稀有金属(英文版) 2020,39(12),1353-1355

Unraveling molecular-level mechanisms of reactive facet of carbon nitride single crystals photocatalyzing overall water splitting

Cong-Yong Wang Chen-Huai Yang Zhi-Cheng Zhang

Joint School of National University of Singapore and Tianjin University,International Campus of Tianjin University

Department of Chemistry,National University of Singapore

Tianjin Key Laboratory of Molecular Optoelectronic Sciences,Department of Chemistry,School of Science,Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering

作者简介：*Zhi-Cheng Zhang,e-mail:zczhang19@tju.edu.cn;

Unraveling molecular-level mechanisms of reactive facet of carbon nitride single crystals photocatalyzing overall water splitting

Cong-Yong Wang Chen-Huai Yang Zhi-Cheng Zhang

Joint School of National University of Singapore and Tianjin University,International Campus of Tianjin University

Department of Chemistry,National University of Singapore

Tianjin Key Laboratory of Molecular Optoelectronic Sciences,Department of Chemistry,School of Science,Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering

Despite the reactive facets play a vital role in designing highly efficient photocatalysts,it has long been a tremendous challenge to reveal the principle that reactive facets favor photocatalysis at the molecular level.In a recent work published in Nature catalysis,Lin et al. [ 1] investigated photocatalytic properties of the and{0001}planes of polytriazine imide (PTI)/Li⁺Clsingle crystals by photodeposition of co-catalysts,which paves the way to develop high efficiency polymer photocatalysts for overall water splitting.

Photocatalytic water splitting has been considered as a promising approach to produce renewable and clean energy as it can generate hydrogen gas for supplying energy and preparing hydrocarbon and alcohol as the raw materials [ 2, 3, 4] .Remarkably,the identification of reactive facets of photocatalysts is critical for developing highly efficient photocatalysts because they can affect not only photocatalytic activity but also charge separation during the water photolysis process [ 5, 6] .Therefore,it is highly desirable to determine the reactive facets of photocatalysts and study the underlying mechanism that the reactive facets of photocatalysts improve photoactivity.

Specially,the exploration of the reactive facets of polymeric carbon nitride (PCN) has attracted extensive attention due to its intriguing features and potential application in photocatalytic water splitting [ 7, 8, 9] .The two-dimensional (2D) planes of PCN exhibited greatly exposed surface areas and accessible introduction of co-catalysts,rendering them to be the ideal reactive facets in photocatalytic water splitting [ 10, 11] .However,it remains a significant challenge to understand atomic structure and the reactive planes of PCN,which can be primarily attributed to the low crystallinity of PCN synthesized by classical thermal polymerization,as well as poor stability under high-energy electron irradiation [ 12] .Publishing in Nature catalysis,Lin et al. [ 1] reported the preparation of PTI/Li⁺Clvia ionothermal methods,achieving high crystallinity and effective photocatalytic overall water splitting by loading Pt/Co co-catalysts (Fig.1).Moreover,the atomic structure and reactive facets of PTI/Li⁺Cl^-have been systematically investigated,which could be beneficial for promoting the development of high-performance polymeric photocatalysts for overall water splitting.

In recent years,most efforts have been devoted to investigating the reactive facets of inorganic photocatalysts.However,the study of the reactive facets and the understanding of atomic structure of the conjugated polymer (PTI) have lagged behind the inorganic counterparts due to the shortcoming of poor stability under the irradiation of high-energy electron beams.Lin et al. [ 1] overcome this limitation by analyzing the atomic and surface structure of PTI/Li⁺Clthrough an advanced transmission electron microscopy (TEM) technique of aberration-corrected integrated differential phase contrast (AC-iDPC) imaging,which was performed at a very low electron dose rate.The key advantages of this technique are the low-dose imaging and the high-contrast imaging of heavy and light atoms,thereby minimizing the irradiative damage of PTI/Li⁺Clcrystals and realizing the structural observation at the atomic level.

Fig.1 Morphology and atomic structure of PTI/Li⁺Cl^-:a SEM image of PTI/Li⁺Cl^-crystals with well-defined prism-like shape;b schematic illustration of the PTI/Li⁺Cl^-crystal with the basal plane terminated by two{0001}planes and side faces terminated by six equivalent

facets (left panel) and crystal structure of PTI/Li⁺Cl^-(Li locates at 1/3 occupation)(right panel);c AC-iDPC image of a typical PTI/Li⁺Cl^-crystal aligned along[0001]direction with inset exhibiting corresponding diffraction pattern,d Enlarged observation of red box in c,where positions of C,N and Cl atoms are superimposed,and some typical Li ions are indicated by red arrows,e AC-iDPC image of a typical PTI/Li⁺Cl^-crystal aligned along[

]direction.f HAADF-STEM image of a PTI crystal aligned to[

]direction and inset exhibits a schematic representation of distribution of Pt particles on surfaces of PTI crystals,where blue circles show positions of Pt particles.Evaluation of photocatalytic properties:g plot of maximum hydrogen and oxygen evolution rate versus average surface area ratio of

and{0001}planes(

/S_{0001}) of samples and insets exhibit three representative SEM images of PTI crystals prepared at different temperatures (500,550 and600℃),where colored diagram and equation suggest that

/S_{0001}is proportional to aspect ratio.h AQY and Kubelka-Munk function with reflectivity (R) of PTI-550 under different wavelengths.i Quantity of produced gas of PTI-550,where error bars exhibit standard deviation(evacuated at each hour after sampling).Reproduced with permission [1] .Copyright 2020 Nature Publisher

In detail,Lin et al. [ 1] applied both scanning electron microscopy (SEM) and AC-iDPC techniques to characterize the morphology and atomic structure of the as-prepared PTI/Li⁺Clcrystals (Fig.1a-e),showing the crystalline size of several hundred nanometers,good crystallinity,high atomic ordering and the precise positions of constitutional atoms (C,N,Cl,Li).Owing to small atomic number,the positions of H atoms could not be determined by AC-iDPC,which were pre^viously reported to exist among an amino group [ 13] .The distribution of photodeposited co-catalysts (Pt or Co) on the planes of PTI crystals has been observed by high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM) image (Fig.1f),in which Pt and Co cocatalysts tend to be photodeposited on the{1010}planes rather than the{0001}planes.Unlike BiVO₄ whose photoreduction and photo-oxidation reactions selectively occurred in different facets [ 6] ,both the photo-reduction and photooxidation reactions of PTI/Li⁺Cl^-crystals happened on the same crystal planes.

Theoretical calculations indicated that photogenerated carriers could favor in-plane transfer,and the planes of PTI/Li⁺Clare the primary reactive facets.This result was further confirmed by carrying out the photocatalytic properties analysis of the as-prepared PTI crystals(Fig.1g-i).Three typical PTI/Li⁺Clcrystals were synthesized at 500,550 and 600℃,denoted as PTI-500,PTI-550 and PTI-600,respectively,showing different morphologies with the average surface area (S) ratios of the plane to the{0001}plane ( /S{0001}) of 0.87,4.70 and 2.48.The Co and Pt co-catalysts were photodeposited on PTI crystals to study photocatalytic performance of reactive facets for overall water splitting.It was observed that the photocatalytic overall water splitting properties,including the H₂ and O₂ evolution rate,gradually enhanced with the increase of /S{0001},suggesting that increasing the exposed surfaces of the reactive{1010}facets is an effective approach to improve the photoactivities of the PTI/Li⁺Clcrystals.Notably,the PTI-550 with a rod shape showed excellent photocatalytic overall water splitting properties with a maximum H₂evolution rate of 189μmol·h^-1 and a maximum O₂ evolution rate of 91μmol·h^-1 under a /S_{0001}value of4.7,which is much superior to the previous report [ 14] .Besides,good consistency between the apparent quantum yield (AQY) value and diffuse reflectance spectra (DRS)pattern was demonstrated in PTI-550,implying that the absorption of the incident light could be responsible for the overall water splitting.

In a word,Lin et al. [ 1] successfully identify the reactive facets of PTI/Li⁺Clcrystals by utilizing the photodeposition of Co and Pt co-catalysts for overall water splitting.This work presents a novel strategy for improving the photoactivity of polymer photocatalysts,thus boosting the development of high-performance photocatalysts for overall water splitting.In the future works,more attention can be paid to extend this design strategy to other layered conjugated polymers,increase the photocatalytic performances and enhance the stability of co-catalysts.

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