesting ability compared to the PCN

and post-PCN. The band gap estimated from Tauc plots (inset in Fig. 5a) are 2.78, 2.76 and 2.73 eV for PCN, post-PCN and KPCN, respectively. Besides, the abrupt absorption edge of KPCN indicates an improved crystallinity and no obvious phase defects compared to PCN (Fig. S6). XPS valence band (VB) spectrum determined the band structure of synthesized carbon nitrides (Fig. S7). Both post-PCN and KPCN show the same VB potential (1.96 V). As the schematic illustration of the electronic

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band alignment presented in Fig. S8, structure remodeling process did not notably change the electronic structure. Only about 0.03 eV lowered conduction band (CB)

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potential in KPCN was achieved with respect to PCN and post-PCN.

Subsequently, steady-state photoluminescence (PL) spectroscopy was performed

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(Fig. S9). KPCN has a weak emission peak placed around 462 nm while the PCN and

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post-PCN have strong adsorption centered about 470 nm, suggesting a low radiative recombination of photoexcited electrons and holes in KPCN. For better understanding

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the transfer of the photogenerated charge carriers, the time-resolved transient fluorescence decay spectrum was achieved (Fig. 5b). The kinetic lifetimes were fitted

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with a bi-exponential decay model[51] according to: τ = (A1*τ12+ A2*τ22)/(A1*τ1+ A2*τ2). Two decay components of τ1 and τ2 are derived and listed in Fig. 4b. The

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lifetimes of both components (τ1 = 0.472 ns, τ2 = 3.759 ns) in KPCN are much shorter than that of PCN (τ1 = 1.365 ns, τ2 = 5.746 ns). The average PL lifetime of KPCN is down to 1.17 ns, about 2.66 ns shorter than that of PCN (3.83 ns). The observation of significant PL quenching is together with the reduction of PL lifetimes. This confirms the enhanced exciton dissociation in KPCN, [52, 53] which is convenient for the

charge separation and make more charge carriers available for interfacial transfers.[54] To fully approve the efficient separation and rapid migration of charge carriers to the surface in KPCN, surface photovoltage response (SPV) (Fig. 5c) and photocurrent response (Fig. 5d) in PCN and KPCN were performed. Compared to PCN, the surface photovoltage response of KPCN is two orders of magnitude stronger, and photocurrent response is three times improved. Those results strongly confirmed

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that the remodeled crystalline structure can truly contribute to the separation of

electrons and holes, and accelerate the charge transfer from the bulk to the KPCN surface. In this case, the active transferred electrons on KPCN surface can be easily

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captured by substrates such as protons and organic molecules. Consequently, the

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photocatalytic hydrogen evolution and waster-splitting-based alkenes hydrogenation reactions were conducted as model reactions to confirm the enhancement that

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achieved by the crystalline KPCN.

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As displays in Fig. 6a and Fig. S10, almost all KPCN catalysts (i~iv) exhibited higher activity than that of bulk PCN. And the photocatalytic activities were enhanced

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gradually as the increase of KCl loading in the KPCN. The initial hydrogen evolution rate for the optimal KPCN reached 59.4 μmol·h-1 (5oC, ~1wt% Pt cocatalyst), which

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is approximate 20 times higher than that of bulk PCN (3.12 μmol·h-1) and 10-folds of post-PCN (5.93μmol·h-1, Fig. S10). Correspondingly, the apparent quantum efficiency (AQE) of KPCN for the H2 evolution is about 8.6% under visible light (420 ± 5 nm). A color change of the KPCN catalyst suspension (not in PCN) from yellow to turquoise was observed during water splitting (Fig. S11). This phenomenon is

coincident with the discussion above, in which the remodeled crystalline KPCN structure contributes to improve the separation efficiency of electron-hole.[53, 55] Importantly, the product yields of all the obtained KPCN samples are higher than that of post-PCN (without KCl, Fig. S12). The H2 production activities displayed in Fig.S13 demonstrated that the KPCN samples prepared from melamine, urea, and melem greatly enhanced activity than that of PCN in photoreduction of water for H2

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evolution, which further justified that it is a universal method by salt to remodel the

strucure of amorphous PCN for photocatalysis enhancement (as conclued in Table

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S1).

Fig. 6 (a) H2 evolution rate from water under visible light irradiation (420 nm ≤λ≤780 nm) with 300 W Xe lamp at 278K over KPCN, the KCl loading ratio in KPCN from i to iv is 3/4,

4/4, 2/4, 1/4, respectively; v and vi are post-PCN and PCN. (b) The photocatalytic hydrogenation of alkenes over Pd/KPCN(PCN) catalysts. Reaction conditions: 0.1 mmol reactant, 10 mg 1.0 wt% Pd/KPCN catalyst, Ethyl acetate/H2O/CH3OH=2 mL/1.5 mL/1.5 mL, additive 0.1mmol NaHSO4, reaction time 4 h, 420 nm LED light 20 W at room temperature, yields were calculated by GC-MS using standard curve.

The crystalline KPCN can also accelerate more complicated organic transformations under ambient conditions, for example, WSH reaction of alkenes. As

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shown in Fig. 6b, only trace yields of substrates such as styrene (a’), 4-methylstyrene

(b’), 1,2-diphenylethene (c’) and acetocinnamone (d’) were detected over Pd/PCN catalyst under LED light (420 nm) irradiation, while more than 90% yields was

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achieved over the Pd/KPCN catalyst. This revealed that crystalline KPCN was an

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excellent bifunctional photocatalyst for combining water splitting and hydrogenation reactions. Then the reaction scope and effectiveness of KPCN catalyst were further The

derivatives

of

styrene

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performed.

with

both

electron-donating

and

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electron-withdrawing groups such as 4-methoxy styrene (e), 4-chloro-styrene (f), α-methyl styrene (g) and 1,1-diphenylethene (i), could achieve yields greater than

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90%. Furthermore, typical α,β-unsaturated ketones such as chalcone (j) and cyclohexenone (k) can also convert to the cressponding ketones with high yields

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demonstrating a selective hydrogenation of C=C bond. Here, the structure remodeled crystalline KPCN with ordered arrangement of framework and fewer phase defects contribute to accelerate the charge transfer to the KPCN surface, finally promote the surface electrons transfer, hydrogen-evolution and hydrogenation reactions.

4. Conclusions

Herein, we disclosed a sustainable strategy for repairing and reordering the structure of amorphous PCN by incorporating potassium ions via simple-annealing with KCl. Owing to the highly crystalline structure and ordered electronic transmission channel, the physicochemical and photoelectronic properties such as light harvesting, electron lifetime, charge separation and migration to the material surface were all improved remarkably. As a result, KPCN exhibited great

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enhancement of photocatalytic activity than pristine PCN in hydrogen evolution and

water splitting-based alkenes hydrogenation reaction under visible light irradiation. The present work not only highlights the strategy for ordering the structure of PCN by

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K+, the importance of crystalline structure for the promotion of photoelectronic

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properties and photocatalytic activities, but also illuminates promising and practical

fine chemicals.

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Supporting Information

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solution to employ water-splitting-technology for artificial synthesis of value-added

Supporting Information is available from the Wiley Online Library or from the

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author.

Author Contributions

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Y. Xu and C. Qiu contributed equally to this work. Y. Xu, C. Qiu and C. Su designed this work. Y. Xu synthesized the PCN and KPCN materials; C. Qiu prepared the catalysts. H. Ju characterized NEXAFS. C. Qiu and X. Fan measured photocatalytic hydrogenation reactions. Y. Xu, C. Qiu and C. Su co-wrote the manuscript. All the authors gave useful suggestions, discussed the results and commented on the manuscript.

Conflict of Interest The authors declare no conflict of interest.

Declaration of Interest Statement

We wish to confirm that there are no known conflicts of interest associated with this

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publication. We further confirm that the order of authors listed in the manuscript has

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been approved by all of us.

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Acknowledgements

This work was financially supported by the National Natural Science Foundation of

Shenzhen

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China (21401190, 51502174, 21902105), Guangdong Special Support Program, Peacock

(KQJSCX20170727100802505 Shenzhen

Innovation

and Program

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KQTD2016053112042971),

Plan

Jo

(JCYJ20170818142642395), Educational Commission of Guangdong Province (2016KTSCX126 and 2016KCXTD006) ， Pengcheng Scholar Program, and Foundation for Distinguished Young Talents in Higher Education of Guangdong (2018KQNCX221).

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