The Effect of Axial Coordination of Iron Porphyrin on Their

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The Effect of Axial Coordination of Iron Porphyrin on Their Nanostructures and Photocatalytic Performance Xuemin Tian, Chensheng Lin, Zhou Zhong, Xiaoxin Li, Xiao Xu, Jingjing Liu, Longtian Kang, Guoliang Chai, and Jiannian Yao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00125 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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Crystal Growth & Design

The Effect of Axial Coordination of Iron Porphyrin on Their Nanostructures and Photocatalytic Performance Xuemin Tian,†,‡ Chensheng Lin,§ Zhou Zhong,†,‡ Xiaoxin Li,†,‡ Xiao Xu,† Jingjing Liu,† Longtian Kang,*,† Guoliang Chai,*,§ and Jiannian Yao*,‖ †Key

Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Provincial

Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China. ‡University

of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing, 100049, P.

R. China. §State

Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Science, Fuzhou, Fujian 350002, P. R. China. ‖Beijing

National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese

Academy of Sciences, Beijing 100190, P. R. China. ABSTRACT: The enough exposure of active face is a key factor of nanocatalysis for the sustainable energy conversion. Here, we exhibit the effect of axial coordination of organic metal complex molecules on the morphology evolution and photocatalytic hydrogen evolution (PHE) activity of organic nanocrystals (ONCs). The three series of iron porphyrin (FeTPPX, X = Cl, O

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and OH) ONCs are controllably synthesized via the cetyltrimethyl ammonium bromide (CTAB)assisted chemical reaction at different pH. The uniform zero-dimension (0D) FeTPPCl ONCs, ultrafine one-dimension (1D) [FeTPP]2O ONCs with the diameter of ~35 nm, and the ultrathin two-dimension (2D) FeTPPOHH2O ONCs with the thickness of a crystal cell (< 1 nm) can be obtained by adjusting the concentration and volume of CTAB during the hydrolysis reaction of iron porphyrin perchlorate (FeTPPClO4). The mechanism of morphology evolution is carefully investigated, revealing the synergistic effect of axial ligand of FeTPPX and CTAB on the exposure of hydrophilic active face parallel to porphyrin ring. The size-, shape- and axial liganddependent photocatalysis can be clearly observed. Without using cocatalyst, the FeTPPOHH2O ultrathin nanoflakes display the highest PHE rate (~0.75 mmol/h/g), followed by FeTPPCl octahedrons (~0.48 mmol/h/g) and [FeTPP]2O ultrafine nanorods (~0.20 mmol/h/g). This work provides a new strategy to apply the conjugated organic compounds in nanocatalysis. KEYWORDS: organic nanocatalysis, photocatalytic hydrogen evolution, axial coordination, iron porphyrin, self-assembly, crystal engineering, 1 Introduction To alleviate the global energy crisis and environmental pollution, the PHE from water splitting has attracted extensive attention over the past decades owing to the effective conversion of solar energy into the storable and clean hydrogen energy. In principle, the enough light absorption, the effective separation of photoinduced electrons and holes as well as the quick durable surface reactions are the three crucial factors for designing and fabricating a highly-efficient photocatalyst.1-8 To date, the remarkable progress in the preparation of photocatalyst has been made on the basis of traditional semiconductor materials, such as transition metal oxides, metal


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sulfides, and organic conjugated polymers like graphitic carbon nitrides (g-C3N4).3-11 However, the inorganic semiconductors or g-C3N4 based photocatalysts intrinsically suffer from a low catalytic activity due to the insufficient visible light absorption and the quick recombination of photoinduced electrons and holes. Moreover, the poor hydrophily related to the dispersion of these photocatalysts in water and the contact of active face with water also results in unsatisfied photocatalitic peformance.3 Organic dyes with a large π-conjugated scaffold and tailorable photoelectric properties are important members of semiconductor materials. They are often used in field-effect transistors, light–emitting displays or solar cells.12-16 Nevertheless, the direct use of organic dyes nanomaterials as photocatalysts seems to be neglected in the past decades.17-19 Metal porphyrin (MTPP), as a kind of typical p-type organic semiconductors or dyes with a stable conjugated structure, possesses a wide absorption of visible light from porphyrin ring and metal-ligand charge transfer (MLCT), more positive LUMO than hydrogen (vs Vac.) and longlifetime of triplet excitons.8,9,17-21 Theoretically, they can be used as not only the good photosensitizers but also the direct nanocatalysts for PHE via controlling the size- and shapedependent photoelectric properties.8,9,18,19 In these ONCs, the generation of J- or H- aggregates after the self-assembly of molecules are intrinsically favor of photocatalysis due to the wider light absorption, longer exciton lifetime of dimers and intermolecular charge transfer (CT) state, and a better hydrophilic ability, as compared to single molecule. Moreover, it has been widely proved that the catalytic active site of MTPP is usually situated in the central metal atom of the large conjugated ring.17-21 Therefore, the key step for improving photochemical reaction on the surface of MTPP is to enough expose the hydrophilic active face of ONCs parallel to the conjugate ring via the controllable self-assembly of molecules, similar to the crystal engineering of TiO2.3-5,19 However, the corresponding active crystal face usually has high surface energy and



thus easily disappears during the kinetic and thermodynamic growth of ONCs. Benefiting from the variety of molecular structures and the recent developments of crystal engineering, it is believed that the hydrophilic axial ligand may provide us a good chance to selectively limit the growth of active surface with the help of a suitable capping agent.4,5,18,19,22 Thereafter, the crystal face parallel to the conjugate ring can be fully exposed and has a strong interaction with water. Unfortunately, the corresponding works are rarely reported until now because of the possible difficulty in the controllable large-scale preparation of uniform ONCs. Iron porphyrin and their derivatives are famous for hemoproteins, and actually have an important impact on a variety of biological process as the catalytic active site of enzymes, e.g. peroxidases, oxidases and halogenases.21,23,24 In the catalytic process of oxygen-containing groups, iron porphyrin with the change of axial ligand and the valence state of Fe atom exhibits excellent activity. In addition, it must be noticed that the axial ligand of FeTPPX like -OH, -Cl etc. intrinsically has a strong hydrophily due to the easy formation of hydrogen bond (H-bond) with water (Figure S1). The advantages of FeTPPX on photocatalysis, such as the extensive absorption of visible light, long-lifetime exciton, activation for O-H bond, and the close contact with water via axial ligand, strongly imply their potential photocatalytic activity for PHE. In this paper, three series of FeTPPX (X = Cl, O and OH) ONCs were successfully synthesized in the CTAB-assistant chemical reactions at different pH.25-27 The self-assembly mechanism of different iron porphyrin molecules and the importance of surfactant were carefully investigated on the basis of thermodynamical and kinetic theory of crystal nucleation and growth in the reaction-diffusion system. The effects of axial ligand on the size and morphology of FeTPPX ONCs as well as catalytic activity of PHE were explored. This work reveals that a hydrophilic


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axial ligand is advantageous to enhance the exposure of active face and catalytic activity of organic metal complex ONCs. 2 Results and discussion 2.1 Synthesis and characterization

Figure 1. (a) MALDI-TOF mass spectra, (b) UV−visible absorption spectra in CH2Cl2, (c) FTIR spectra and (d) XRD patterns of samples obtained at pH=1.2 (S1, black line), pH=6.5 (S2, red line) and pH=13.7 (S3, blue line), respectively. FeTPPClO4 was firstly synthesized as a precursor according to the reaction (1). The three kinds of FeTPPCl, [FeTPP]2O and FeTPPOHH2O molecules in Figure S1 can be obtained through the reaction (2), (3) and (4) when the pH value was 1.2, 6.5 and 13.7, respectively.25-27 The corresponding products are called as S1, S2 and S3 here. The detailed process can be found



in Experimental section. In a typical experiment, 1.0 mL of 1.0 mM FeTPPClO4 solution was quickly injected into CTAB aqueous solution at a fixed pH. After centrifuging and washing several times with ultrapure water, the products were collected for further analysis. A series of characterizations were used to explore the composition and crystal structure of the as-obtained samples. The MALDI-TOF mass spectra in Figure 1a implies the possible existence of [FeTPPCl]+ (m+/z = 702.7 (Exp.), 704.2(Cal.) ) in S1, [(FeTPP)2O]+ (m+/z = 1353.1 (Exp.), 1353.1(Cal.) ) in S2 and [FeTPPOH]+ (m+/z = 684.0 (Exp.), 685.6 (Cal.) ) in S3. A weak peak of m+/z = 1353.9 in S3 may be from a little conversion of FeTPPOHH2O to [FeTPP]2O, as displayed in the reaction (5).25,26 Figure S2 exhibits the color change of the S1, S2 and S3 in dichloromethane (CH2Cl2) from brown to green. The typical Soret band (B-band) and Q-band of metal porphyrin (MTPP) can be clearly observed from their UV−vis spectra in Figure 1b.28 The UV-vis spectrum of S1, including a Soret band at 416 nm and three obvious Q-bands at 509, 572 and 690nm, is according with that of FeTPPCl molecule. The four typical peaks of [FeTPP]2O molecule at 318, 407, 570 and 610 nm can be found in the UV-vis spectrum of S2. The appearance of three peaks at 323, 411 and 570nm mean the generation of FeTPPOHH2O in S3. The Fourier-transform infrared spectra (FT-IR) in Figure 1c provide more information on the three samples. All of them have two obvious bands at 1003 and 994 cm-1 derived from the vibration modes of the skeleton of porphyrin ring.29 The moderate intensity bands at 874 and 891 cm-1 in S2, as well as a broader and weaker vibration band at 872 cm-1 in S3 are the stretching vibration of Fe-O-Fe.28 The vibration band at 3645 cm-1 in S3 may be assigned to the stretching vibration of O-H from the Fe-O-H,30,31 while the vibration band at 3742 cm-1 is assigned to the stretching vibration of O-H from the weak coordination of water in crystal. This water takes part in the formation of crystal but has weak interaction with the main structure owning to the long


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atomic distance. As a result, the infrared vibration of O-H wave number moves higher than FeO-H but still lower than the absorbed water.32 Besides, the crystal structures of the three samples were also investigated by powder X-ray Diffraction (XRD), as shown in Figure 1d. S1 may be assigned to tetragonal FeTPPCl single crystal (CCDC No. KANYUT02) with the space group I4 and the unit cell parameters of a = 13.563 Å, b = 13.563 Å, c = 9.837 Å. S2 can be ascribed to the formation of orthorhombic [FeTPP]2O single crystal (CCDC No. PPORFE10) with the space group C2ca and the unit cell parameters of a = 15.197 Å, b = 25.077 Å, c = 18.074 Å. The XRD pattern of S3 can also be perfectly indexed to the tetragonal FeTPPOHH2O single crystal (CCDC No. FHTPOR) with the space group I4 and the unit cell parameters of a = 13.534 Å, b = 13.534 Å, c = 9.82 Å. All of the above experimental results prove that the samples of S1, S2 and S3 are mainly composed of FeTPPCl, [FeTPP]2O and FeTPPOHH2O crystals in order, indicating that iron porphyrin ONCs with different axial ligands can be synthesized via adjusting pH during the hydrolysis reaction of FeTPPClO4. 2.2 Controllable self-assembly of iron porphyrin molecules



Figure 2. (a) Atomic model of FeTPPCl. (b) XRD patterns of different FeTPPCl ONCs. (c) Size distribution of FeTPPCl ONCs with the change of VCTAB. SEM and TEM images as well as SAED patterns of (d) snowflake-like, (e) tetrapod-like and (f) octahedral FeTPPCl ONCs obtained when 1 mL of 1 mM FeTPPClO4 was added into (d)1 mL, (e) 2 mL and (f) 3 mL of 50 mM CTAB aqueous solution at pH 1.2, respectively. The controllable synthesis of FeTPPX ONCs was explored via adjusting the concentration of CTAB (CCTAB) and the volume of CTAB (VCTAB) in the reaction (2), (3) and (4) systems. Here, the increase of VCTAB means the decrease of FeTPPX monomer concentration and vice versa, when the molar amount of FeTPPClO4 is fixed. Figure 2a shows atomic model of FeTPPCl molecule obtained in acid condition (pH=1.2). When VCTAB is 1 mL, Figure S3 shows that the morphologies of as-prepared ONCs vary from irregular particles to snowflake-like structures with the increase of CCTAB from 0 to 50 mM, indicating the importance of CTAB in the controllable preparation of uniform FeTPPCl ONCs. The appearance of regular morphology can be found when CCTAB is higher than 20 mM. A higher CCTAB will induce the production of more uniform structures. Figure S4 displays the morphology evolution of these FeTPPCl ONCs with the VCTAB increased. The XRD patterns of as-obtained samples in Figure 2b further confirm that the above processes generate the tetragonal FeTPPCl single crystals (CCDC No. KANYUT02). Figure 2c reveals that the average size of these ONCs gradually decreases form 4.2 to 1 m as increasing the VCTAB from 0.5 to 4 mL. Three typical morphologies can be found in Figure 2d-f and Figure S4 like snowflake, tetrapod and octahedron with the change of VCTAB. When VCTAB is smaller than 1 mL, the crystal shape is always like snowflake. The tetrapod-like crystals can be observed when VCTAB is 2 mL. The octahedral crystals appear once the VCTAB is more than 3 mL. According to the selected area electron diffraction (SAED) patterns in Figure 2 and the structure


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of single crystal, the C4 symmetry axis of the three ONCs can be easily assigned to the [001] direction. The generation of snowflake-like crystal means the kinetic preferential growth of FeTPPCl along the four directions of ± [110], ±[110] under a high monomer supersaturation. The growth along the [001] direction becomes fast when VCTAB is increased, until the formation of octahedral single crystal surrounded by eight equivalent crystal faces of ±{101}, ±{101}, ±{011} and±{011} after the defect sites are filled.19 This process also implies the gradual disappearance of (002) plane, which is consistent with the change of XRD patterns. As exhibited in Figure 2b, the intensity ratio of diffraction peaks between (002) and other crystal faces, such as (110) and (101), decreases with the shape evolution from snowflake to octahedron.

Figure 3. (a) Atomic model of [FeTPP]2O. (b) XRD patterns of Rod 1, Rod 2 and Rod 3 obtained when 1 mL of 1 mM FeTPPClO4 was added into (d) 1 mL, (e) 2 mL and (f) 3 mL of 50 mM CTAB aqueous solution at pH 6.5, respectively. (c) Size distribution and aspect ratio of



[FeTPP]2O crystals obtained with the change of VCTAB. (d-f) SEM images, TEM images and SAED patterns of Rod 1, Rod 2 and Rod 3. [FeTPP]2O molecules can be prepared and tend to 1D self-assemble at pH 6.5 with the aid of CTAB, as exhibited in Figure 3. When VCTAB is 1 mL, Figure S5 reveals that the morphologies of as-prepared ONCs evolve from irregular nanoparticles to nanorods with the CCTAB changing from 0 to 50 mM, suggesting that CTAB can regularize the self-assembly of [FeTPP]2O molecules. A low CCTAB (< 0.5 mM) goes against the formation of the uniform 1D ONCs. When CCTAB is up to 10 mM, the width of products increases from 180 to 260 nm, and the two tips of nanorods gradually become concave. The effect of the monomer concentration of [FeTPP]2O on the molecular self-assembly was also investigated via the change of VCTAB when CCTAB is kept at 50 mM, as shown in Figure S6. The XRD patterns in Figure 3b confirm that a series of asobtained nanorods have the same structure as [FeTPP]2O single crystal (CCDC No. PPORFE10), and reveal that the diffraction peak intensity ratio of (020) to (111) reduces with the increasing of aspect ratio of nanorods. As generalized in Figure 3c, their diameters decrease to even 35 nm, while the aspect ratio obviously increases with VCTAB increased. All of nanorods may be classified as three types according to their terminal shape (Figure 3d-f.). Here, the nanorods with obviously opened ends are named as Rod 1, those with the slight concave ends are Rod 2, and those with the regular ends are called as Rod 3. Rod 1 can be found when VCTAB was less than 1 mL, Rod 3 appears when VCTAB is more than 3 mL, while Rod 2 is easily obtained when VCTAB is between 1 and 3 mL. The TEM image in Figure 3d proves that Rod 1 is not a nanotube structure. The opened ends may be attributed to the formation of etch pit. It may generate during the equilibrium between dissolution and recrystal, and is closely associated with the property of crystal face.33,34 In our experiments, the critical size of etch pit is about 150 nm. Further analysis


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from the XRD and SAED patterns manifests that these nanorods preferentially grow along the [101] direction, as reported in our previous work.35

Figure 4. (a) Atomic model of FeTPPOHH2O molecular, (b) XRD patterns of as-obtained FeTPPOHH2O structures, (c-e) typical SEM images of FeTPPOHH2O nanoflakes, (f) TEM image and relevant SAED pattern (g) element mapping images as well as (h) AFM image and (i) the corresponding height profiles of FeTPPOHH2O nanoflakes prepared via the ultrasonic treatment .



In alkaline solution (pH=13.7), FeTPPOHH2O molecules in Figure 4a could be synthesized. The effect of CCTAB on the structure and shape of FeTPPOHH2O crystals was firstly investigated. The XRD patterns in Figure 4b confirm that the as-obtained products with or without CTAB have the same structure as FeTPPOHH2O single crystal (CCDC No. FHTPOR). The widening diffraction peaks indicate the CTAB-induced formation of FeTPPOHH2O nanostructures. When CCTAB increases from 0, 10, 20 to 50 mM, the regular nanostructures can not be observed rather than the layer-by-layer structures in Figure 4c and Figure S7. Atomic force microscopy (AFM) image in Figure S8 shows the self-organization of FeTPPOHH2O nanoflakes with the thickness of 15 nm. A series of experiments were also carried out to investigate the influence of VCTAB in the self-assembly of FeTPPOHH2O molecules when CCTAB was kept at 50 mM, as seen in Figure S9. Unlike the distinct shape evolution of FeTPPCl and [FeTPP]2O ONCs, the shape transition of FeTPPOHH2O ONCs was not observed with the increasing of VCTAB. However, the SEM images in Figure 4d and 4e clearly prove that the generation of many looser laminar structures, which derive from either layer-by-layer growth or the self-organization of ultrathin 2D structure, suggesting the existence of strong interaction between CTAB and exposed crystal face. The TEM image and relevant SAED pattern of nanoflake in Figure 4f confirm that the exposed crystal face of FeTPPOHH2O nanoflakes is (002) plane. The element mappings of FeTPPOHH2O nanoflake in Figure 4g show not only the homogeneous dispersion of C, N, O and Fe atoms, but also the existence of Br atom, further verifying the significant role of CTAB on the formation of lamellar FeTPPOHH2O nanostructures. Here, the exfoliation of lamellar nanostructure was tried via the ultrasonication with the power of 800 W for 30 min. A large number of pieces with the width between 100 and 1000 nm can be seen on the surface of clean Si substrate for AFM measurement (Figure 4h). The corresponding height profile in Figure 4i


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reveals that the thickness of these pieces is about 1 nm. The surprising size is same as that of a unit cell along the c-axle direction (0.982 nm) and confirms that the selective adsorption of CTAB on the (002) plane may completely prevent the growth of 2D crystal nucleus along the axial direction after the nucleation stage. Theoretically, these ultrathin nanoflakes are the thinnest 2D small molecule crystal. 2.3 Self-assembly mechanism of iron porphyrin molecules Here, the effect of CTAB, surface energy of crystal, monomer supersaturation and surface hydrophily of crystal on the reaction, nucleation and growth of iron porphyrin are systematically investigated. As above-mentioned, the FeTPPX molecules with three axial ligands exhibit the different self-assembly modes by adjusting the CCTAB. Indeed, we can not obtain any regular and uniform crystals in the reaction (2), (3) and (4) without CTAB, strongly indicating the key functions of CTAB. As we know, CTAB has been widely used in the preparation of nanomaterials as amphipathic stabilizer that reduces the free energy of target via the hydrophilic and hydrophobic action,35,36 or as the capping agent that limits the growth of some crystal faces ,4,37,38 or as a template that induces the nucleation and growth on the surface of its micelles as well as subsequent self-organization,35,39 even as a reactant due to the existence of bromide ion.39,40 In our experiments, the most obvious function of CTAB is as a stabilizer. 1) It can cause the homogenous dispersion of precursor FeTPP+ ions via the electrostatic repulsion force with CTA+ ions. 2) It may efficiently reduce the driving force of the self-assembly of iron porphyrin molecules with a big and anisotropic molecular structure, and in turn avoids the quick formation of irregular nanostructures. 3) After nucleation, it may also relieve the aggregation of nanostructures. Besides, CTAB also acts as a capping agent in our experiments. The most convincing examples are the synthesis of 2D FeTPPOHH2O and quasi-2D snowflake-like 13 Environment ACS Paragon Plus


FeTPPCl ONCs because the strong electrostatic interaction between ligand -Cl / -OH and CTA+ seriously limits the growth of crystal nucleus along the axial direction, causing the more exposure of (002) plane parallel to porphyrin ring. The template effect of CTAB micelle can be observed during the production of 1D rod-like [FeTPP]2O ONCs, which has been clearly illustrated in our previous work.35 It is believed that the achievement of further exfoliation of lamellar FeTPPOHH2O ONCs may be attributed to the strong interaction between CTAB micelle and (002) plane. In this case, CTAB acts as both capping agent and template. As a result, the thinnest 2D small molecule crystals can be exfoliated after the CTAB-assisted layer-andlayer growth of smooth face with low surface energy.38 The last but not least role of CATB may be as a reducing agent owing to the strong oxidation of FeTPPClO4, which ensures that the FeTPPX molecules are immediately enclosed after the occurrence of the reaction (2), (3) and (4), followed by the homogenous nucleation and growth.40 Generally, the importance of CTAB can be observed during the formation of FeTPPX ONCs in the diffusion-reaction system. However, it must be known well that the most important factor, which determines the final shape of thermodynamical crystal is the intrinsic structure or surface energy of single crystal once the 2D crystal nucleus formed.4,5,8,9,18,19,22 To verify the mechanism, the surface free energy of different crystal faces of FeTPPCl and FeTPPOHH2O ONCs were calculated using the VASP software on the basis of the first-principle and density functional theory (DFT),41 as listed in Table S1. At high supersaturation of iron porphyrin monomer, the quick kinetic growth of high energy facets usually occurs, such as the growth of FeTPPCl along the four directions of ±[110] and ±[110], and the quick growth of (100) and (010) planes of FeTPPOHH2O ONCs after the weakening of (002) affinity energy due to the selective adsorption of CTAB.19 Under a low supersaturation condition, the octahedral thermodynamic crystal, which is surrounded by eight


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equivalent facets with the lowest surface energy, can be often found with the decreasing of supersaturation, e.g. the big octahedron of FeTPPCl in Figure 2f and many small ones in four branches of snowflake-like FeTPPCl ONCs in Figure 2d. The appearance of 1D [FeTPP]2O ONCs with two concave ends may be also attributed to the CTAB-assistant dissolution of lowenergy facet.33,34,42 In this work, the most interesting phenomenon is that the difference of axial ligands in iron porphyrin molecules leads to the shape change from 0D, 1D to 2D nanostructures under the same CCTAB and monomer supersaturation conditions, even although the single crystal structures of FeTPPCl and FeTPPOHH2O are almost same. In this case, the hydrophilic interaction between the ligand of Fe atom and water on the existence of CTAB has to be thought about.43 The crystal face with a better hydrophily usually means its slower growth in aqueous solution due to the limitation of H2O layers after the formation of H-bond. Here, water is more like a template and/or a capping agent due to the synergistic effect with the hydrophilic group of CTA+ during the growth of crystal nucleus. On the surface of crystal nucleus in the aqueous solution, the FeTPPCl molecule with intrinsic hydrophobicity easily form a FeClHOH H-bond, while a FeTPPOHH2O molecule generate a Fe(H2)OHOH and three FeOHO(H2) H-bonds, as shown in Figure S10. Therefore, the hydrophily of iron porphyrin molecule follows the order of FeTPPOHH2O > FeTPPCl > [FeTPP]2O. For the crystal face parallel to conjugated ring, the hydropholic action drives its fast growth, e.g. the 1D growth of [FeTPP]2O, while the hydrophilic action limits the axial growth, especially when the amphiphilic surfactant exists like CTAB. Hence, the crystal morphology of iron porphyrin is also closely associated with the interaction between the axial ligand and dispersing agent.



2.4 Photocatalytic activity

Figure 5. (a) PHE rate when the different morphologies of iron porphyrin ONCs as photocatalysts. (b) Stability test of PHE for the FeTPPOHH2O nanoflakes. To gain insight into the influence of shape, size and axial ligand in the photocatalysis of iron porphyrin ONCs, the PHE performance was measured upon a 300 W Xe lamp irradiation. In a typical experiment, 1.0 mg sample was dispersed into 100 mL water containing 10.0 mL triethanolamine (TEOA) as a sacrificial electron donor (SED). Figure S11 and Figure 5a display that all of FeTPPX ONCs have a wide absorption for visible light and exhibit the PHE activity. The impact of shape on PHE rate can be clearly found in FeTPPCl ONCs. Here, the snowflakelike ONCs show a higher PHE performance (0.66 mmol/h/g) than quasi-octahedral (0.56 mmol/h/g) and octahedral (0.48 mmol/h/g) ONCs. The PHE rate of different [FeTPP]2O nanorods reveal the size-dependent PHE and a lower activity compared with FeTPPCl and FeTPPOHH2O. The thin Rod 2 displays a better PHE activity than thick Rod 1, which can be attributed to the more exposure of active sites. Interestingly, the thinnest Rod 3 exhibits the lowest PHE rate. It may be owing to the poor crystalline related to the crystal size and the


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weaker absorption for visible light than those of Rod 1 and Rod 2 (see Figure S11b). In the whole experiments, the FeTPPOHH2O ONCs has the best excellent PHE activity (0.75 mmol/h/g). The corresponding durability was investigated via the cycling PHE experiments. Figure 5b reveals that the PHE activity is maintained well over five cycles for 25 h. As shown in Figure S12-17, the SEM images and XRD patterns of FeTPPOHH2O nanoflakes, FeTPPCl snowflakes and [FeTPP]2O nanorods before and after PHE don’t have obvious change in morphology and structure, that indicates these ONCs possess the excellent stability. For the three kinds typical FeTPPX ONCs, Figure S11d and Table S2 reveal that there are only slight differences in the absorption for visible light and fluorescence lifetime, indicating the similar ability of light absorption as well as the separation and transfer of photoinduced electrons and holes. Therefore, the exposure of hydrophilic active face has a vital effect on the PHE activity. Indeed, the SEM images and XRD patterns in Figure 2, 3 and 4 reveal that the PHE activity of the as-obtained samples is completely consistent with the exposure and hydrophily of (002) plane in FeTPPX ONCs. As illustrated in Figure S10, FeTPPOHH2O nanoflakes have the completely exposed active (002) plane with the best hydrophiliy and thus exhibit the highest PHE rate, FeTPPCl snowflake-like structure is more active than its octahedron, while the PHE rate of [FeTPP]2O nanorods are the lowest as a result of little exposure of (002) plane. To some extent, the enough exposure of (002) plane should be the decisive factor for improving PHE performance of FeTPPX ONCs, further confirming that the (002) planes parallel to porphyrin ring should be the photocatalytic active face.19 In other words, the better hydrophilic axial ligand of FeTPPX molecules may induce the production of ONCs with more (002) planes exposed in CTAB-assisted chemical reactions, and in turn results in a better PHE activity of ONCs. Here, the effect of the axial ligand of FeTPPCl and FeTPPOHH2O on the PHE may be negligible in



photocatalysis due to the occurrence of photo-reduction, and the subsequent production of real photocatalyst Fe(II)TPP, as shown in the reaction (FeTPPX + ℎ𝑣 → Fe(II)TPP + X۰),44 The photo-reduction of [FeTPP]2O is difficult because of the demand of more photoelectrons, as seen in the reaction ([FeTPP]2O + 2ℎ𝑣 → 2FeTPP + O۰). The relevant photocatalysis mechanism for PHE of the FeTPPX is illustrated in Scheme 1. Therefore, the FeTPPCl snowflake-like ONCs have a similar photocatalytic activity as FeTPPOHH2O nanoflakes, but more better than [FeTPP]2O nanorods.

Scheme 1. Proposed photocatalysis mechanism for PHE of the axial coordination iron porphyrins. 3. Conclusions In summary, the enough exposure of hydrophilic active faces of organic nanocatalysts for the PHE from water splitting has been achieved via adjusting the hydrophilic axial ligand of iron porphyrin during the synthesis of ONCs in a facile CTAB-assisted chemical reaction. In this paper, the uniform and regular 0D FeTPPCl, ultrafine 1D [FeTPP]2O and ultrathin 2D FeTPPOHH2O ONCs were prepared by adjusting pH, monomer supersaturation and the concentration of CTAB, respectively. The SEM, TEM, SAED and XRD results confirmed the


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importance of CTAB, monomer supersaturation, surface energy of single crystal and hydrophily of axial ligand in the morphology evolution of FeTPPX ONCs, especially in the exposure of (002) plane parallel to porphyrin ring. The UV-vis spectra, fluorescent lifetime and PHE performance of different iron porphyrin ONCs revealed that the photocatalytic center is situated on the surface of hydrophilic (002) plane. The nanoflakes of FeTPPOHH2O formed in the alkaline solution possess the best PHE activity and excellent durability. This work not only proves the important effect of hydrophilic axial ligand of iron porphyrin on the production of nanostructure, but also provides a new strategy to apply organic metal complex in nanocatalysis. Experimental section Materials. 5,10,15,20-Tetraphenyl-21H,23H-porphine iron(III) chloride (FeTPPCl, C44H28N4FeCl), silver perchlorate (AgClO4, anhydrous, 97%), dichloromethane (CH2Cl2, 99.9%), and acetonitrile (CH3CN, 99.9%) were purchased from J&K Chemicals Co. Cetyltrimethyl ammonium bromide (CTAB, AR), hydrochloric acid (HCl, AR), and sodium hydroxide (NaOH, AR) were purchased from Sinopharm Chemical Reagent Co. Ultrapure water was from a Water Purifier apparatus (WP-UP-IV-20) with a resistivity of 18.2 MΩcm-1. All reagents were used without further purification. Synthesis of precursor. The precursor of iron porphyrin was synthesized according to reaction (1)35. FeTPPCl + AgClO4→FeTPPClO4 + AgCl

(1)

In this experiment, 1.2 mL of 0.5 M silver perchlorate (0.6 mmol) solution in acetonitrile was added to 30 mL of 0.018 M FeTPPCl (0.54 mmol) solution in CH2Cl2 under vigorous stirring. A brownish red solution with a white precipitate formed immediately. After stirring for 4 hours, the



white precipitate of silver chloride (AgCl) was separated out of the reaction mixture by centrifuging at 10 000 rpm. Then the filtrate was poured into 100 mL of n-hexane and cooled for 24 hours at -22 oC. The black powder of FeTPPClO4 was obtained through centrifuging and was washed with n-hexane and dried under vacuum for further experiments. Preparation of iron porphyrin ONCs. The three iron porphyrin crystals were synthesized via the reaction (2) – (4). FeTPPClO4 FeTPPClO4 FeTPPClO4

HCl, H2O

H2O

FeTPPCl

[FeTPP]2O

NaOH, H2O

(2) (3)

FeTPPOH ∙ H2O (4)

In a typical experiment, 1.0 mL of 1.0 mM FeTPPClO4 in anhydrous acetonitrile was quickly injected into 1 mL of 50 mM CTAB aqueous solution. The pH was adjusted at 1.2, 6.5 and 13.7 by hydrochloric acid (HCl) or sodium hydroxide (NaOH) before injecting. After one minute of stirring, the reaction solution was kept in a glass tube at room temperature for 3h. The products were separated by centrifugation and then washed several times with ultrapure water to remove residual surfactant. It is necessary to point out the conversion of FeTPPOHH2O to [FeTPP]2O may occur via the reaction (5)25,26.

FeTPPOH ∙ H2O

H2O

[FeTPP]2O

(5)

Characterization. UV-Vis absorption spectra were recorded using a UV2600 (Shimadzu) Spectrophotometer. Fourier transform-infrared data were collected with VERTEX70. The crystal structure was characterized by powder X-ray Diffraction (XRD, MiniFlex 600, Cu Kα1). MALDI-TOF-MS Spectrometry (Bruker, Autoflex III) were used to confirm the products


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compositions. Fluorescence lifetime tests were performed on a fluorescence spectrometer (FLS980, Edinburgh Instruments). The morphologies and structures of as-obtained samples were investigated using a field emission scanning electron microscope (FESEM, JEOL, JSM-6700F), transmission electron microscopy (TEM, Hitachi HF2000). The selected-area electron diffraction (SAED) patterns were obtained under high-resolution TEM (HRTEM, Hitachi HF2000). The height profiles of iron porphyrin nanoflakes were obtained with atomic force microscope (AFM, Bruker Dimension ICON). Photocatalytic hydrogen evolution measurements. The photocatalytic hydrogen evolution (PHE) reactions were carried out in an outer Pyrex photo-reactor connected to a closed glass gas circulation system (Labsolar VIAG, Beijing Perfectlight Technology Co. Ltd.). 1 mg sample was dispersed in 100 mL aqueous solution that contains 10.0 mL TEOA. Acontinuous magnetic stirrer was applied at the bottom of the reactor in order to keep the photocatalytic particles suspended in water during the whole experiment. The reaction solution was thoroughly degassed. After that, the reaction solution was illuminated using a 300 W Xe-lamp (PLS-SXE300, Beijing Perfectlight Technology Co. Ltd.). The PHE was detected by gas chromatograph (GC7900, Techcomp Instrument Ltd.) equipped with a thermal conductivity detector (TCD), where Ar was used as the carrier gas. The amount of H2 was counted by standard curve of H2. Simulation methods. The first-principles simulations were performed by using Vienna ab initio simulation package (VASP) code.41 The exchange-correlation energy in density functional theory (DFT) is described by generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE) functional. The plane-wave cutoff energy was set to 480eV. Monkhorst-Pack k points sampling with 2×2×1 k-meshes was adopted in the Brillouin zone. The vacuum region in the slabs of different surfaces is larger than 10 Å. 21 Environment ACS Paragon Plus


ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional SEM images, AFM images, UV-vis absorption spectra and fluorescent lifetime decay of three kinds of iron porphyrin nanomaterials. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: g.chai@ fjirsm.ac.cn *E-mail: [email protected] ORCID Longtian Kang: 0000-0002-3710-6285 Guoliang Chai: 0000-0003-3792-2204 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21473204), the Natural Science Foundation of Fujian Province (No. 2015J01070), and the Science and Technology Planning Project of Fujian Province (Grant No. 2014H2008). REFERENCES


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For Table of Contents Use Only

The Effect of Axial Coordination of Iron Porphyrin on Their Nanostructures and Photocatalytic Performance Xuemin Tian,†,‡ Chensheng Lin,§ Zhou Zhong,†,‡ Xiaoxin Li,†,‡ Xiao Xu,† Jingjing Liu,† Longtian Kang,*,† Guoliang Chai,*,§ and Jiannian Yao*,‖

The active (002) crystal face of iron porphyrin (FeTPPX) nanocrystals parallel to conjugated ring can be enough exposed for photocatalytic hydrogen evolution via changing the axial ligand (X: Cl, O and OH) in CTAB-assisted wet chemical reaction at different pH value.


The Effect of Axial Coordination of Iron Porphyrin on Their

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