Because of the correlation between the surface and the performance of the catalyst, the control of the crystal surface of the nano scale catalyst has become very important. However, MOFs materials have been widely used in various catalytic fields, but there are few reports on their surface mechanism and crystal surface control. Professor Sun Weiyin and Professor Zhao Jin, National Key Laboratory of coordination chemistry, Nanjing University, firstly calculated the surface energy of different crystal surfaces of nh2-mil-125 (TI) by using density functional theory (DFT), and then designed and synthesized nano-sized nh2-mil-125 (TI) with controllable crystal surface by simple method. It is worth noting that the {110} surface of the MOF material shows excellent photoelectric properties and photocatalytic hydrogen evolution activity (60.8? Mol g-1 h-1).
Relevant work was previously published in the form of Edge article in chemical science, the flagship journal of Royal Society of chemistry.
Paper information: facet dependent photographic hydrogen production of metal – organizational framework nh2-mil-125 (TI)
Fan Guo, Jing Zhao * (Zhao Jin, Nanjing University) and Wei Yin sun * (Sun Weiyin, Nanjing University)
Chem. Sci., 2019
http://dx.doi.org/10.1039/c8sc05060k
The surface structure and surface energy of nh2-mil-125 (TI) were studied by density functional theory (DFT) for the first time.
Based on the theoretical calculation, the nanoscale nh2-mil-125 (TI) MOF with controllable crystal surface was successfully designed and synthesized, and the materials with high surface energy {110} surface were successfully prepared.
The truncated quadrilateral plate MOF with high surface energy {110} has excellent photoelectric performance and photocatalytic hydrogen evolution activity, which proves that the crystal surface control is one of the most effective strategies to realize the modification and efficient photocatalytic performance of MOFs materials.
Background: due to the correlation between the surface and the performance of the catalyst, the crystal surface control of the nano catalyst has become very important. Since the high-quality anatase TiO2 single crystal exposed on {001} crystal surface has been reported, the reaction surface exposure, the activity of crystal surface controlled catalyst and the mechanism related to hard materials such as Pt and metal oxides have attracted extensive attention. These types of nanomaterials can usually sense the suspended bond and reconstruct the surface.
As a new kind of unique hybrid nano materials, MOFs can combine the advantages of MOFs and nano materials. However, compared with metal and metal oxide catalysts, the study of MOFs crystal mainly focuses on its structure and performance, while the study of crystal surface control and surface mechanism of MOFs materials is very little. Because of the diversity and complexity of the structure and the coexistence of various chemical bonds in MOFs, it is still challenging to control the crystal surface of MOFs.
Among Ti MOFs, nh2-mil-125 (TI) is one of the most thoroughly studied MOF systems, which shows a variety of semiconductor behaviors and has potential applications in the fields of light response, photocatalytic CO2 reduction and amine to imine oxidation. In these applications, the morphology of the prepared nh2-mil-125 (TI) is single, so it is a good idea to control its crystal surface. However, the growth of MOFs crystal surface is generally realized by two-dimensional surface nucleation or "growth and diffusion" mechanism. The metal clusters in MOFs act as the active sites of catalysts. When a large proportion of metal clusters exist on a surface, it indicates that the crystal surface has high activity and surface energy.
Based on this, in this paper, the surface energy of different crystal faces of nh2-mil-125 (TI) is calculated by density functional theory (DFT) for the first time, and the nano-sized nh2-mil-125 (TI) with controllable crystal faces is successfully designed and synthesized by a simple method.
Based on the density functional theory (DFT), the author studies the surface structure of nh2-mil-125 (TI) for the first time, and gives the simulation diagrams of {001}, {110}, {100} and {111} four main crystal planes and their cutting planes, as shown in Figure 1. The order of surface energy is as follows: {111} (0.51 jm-2) & lt; {001} (0.74 jm-2) & lt; {100} a (0.80 jm-2), {100} B (0.81 jm-2) & lt; {110} (1.18 jm-2). The reason for the high surface energy of {110} plane is that the number of metal clusters exposed to {110} plane is more than that of other crystal planes, which can also be confirmed from the diagram of cutting plane in Fig. 1. Therefore, the {110} surface of the material is theoretically more reactive than other surfaces.
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Figure 1. Surface structure and tangent plane of {001}, {110}, {100} and {111} of nh2-mil-125 (TI).
Based on the above theoretical prediction, a series of crystal controlled NH2-MIL-125 (Ti) materials were successfully designed and synthesized by adjusting the concentration of surfactant sixteen alkyl three methyl ammonium bromide (CTAB) in DMF-CH3OH solution, as shown in Figure 2. With the increase of CTAB concentration, the morphology of nanoscale nh2-mil-125 (TI) changes continuously, and finally becomes a complete octahedral structure.
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Fig. 2. SEM, enlargedsem and TEM of nh2-mil-125 (TI) were obtained at different CTAB concentrations, and the corresponding three-dimensional models were: (a) 0, (b) 1mm, (c) 2mm, (d) 3mm and (E) 4mm.
It can be seen from the PXRD diagram of the sample that a series of prepared nh2-mil-125 (TI) samples are highly consistent with mil-125 (TI) samples, and the use of h2bdc-nh2 as an organic linker will not affect the original skeleton structure of the sample. It is worth noting that the ratio of diffraction intensity of {110} and {220} planes in T110 is significantly higher than that of other samples, which indicates that {110} planes in T110 are exposed more. Similarly, t001-1 and t111 have obvious diffraction intensity ratio at {002} and {222} respectively, so the {001} crystal surface of t001-1 and {111} crystal surface of t111 are exposed more. Moreover, it is known from PXRD that a series of samples show the characteristics related to the exposed crystal surface, which is mutually confirmed with the crystal morphology observed by SEM.
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Figure 3. PXRD of nh2-mil-125 (TI) was obtained at different CTAB concentrations: (a) 0, (b) 1 mm, (c) 2 mm, (d) 3 mm and (E) 4 mm.
The FT-IR spectra of nh2-mil-125 (TI) samples show that the characteristic spectra of the samples are similar, and the presence of CTAB group is not detected. The presence of Br - in CTAB was not found by XPS, indicating that there was no surfactant in the sample.
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Figure 4. FT-IR (left) and XPS (right) of nh2-mil-125 (TI) were obtained at different CTAB concentrations: (a) 0, (b) 1 mm, (c) 2 mm, (d) 3 mm and (E) 4 mm.
In addition, table 1 lists the atomic concentrations of the samples detected by XPS. The concentration of Ti2P in T110 is significantly higher than that in other samples, indicating that the metal clusters on T110 crystal surface are higher than that in other samples, which is consistent with the results of EDS element distribution diagram.
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Table 1. Atomic concentration of samples detected by XPS
As shown in Fig. 5 (a), the UV-vis diffuse reflectance spectrum shows that the light response wavelengths of all samples are transferred from the UV region to the visible region. In Figure 5 (b), among the above samples, T110 has the fastest electron transfer speed and the highest efficiency. In addition, figure 5 (c) shows that T110 has the best light response ability and interface charge separation efficiency, which is confirmed by EIS.
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Fig. 5. (a) UV-Vis diffuse reflectance spectrum; (b) PL spectrum; (c) photocurrent response spectrum and (d) electrochemical impedance spectrum.
In order to evaluate the photocatalytic hydrogen evolution performance of nh2-mil-125 (TI) nano materials, the above samples were added to the water system containing CH3CN and teoa as sacrificial agents for hydrogen evolution test. T110 showed the highest hydrogen evolution rate (60.8? Mol g-1 h-1), about three times of t111. And even in the 30 h cycle test, T110 has good cycle stability. It shows that T110 has excellent cycling and stability.
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Fig. 6. (a) photo catalytic hydrogen evolution of NH2-MIL-125 (Ti) with different morphology; (b) cycle test of T110 material.
Table 2 summarizes the reported hydrogen evolution effects of nh2-mil-125 (TI) and other MOFs based photocatalysts. Obviously, compared with the reported nh2-mil-125 (TI) materials, and even with some noble metal mixed photocatalysts based on MOFs, nano-sized nh2-mil-125 (TI) MOFs with crystal surface control can significantly enhance its photocatalytic activity.
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Table 2. Summary of MOF based photocatalysts
In conclusion, the author concludes that in this study, by combining theoretical prediction with experimental verification, the concentration of CTAB can be changed to prepare nano-sized nh2-mil-125 (TI) materials with controllable crystal surface. The DFT calculation for the first time shows that the {110} crystal surface has the highest surface energy and shows high surface activity. The experimental results show that the {110} crystal surface has the highest photocatalytic hydrogen evolution performance and apparent quantum yield, which is consistent with the theoretical prediction. This work shows that the crystal surface control is one of the most effective strategies to realize the modification of MOF materials and efficient photocatalytic performance.
Professor Zhao Jin, National Key Laboratory of coordination chemistry, School of chemistry and chemical engineering, Nanjing University
In 2005, he received a Ph.D. from Yale University, and then went to the University of California, Berkeley and the University of Chicago for postdoctoral research. From 2007 to 2008, he worked as a senior researcher in Rohm and Haas company. 2008 – 2014 Professor, School of life sciences, Nanjing University. 2014 Professor, School of chemistry and chemical engineering, Nanjing University. Selected as the national excellent youth in 2016, the second batch of national "ten thousand talents plan" leading talents. The research field involves many aspects such as chemistry, life science, material science, etc. dozens of research papers have been published in international famous academic journals such as J. am. Chem. SOC., angel. Chem. Int. ed., NAT. Comm., SCI. Adv., etc., and won the first prize of 2019 University science research outstanding achievement award (Science and Technology). Presided over the cultivation project of NSFC major research program (91753121), the chemical recognition and functional regulation of tyrosine dynamic modification, and the bioinorganic chemistry project of NSFC Youqing (2090245).
Email: jingzhao@nju.edu.cn
Professor Sun Weiyin State Key Laboratory of coordination chemistry, School of chemistry and chemical engineering, Nanjing University
Sun Weiyin, Professor, doctoral supervisor, distinguished professor of Nanjing University. Winner of National Science Foundation for Distinguished Young Scholars. In 1986, he graduated from chemistry department of Shanghai East China Normal University and was admitted to Dalian Institute of Chemical Physics, Chinese Academy of Sciences. In 1987, he was sent by the state government to study in Japan. In 1990 and 1993, he obtained master's degree and doctor's degree in Osaka University, Japan. From 1993 to 1995, he worked as a postdoctoral researcher in the Central Research Institute of yanyeyi pharmaceutical company of Japan, returned to China at the beginning of 1996, and has worked in the school of chemistry and chemical engineering, the Institute of coordination chemistry and the State Key Laboratory of coordination chemistry of Nanjing University until now, mainly in coordination chemistry and supramolecular chemistry. To undertake key and general projects of NSFC. More than 350 SCI papers have been published. In 2014, he was selected as FRSC, and in 2014-2018, he was selected as Elsevier's most cited Chinese researchers.
Email: sunwy@nju.edu.cn
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