Khảo sát cấu trúc hình học, điện tử và độ bền cơ học của MOF 901 dưới sự ảnh hưởng của phân tử hấp phụ methanol

ĐẠI HỌC QUỐC GIA TP HỒ CHÍ MINH TRƯỜNG ĐẠI HỌC BÁCH KHOA THÀNH PHỐ HỒ CHÍ MINH ĐỒNG CAO HIẾU KHẢO SÁT CẤU TRÚC HÌNH HỌC, ĐIỆN TỬ VÀ ĐỘ BỀN CƠ HỌC CỦA MOF-901 DƯỚI SỰ ẢNH HƯỞNG CỦA PHÂN TỬ HẤP PHỤ METHANOL Chuyên ngành: Vật Lý Kỹ Thuật Mã số: 60 52 04 01 LUẬN VĂN THẠC SĨ TP HỒ CHÍ MINH, Tháng năm 2020 ĐẠI HỌC QUỐC GIA TP HỒ CHÍ MINH TRƯỜNG ĐẠI HỌC BÁCH KHOA THÀNH PHỐ HỒ CHÍ MINH ĐỒNG CAO HIẾU KHẢO SÁT CẤU TRÚC HÌNH HỌC, ĐIỆN TỬ VÀ ĐỘ BỀN CƠ HỌC CỦA MOF-901 DƯỚI SỰ ẢNH HƯỞNG CỦA PHÂN TỬ HẤP PHỤ METHANOL Chuyên ngành: Vật Lý Kỹ Thuật Mã số: 60 52 04 01 LUẬN VĂN THẠC SĨ TP HỒ CHÍ MINH, tháng 04 năm 2020 Cơng trình hồn thành tại: Trường Đại học Bách Khoa – ĐHQG HCM Cán hướng dẫn khoa học 1: TS Lê Minh Hưng Cán hướng dẫn khoa học 2: TS Trần Thị Thu Hạnh Cán chấm nhận xét 1: TS Nguyễn Thị Thúy Hằng Cán chấm nhận xét 2: TS Phan Hồng Khiêm Luận văn thạc sĩ bảo vệ Trường Đại học Bách Khoa, ĐHQG Tp HCM, ngày 29 tháng năm 2020 Thành phần Hội đồng đánh giá luận văn thạc sĩ gồm: PGS.TS Huỳnh Quang Linh……………………………………….Chủ tịch TS Lý Anh Tú…………………………………………………….Thư ký TS Nguyễn Thị Thúy Hằng……………………………………….Phản biện TS Phan Hồng Khiêm…………………………………………….Phản biện TS Lê Thị Cẩm Tú……………………………………………… Ủy viên Xác nhận Chủ tịch Hội đồng đánh giá LV Trưởng Khoa quản lý chuyên ngành sau luận văn sửa chữa (nếu có) CHỦ TỊCH HỘI ĐỒNG TRƯỞNG KHOA KH ỨNG DỤNG ĐẠI HỌC QUỐC GIA TPHCM CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM TRƯỜNG ĐẠI HỌC BÁCH KHOA Độc lập – Tự – Hạnh phúc NHIỆM VỤ LUẬN VĂN THẠC SĨ Họ tên học viên: ĐỒNG CAO HIẾU MSHV: 1770187 Ngày, tháng, năm sinh: 01/01/1994 .Nơi sinh: QUẢNG NGÃI Chuyên ngành: VẬT LÝ KỸ THUẬT Mã số: 60 52 04 01 I TÊN ĐỀ TÀI KHẢO SÁT CẤU TRÚC HÌNH HỌC, ĐIỆN TỬ VÀ ĐỘ BỀN CƠ HỌC CỦA MOF-901 DƯỚI SỰ ẢNH HƯỞNG CỦA PHÂN TỬ HẤP PHỤ METHANOL II NHIỆM VỤ VÀ NỘI DUNG - Tối ưu hóa cấu trúc MOF-901 MOF-901 hấp phụ 12 phân tử methanol phương pháp phiếm hàm mật độ Từ phân tích thay đổi cấu trúc hình học ba cấu trúc trước sau tối ưu hóa - Tính tốn phân tích cấu trúc điện tử độ bền học MOF-901 trước sau hấp phụ methanol Tính tốn lượng hấp phụ methanol MOF-901 III NGÀY GIAO NHIỆM VỤ: 19/08/2019 IV NGÀY HOÀN THÀNH NHIỆM VỤ: 01/02/2020 V CÁN BỘ HƯỚNG DẪN: - TS LÊ MINH HƯNG - TS TRẦN THỊ THU HẠNH Tp HCM, ngày tháng năm 2020 CÁN BỘ HƯỚNG DẪN CHỦ NHIỆM BỘ MÔN ĐÀO TẠO TRƯỞNG KHOA KHOA HỌC ỨNG DỤNG Đồng Cao Hiếu Luận văn thạc sĩ Lời cảm ơn Tôi xin gửi lời cảm ơn chân thành sâu sắc đến TS Lê Minh Hưng TS Trần Thị Thu Hạnh tận tâm hướng dẫn, truyền thụ cho kiến thức bổ ích, kinh nghiệm vơ q báu tạo điều kiện thuận lợi cho thực hoàn thành luận văn Cảm ơn ThS Đặng Thị Xuân Diễm, bạn Hoàng Trung Hiếu Nguyễn Thị Thu Huế (INOMAR) có góp ý vào luận văn Tôi biết ơn GS Nguyễn Mạnh Đức (Culham Science Centre, Oxfordshire, UK), TS Nguyễn Lạc Hà (Berkeley Global Science Institute) cho lời khuyên bổ ích trình nghiên cứu Cảm ơn Trung tâm Nghiên cứu Vật liệu Cấu trúc Nano Phân tử (INOMAR) hỗ trợ phần kinh phí để thực luận văn Tôi xin cảm ơn quý Thầy cô Khoa Khoa Học Ứng dụng, Trường Đại học Bách Khoa Thành phố Hồ Chí Minh tạo điều kiện để tơi hồn thành luận văn Và cuối cùng, xin cảm ơn gia đình tạo điều kiện hỗ trợ mặt tinh thần giúp hoàn thành luận văn Xin chân thành cảm ơn! Tp Hồ Chí Minh, ngày 01 tháng 02 năm 2020 Học viên Đồng Cao Hiếu i Đồng Cao Hiếu Luận văn thạc sĩ Tóm tắt Vật liệu khung hữu kim loại (Metal–organic frameworks-MOFs) cấu tạo từ tâm kim loại cầu nối hữu (với vai trị làm điểm kết nối) Trong đó, MOFs với tâm kim loại Ti thường có độ bền cao, có khả hấp phụ khí hoạt tính xúc tác cho phản ứng tốt Từ động lực đó, chúng tơi tiến hành nghiên cứu cấu trúc hình học, điện tử độ bền học vật liệu MOF-901 phương pháp DFT Kết tính tốn thu giá trị vùng cấm MOF-901 2.07 eV Bên cạnh đó, cấu trúc có khả hấp phụ methanol (MeOH) với nồng độ khác Độ rộng vùng cấm MOF-901 hấp phụ 12 MeOH 1.84 1.63 eV Kết tính tốn số đàn hồi cho thấy độ bền học MOF-901 cải thiện thơng qua q trình hấp phụ MeOH Hơn nữa, MOF-901 thể đặc trưng vật liệu 2D Abstract (Investigating the geometrical, electronic structure and mechanical stability of MOF-901 under the influence of methanol guest-molecules encapsulation) Metal-organic frameworks (MOFs) are composed of metal sites and organic linkers (as connection points) In particular, MOFs materials based on Ti metal sites are regularly stable, capable of adsorbing gas and have good catalytic activity for some reactions From those motivations, we conducted a study about the geometric, electronic structure and the mechanical stability of MOF-901 material using DensityFunctional Theory (DFT) The calculation results achieve a bandgap value equal to 2.07 eV for MOF-901 Besides, this structure can adsorb methanol with different concentrations The bandgap value of MOF-901 with and 12 methanol molecules is 1.84 and 1.63 eV, respectively The calculations of elastic constants illustrate that the mechanical stability of MOF-901 can be improved by methanol adsorption Moreover, MOF-901 also indicates the characteristics of a 2D material ii Đồng Cao Hiếu Luận văn thạc sĩ Lời cam đoan Học viên cam đoan kết báo cáo luận văn thạc sĩ có tên “Khảo sát cấu trúc hình học, điện tử độ bền học MOF-901 ảnh hưởng phân tử hấp phụ methanol” cơng trình nghiên cứu cá nhân học viên, hướng dẫn TS Lê Minh Hưng TS Trần Thị Thu Hạnh Các liệu, hình ảnh, so sánh thực luận văn hoàn toàn khách quan trung thực Tp Hồ Chí Minh, ngày 01 tháng 02 năm 2020 Học viên Đồng Cao Hiếu iii Đồng Cao Hiếu Luận văn thạc sĩ Mục lục Lời cảm ơn i Tóm tắt ii Lời cam đoan iii Mục lục iv Danh mục kí tự viết tắt vi Danh mục hình ảnh vii Danh mục bảng viii Chương 1: TỔNG QUAN 1.1 Vật liệu khung hữu kim loại 1.1.1 Cấu trúc hình học 1.1.2 Tính chất 1.1.3 Ứng dụng 1.2 Mục đích nghiên cứu 1.3 Đối tượng nghiên cứu Chương 2: PHƯỚNG PHÁP TÍNH TỐN .9 2.1 Lý thuyết tính tốn 2.1.1 Phương trình Schrưdinger hệ nhiều hạt 2.1.2 Phép gần Born-Oppenheimer 11 2.1.3 Phương pháp Hartree-Fock 13 2.1.4 Phương pháp phiếm hàm mật độ 14 iv Đồng Cao Hiếu Luận văn thạc sĩ 2.1.5 Phương trình Kohn-Sham 15 2.1.6 Hàm trao đổi – tương quan 16 2.1.7 Hàm sóng phẳng vùng Brillouin 18 2.1.8 Bộ giả 19 2.1.9 Hằng số đàn hồi 21 2.2 Chương 3: Chi tiết tính tốn 24 KẾT QUẢ VÀ THẢO LUẬN 27 3.1 Cấu trúc hình học 27 3.2 Cấu trúc điện tử 32 3.3 Độ bền học 36 3.4 Năng lượng hấp phụ 38 Chương 4: Kết luận 40 Danh mục cơng trình khoa học .41 Tài liệu tham khảo 42 v Đồng Cao Hiếu Luận văn thạc sĩ Danh mục kí tự viết tắt Kí hiệu MOFs MeOH FCU PCU LIL SNC HKUST ZIL MIL PCN UIO HXL USPP NCPP DFT GGA LSDA LDA PAW OPW VASP PBC SCF PDOS Tiếng Anh Metal–organic frameworks Methanol molecule Face Centered Cubic Lattice Unit Primitive Cubic Lattice Unit Lvt-Derived Lattice Unit S-nitrosocysteine Hong Kong University of Science and Technology Zeolitic Imidazolate Framework Material Insitude Lavoisier Porous Coordination Network University of Oslo Hexagonal Lattice Ultrasoft (Vanderbilt) Pseudopotentials Nonlocal Normconserving Pseudopotential Density Functional Theory Generalized Gradient Approximation The Local Spin Density Approximation Local Density Approximation Projector Augmented Wave Orthogonalised Plane Wave Vienna Ab-initio Simulation Package Periodic Boundary Conditions Self-Consistent-Field Partial Density of States vi Tiếng Việt Vật liệu khung kim Phân tử Methanol Đơn vị mạng lập phương tâm mặt Đơn vị mạng sở lập phương Đơn vị mạng xuất phát từ lvt Hợp chất C3H6N2O3S Đại học Khoa học Công nghệ Hồng Kông Khung Zeolitic imidazolate Viện vật liệu Lavoisier Mạng lưới phối trí lỗ xốp Đại học Oslo Mạng lục giác Giả Ultrasoft (Vanderbilt) Giả tiêu chuẩn lưu trữ bất định xứ Lý thuyết phiếm hàm mật độ Gần gradient suy rộng Gần mật độ quay địa phương Gần mật độ định xứ Phương pháp sóng chiếu mở rộng Sóng phẳng trực giao Gói phần mềm VASP Điều kiện biên tuần hoàn Trường tự hợp Mật độ trạng thái phần Đồng Cao Hiếu Luận văn thạc sĩ [39] J C Slater, “Note on Hartree's Method,” American Physical Society, vol 35, no 2, pp 210-211, 1930 [40] L Brillouin, “Self-Consistents de Hartree et de Fock,” Actualités Scientifiques et Industrielles, chapter 159, 1934 [41] D R Hartree and W Hartree, “Self-consistent Field, With Exchange, for Berylliu,” Proceedings of the Royal Society, vol 150, no 869, pp 9-33, 1935 [42] P Hohenberg and W Kohn, “Inhomogeneous Electron Gas,” Physical Review, vol 136, no 3B, pp B864-B871, 1964 [43] W Kohn and L J Sham, “Self-Consistent Equations Including Exchange and Correlation Effects,” Physical Review, vol 140, A1133-A1138, 1965 [44] A D Becke, “Perspective: Fifty Years of Density-Functional Theory in Chemical Physics,” The Journal of Chemical Physics, vol 140, no 4A, pp 18A301, 2014 [45] D M Cox; R Brickman; K Creegan and A Kaldor, “Gold clusters: reactions and deuterium uptake in Clusters of Atoms and Molecules,” Zeitschrift fuer Physik, vol 19, no 4, pp 353–355, 1991 [46] J P Perdew; K Burke and M Ernzerhof, “Generalized Gradient Approximation Made Simple,” Physical Review Letters, vol 77, no 18, pp 3865-3868, 1996 [47] J P Perdew; K Burke and M Ernzerhof, “Generalized Gradient Approximation Made Simple,” Physical Review Letters, vol 78, no 18, pp 1396-1396, 1997 [48] G Kresse and J Hafner, “Ab Initio Molecular Dynamics for Liquid Metals,” Physical Review B, vol 47, no 1, pp 558-561, 1993 [49] G Kresse and J Hafner, “Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium,” Physical Review B, vol 49, no 20, pp 14251-14269, 1994 47 Đồng Cao Hiếu Luận văn thạc sĩ [50] G Kresse and J Furthmüller, “Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set,” Computational Materials Science, vol 6, pp 15-50, 1996 [51] G Kresse and J Furthmüller, “Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set,” Physical Review B, vol 54, no 1, pp 11169-11186, 1996 [52] J H Monkhorst and D J Pack, “Special points for Brillouin-zone integrations,” Physical Review B, vol 13, no 12, pp 5188-5192, 1976 [53] P E Blöchl, “Projector Augmented-Wave Method,” Physical Review B, vol 50, no 24, pp 17953-17979, 1994 [54] G Kresse; D Joubert, “From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method,” Physical Review B, vol 59, no 3, pp 17581775, 1999 [55] F Terumi; K Izuru; S Hiroshi and H Mitsuya, “Design of Improved MetalOrganic Framework (MOF) H2 Adsorbents,” Polymers, vol 3, no 4, pp 2133-2141, 2011 [56] K Ohmin; K Y Jin; P Sungbin; L H Jae; H Junsu; P Hyunsoo; M R Hoi and K Jihan, “Computer-aided discovery of connected metal-organic frameworks,” Nature Communications, vol 10, no 1, 3620-3628, 2019 [57] K Momma and F Izumi, “VESTA for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data,” Journal of Applied Crystallography, vol 44, no 6, pp 1272-1276, 2011 [58] Y L Page and P Saxe, “Symmetry-general least-squares extraction of elastic data for strained materials from ab initio calculations of stress,” Physical Review B, vol 65, no 14, pp 104104-14, 2002 [59] R F Zhang; X D Wen; D Legut; Z H Fu; S Veprek; E Zurek and H K Mao, “Crystal Field Splitting is Limiting the Stability and Strength of Ultraincompressible Orthorhombic Transition Metal Tetraborides,” Scientific Reports, vol 6, id 23088, 2016 48 Đồng Cao Hiếu Luận văn thạc sĩ [60] F Mouhat and F-X Coudert, “Necessary and Sufficient Elastic Stability Conditions in Various Crystal Systems,” Physic Review B, vol 90, no 4, pp 224104-4, 2014 [61] R Hill, “The Elastic Behaviour of a Crystalline Aggregate.” Proceedings of the Physical Society, vol 65, no 5, pp 349-354, 1952 [62] M K Y Chan and G Ceder, “Efficient Band Gap Prediction for Solids,” Physical Review Letters, vol 105, no 19, pp 196403-4, 2010 [63] C S Wang and J P Perdew, “Density-Functional Theory of Excitation Spectra of Semiconductors: Application to Si,” Physical Review Letters, vol 51, no 7, pp 597-600, 1983 [64] Z G Gu1; L Heinke; C Wöll1; T Neumann; W Wenzel; Q Li; Ka Fink; O D Gordan and D R T Zahn, “Experimental and theoretical investigations of the điện tửic band structure of metal-organic frameworks of HKUST-1 type,” Applied Physics Letters, vol 107, no 18, pp 183301, 2015 [65] A S Maria; C P Ireland; F M Ebrahim; S Berend; and A O Syzgantseva, “Metal Substitution as the Method of Modifying Electronic Structure of Metal–Organic Frameworks,” Journal of the American Chemical Society, vol 141, no 15, pp 6271–6278, 2019 [66] R Dong; P Han; H Arora; M Ballabio; M Karakus; Z Zhang; C Shekhar; P Adler; P Petkov; A Erbe; S Mannsfeld; C Felser; T Heine; M Bonn and X Feng, “High-mobility band-like charge transport in a semiconducting twodimensional metal-organic framework,” Nature Materials, vol 17, no 11, pp 1027-1032, 2018 49 www.nature.com/scientificreports OPEN Received: May 2018 Accepted: 23 October 2018 Published: xx xx xxxx Monitoring Mechanical, Electronic, and Catalytic Trends in a Titanium Metal Organic Framework Under the Influence of Guest-Molecule Encapsulation Using Density Functional Theory Hieu C. Dong1, Ha L. Nguyen1, Hung M. Le1, Nam Thoai2, Yoshiyuki  Kawazoe3 & Duc NguyenManh4 In this study, we conduct a density functional theory investigation to study the mechanical stability of a titanium-based metal organic framework (MOF-901), which was hypothetically assumed to possess 2D characteristics It is systematically found that the encapsulation of methanol enhances the mechanical stability of MOF-901 as the elastic tensors Cij of MOF-901∙nMeOH are higher than the corresponding Cij quantities reported for solvent-free MOF-901 Moreover, the 2D characteristics of MOF-901 is confirmed by verifying the negative values of C33 At the same time, the band gap of MOF-901 is observed to be solvent-dependent In its pure form, MOF-901 possesses a direct gap (Eg) of 2.07 eV, with the valence and conduction bands mainly constituted by electrons of 4-aminobenzoate linkers Introducing methanol into MOF-901 causes distortion to the 4-aminobenzoate geometry, thereby induces electronic degeneracy to the conduction bands Consequently, Eg is narrowed to 1.84 eV with 5.7 wt% MeOH or 1.63 eV with 11.4 wt% MeOH Hence, it is possible to tailor the band gap of MOF901 by controlling methanol guest, which only acquires van der Waals interaction to the framework In addition, our theoretical prediction shows a Ti(IV) site can undergo electronic hopping to become Ti(III) under the effect of visible light (~440–443 nm) Then, Ti(III) is capable of breaking the C-Br bond in ethyl α-bromophenylacetate spontaneously, which in turn activates the polymerization of methyl methacrylate with an energy barrier of 0.30 eV Metal–Organic Frameworks, a hybrid material comprised of inorganic metal–oxo clusters and organic linking units, have been intensively studied and developed for past few decades, as proven by nearly 700,000 reported MOFs structures1 Possessing well-defined characteristics including high porosity, post-synthetic modification of crystal structure, structural engineering, and surface decoration, MOFs have emerged as promising candidates for key applications of global issues such as renewable energy2, gas storage and separation3, catalysis4–6, and drug release7 Among these applications, utilizing MOFs as highly tunable photocatalytic materials for catalytic transformation such as CO2 reduction, H2 production, water splitting, polymerization, and organic reactions is a key technological component of energy economics8 Hence, the quest for stable MOFs with promising optical properties and redox activities has led to the motivation of the MOF research community in the past decade For those purposes, MOFs based on tetravalent metal–oxo building units (i.e Ti(IV), Zr(IV), and Hf(IV)) have been especially realized as promising candidates due to its high stability under harsh-working conditions9 Center for Innovative Materials and Architectures (INOMAR), Ho Chi Minh City, 721337, Vietnam 2Faculty of Computer Science & Engineering, University of Technology, Vietnam National University, Ho Chi Minh City, Vietnam New Industry Creation Hatchery Center, Tohoku University, Sendai, 980-8579, Japan 4Culham Centre for Fusion Energy, United Kingdom Atomic Energy Authority, Culham Science Centre, Abingdon, OX14 3DB, United Kingdom Correspondence and requests for materials should be addressed to H.M.L (email: hung.m.le@hotmail.com) ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 www.nature.com/scientificreports/ Moreover, MOFs based on so-mentioned secondary building units (SBU) possess suitable band gaps for photocatalytic transformation under UV-vis irradiation which can be tailored due to the nature of MOFs structures In order to engineer band gaps of MOFs materials, post-synthetic modification on linkers which introduces functionalities into the framework of MOFs via direct synthesis or covalent bonding, was reported by experiments as well as modeling approaches In particular, Hendon et al10 synthesized the MIL-125-X analogues (with X = NH2, OH, and halogen) which exhibited a wide-tuning range of band gap from 1.1 eV to 3.6 eV The modification of electronic structures and band gaps of MOFs were further demonstrated by Pham et al11 by employing state-of-the-art density functional theory12–14 (DFT) calculations to investigate the effect of functional groups to the isoreticular structures of MOF-5 Recently, Nguyen and co-workers introduced a high conjugation system of imine linking unit into MOF-90115, a 2D-resembled Ti-MOF constructed through imine linker and hexameric Ti–oxo cluster, to further extend the framework and reduce the band gap of the resulting material, termed MOF902, which subsequently enhanced the photoresponsive properties compared to other competitors16 By taking the advantage of molecular modeling method, band structure and electronic properties of MOFs can be fully explored and studied in details, which provides a new insight to engineer and optimize the optical response of MOFs, an imperative requirement in photocatalytic applications In this article, we utilize molecular modeling through DFT calculations to investigate the mechanical stability of MOF-901 and present a novel approach to tailor the highest-occupied molecular orbital (HOMO)-lowest-unoccupied molecular orbital (LUMO) gap energy of this material by controlling guest molecules in the pores We suspect that the encapsulation of solvent with the MOF structure would enhance the capability to sustain external pressure In particular, we attempt to resolve such a hypothesis by investigating the contribution of guest molecules on the elastic tensors of the framework In another aspect, we seek for an energetic profile of mechanism activation under visible-light irradiation of a polymerization reaction promoted by MOF-901 in the presence of ethyl α-bromophenylacetate co-initiator, which has been mentioned in the previous experimental report15 Results Electronic structure and mechanical stability from condensed-phase calculations.  Electronic structure and mechanical stability of MOF-901.  In a previous report, Nguyen et al15 employed power X-ray diffraction analysis to elucidate the structural information of MOF-901 (shown in Fig. 1) Upon such analysis, the unit cell parameters a and c of the hexagonal cell were reported to be 27.40 Å and 8.94 Å, respectively In this report, we verify the structure of MOF-901 by carrying out two sets of calculations: PBE and PBE-D3 (including D3 correction terms for van der Waals interactions) After a full optimization using PBE calculations without the inclusion of empirical D3 correction, we obtain a to be 27.33 Å, which is in very good agreement with the experiment-reported parameter However, the c lattice parameter is compressed by 7.6% (given as 8.26 Å in our calculation) In the examination of such compressing behavior, we pay most attention to the Ti-O bonds Specifically speaking, there are two types of Ti-O bonds in the MOF-901 structure: Ti-O(methoxy) and Ti-O(carboxylate) The Ti-O(carboxylate) bond is established within the [Ti6O6(O-CH3)6(4-aminobenzoate)6] core cluster as first synthesized by Hong and Chun17, while the Ti-O(methoxy) bond links Ti to a methoxy group The available experimental data also revealed that the two types of Ti-O bonds fell in the range of 1.98–2.00 Å From our DFT optimization, the Ti-O(carboxylate) bond is extended to 2.12 Å, while the Ti-O(methoxy) bond is compressed in comparison with the experimental values The compression of Ti-O(methoxy) bonds is associated with the compression of the c lattice parameter In addition, we also observe that both Ti-O(methoxy) and Ti-O(carboxylate) bonds are tilted (Fig. 1), while in experiment, those two bonds are shown almost parallel to the z axis Introducing the D3 correction for longrange van der Waals interactions, in principle, would enhance long-range binding energetics With D3 correction accounted, we find the a and c lattice parameters to be 27.27 Å and 8.55 Å, respectively It should be noted that the c axis seems to suffer less from lattice narrowing of the PBE-D3 optimization In this case, the Ti-O(methoxy) and Ti-O(carboxylate) bonds are 2.06 Å and 1.90 Å, respectively, slightly smaller than the previous PBE-optimized values As a result, we can verify that the compressed c lattice is caused by Ti-O tilting behavior In addition, it should be noted that in our calculations, the long-range van der Waals interactions cannot be perfectly described by the empirical D3 correction For validation of pseudopotentials, we perform another optimization with a consideration of semicore electron inclusion At convergence, the c lattice parameter is more compressed in comparison with the previous PBE results For convenience, we list the resultant a, c lattice parameters and unit-cell volume in each calculation in Table 1 From electronic structure analysis of PBE calculations, MOF-901 is found to possess a direct HOMO-LUMO gap of 2.07 eV at the Γ point as shown in the band structure in Fig. 2a Compared with the optical band gap (2.65 eV) measured by UV-vis diffuse reflectance spectroscopy15, our calculated Eg herein is underestimated by 22% To our knowledge, the resultant percent error is acceptable, considering the fact that PBE calculations always underestimate Eg as a result of poor electron-hole interaction description in the exchange-correlation functional18 In a typical case of band-gap underestimation, the problematic circumstance arises severely when the 3d electrons of metal directly constitute either the HOMO or LUMO levels Here, our situation is quite different In this particular case of MOF-901, both HOMO and LUMO are, however, localized on the organic linkers Thus, the band gap prediction herein does not suffer much from underestimation When analyzing the HOMO-LUMO gap of PBE-D3 calculations, a direct gap of 2.03 eV is observed, which is slightly lower than the quantity given by conventional PBE calculations Subsequently, we examine the contribution of molecular orbital to the nature of HOMO-LUMO gap opening for MOF-901 by analyzing the partial density of states (PDOS) of four different groups: (1) Ti, (2) O(carboxylate), (3) methoxy, and (4) remaining 4-aminobenzoate (Fig. 2b) Even though Ti is considered as an important recipe to construct the building block for hexagonal cell formation, it is surprisingly revealed that the contribution of Ti to the overall HOMO-LUMO gap is insignificant The cationic charge of Ti is revealed as + 1.26 without D3 correction, or + 1.25 with D3 correction The covalent group, i.e 4-aminobenzoate, plays a very important role ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 www.nature.com/scientificreports/ Figure 1.  Crystal structure of MOF-901 (a) View from the c axis, (b) Staggered layers of MOF-901 viewed from the a axis showing the distance between two layers is ca 4.13 Å, (c) An illustration of the Ti-O tilting angle (shaded color is the experimental structure) Color code: C, brown; O, red; Ti, blue; H, pink Structure MOF-901 MOF-901 with 5.7 wt% MeOH MOF-901 with 11.4 wt% MeOH Calculation method a (Å) c (Å) Volume (Å3) Ti-O(methoxy) (Å) Ti-O(carboxylate) (Å) Experiment 27.40 8.94 5,813 2.00 1.95 PBE 27.33 8.26 5,343 1.79 2.12 PBE + D3 27.27 8.55 5,506 1.90 2.06 Experiment 27.30 8.38 5,409 1.99 1.98 PBE 27.44 8.05 5,249 1.79 2.12 PBE + D3 27.27 7.84 5,049 1.78 2.11 Experiment 27.36 8.04 5,212 1.99 1.98 PBE 27.40 8.13 5,286 1.79 2.10 PBE + D3 27.29 8.20 5,289 1.78 2.10 Table 1.  Lattice parameters and volumes of solvent-free MOF-901, MOF-901 with 5.7 wt% MeOH, and MOF901 with 11.4 wt% MeOH instead As we can see from the PDOS plot (Fig. 2b), the eigenstates of HOMO and LUMO are mainly constituted by the hybridized orbitals of 4-aminobenzoate, whereas the 2p orbitals of O(carboxylate) atoms dedicate a smaller contribution Besides the theoretical investigation of electronic structure, we also validate the mechanical stability of MOF901 by evaluating the Cij elastic tensors19 resulted from our DFT calculations To evaluate the unit-cell elastic response, we employ a central finite difference scheme with two ionic (or lattice) displacements (one positive and one negative) for computational feasibility For a large structure of 336 atoms within a hexagonal unit cell, and thanks to symmetry consideration, we only need to evaluate 168 degrees of freedom for Hessian evaluation in total The elastic tensors with consideration of rigid ions and with ionic response inclusion are reported in Table 2 It should be noted, however, that we only reported those elastic tensors given by conventional PBE calculations ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 www.nature.com/scientificreports/ Figure 2. (a) Band structure of hexagonal MOF-901, (b) PDOS of Ti, O(carboxylate), methoxy, and 4-aminobenzoate given by PBE calculations Elastic mode C11 C12 C13 C33 C44 C66 Symmetrized elastic modulus (without ionic contribution) 2504 526 405 921 526 988 Total elastic modulus (with ionic contribution) 487 150 33 −14 −1 168 Tablee 2.  Elastic tensor moduli (kBar) of pure MOF-901 arising from PBE calculations The PBE-D3 calculations are not chosen in this case because of empirical fitting in the D3 parameterization, which certainly alters the MOF structure from its actual ground state given by PBE calculations For a particular hexagonal system, the mechanical stability can be certified if all four following conditions hold20,21 C11 > C12 (1) 2C13 < C33(C11 + C12) (2) C44 > (3) C66 = (C11 − C12)/2 (4) In the original formalism to calculate elastic constants using the stress-strain relationship , only lattice distortions (with rigid ions) are considered, and the listed Cij values considering rigid ions in Table 2 provide complete satisfaction to the above inequalities In other words, MOF-901 can be confirmed to be stable by the elastic tensors derived from lattice distortions On the other hand, if the approximate harmonic ionic response is taken into account, C33 and C44 become negative after adding the largely negative terms from ionic response, leading to the unsatisfaction of inequalities (2) and (3) The very small value of C44 (−1 kBar) may be a result of calculation deviation from lattice deformation and ionic perturbation in a very large unit cell As we acknowledge the compression of the c lattice parameter in the earlier part of this discussion, obtaining a negative value of C33 with ionic response inclusion is not surprising Upon the compression in the z direction, the two types of Ti-O bonds listed above seem to suffer from bending, not from bond compression More interestingly, the negative value of C33 truly indicates the 2D-characteristics of MOF-901 It should be noted that the z-oriented connection between layers of MOF-901 is established by an alternative zigzag bonding scheme through 4-aminobenzoate, as illustrated in Fig. 1b Overall, upon consideration of elastic tensors given by unit-cell compression, we conclude that MOF-901 is mechanically stable with 2D characteristics 22 Methanol encapsulation with MOF-901: change in electronic structure and effect on mechanical stability.  As a solvent in the chemical synthesis, methanol is naturally encapsulated within the pores of MOF-901 Here, we validate different concentrations of methanol adsorbed inside the network In general, the adsorption energy of multiple methanol molecules inside MOF-901 can be calculated using the following equation: Eadsorption = En ×methanol @ MOF −901 − EMOF −901 − nEmethanol ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 (5) www.nature.com/scientificreports/ Calculation method PBE PBE + D3 5.7 wt% MeOH −0.23 −0.42 11.4 wt% MeOH −0.40 −0.52 13.3 wt% MeOH −0.33 −0.61 Table 3.  Adsorption energy of various MeOH concentration in MOF-901 (eV/molecule) where EMOF-901, Emethanol, and En×methanol@MOF-901 are the total energies of MOF-901, an isolated methanol molecule, and MOF-901 with n methanol molecules, respectively A negative value of Eadsorption indicates attraction between methanol and MOF-901, while a positive value supposingly indicates repulsion There are two levels of methanol adsorption concentration inside MOF-901 as reported in a previous experiment15 In the first case, methanol molecules are encapsulated within a MOF-901 primitive cell (5.7 wt%) In the second case, the adsorption concentration is doubled (12 methanol molecules, 11.4 wt%) The structural configurations obtained from power X-ray diffraction reveals that the hexagonal unit cell is nicely retained in both adsorption cases Those configurations are used in our optimizations Subsequently, we perform optimizations for those two structures using the same calculation method In both cases, methanol tends to establish van der Waals interaction with the six-membered 4-aminobenzoate moiety Interestingly, this long-range interacting behavior is correlated with an impact on electronic structure that we will discuss later on To analyze the favorability of settlement, the adsorption energy is subsequently calculated For PBE calculations without D3 corrections, methanol molecules are shown to expose attraction to the framework (−0.23 eV/molecule) PBE-D3 calculations reveal that methanol molecules find nice dwelling in MOF-901 pores with an improved adsorption energy of −0.42 eV/molecule, as shown in Table 3 The c lattice parameter is shortened, while a almost remains unchanged Looking at the two-dimensional characteristics in Fig. 1b, it is reasonable that MeOH establishes long-range interaction with 4-aminobenzoate linkers along the z direction, and acts as a bridge to connect two parallel panels (layers) Hence, the c lattice parameter is shortened to 7.84 Å The magnitude of binding energy given by PBE-D3 calculations unsurprisingly indicates weak van der Waals interactions At this stage, we are curious to learn how this concentration of solvent would affect the electronic property of MOF-901 According to band structure and PDOS examination, Eg is now narrowed to 1.84 eV as predicted by PBE calculations as shown in Fig. 3a Recall that in pure MOF-901, both HOMO and LUMO are mainly constituted by the hybridized orbitals of 4-aminobenzoate In the MOF-methanol interacting model, methanol establishes long-range interaction with the phenyl ring and distorts 4-aminobenzoate away from the original equilibrium position, thereby causes an incidental alternation on the electronic structure Encountering structural distortion from the equilibrium position, the LUMO band is broadened around the region of 0.3–0.5 eV Interestingly, there is also a split in the LUMO band as shown in the PDOS plot (Fig. 3b), indicating that there is an energetic degeneracy in the virtual eigenstates as a result of structural deformation of 4-aminobenzoate Moreover, an in-gap state appears near the Fermi level, which is constituted by almost-equal contributions of guest methanol and 4-aminobenzoate At this point, we can point out two factors causing Eg narrowing First, guest molecules themselves contribute an intermediate eigenstate in the valence region Second, the spatial occupancy of MeOH induces distortion to 4-aminobenzoate, and eventually broadens LUMO It is also interesting to notice that the Fermi level is shifted to a higher energy level (1.5 eV) under the presence of methanol The most important goal in this study is to verify elevation of mechanical stability under the presence of methanol inside the MOF-901 network This can be achieved by conducting elastic moduli calculations The motivation for elasticity verification of MOF-solvent systems relies on the fact that a MOF product obtained from an experiment usually needs to undergo a stability test after product activation Therefore, we suspect that the calculated elastic tensors Cij would indicate MOF-solvent to be more mechanically-stable than the pure MOF itself (activated sample) Upon analyzing elastic tensors resulted from lattice distortion, we observe that the Cij values for MOF-901 with 5.7 wt% methanol increase from 3% to 17% (see Tables 2 and 4) When ionic relaxation is included, we again observe that C33 is negative (−155 kBar), while C13 is largely reduced This is an implication of 2D characteristics More importantly, the elastic tensor moduli show that MOF-901 with 5.7 wt% methanol is more mechanically stable than pure MOF-901 Under the presence of 12 methanol molecules, the c lattice parameter is even less shortened than in the case of 6-methanol adsorption as listed in Table 1 In this case, while PBE indicates attraction with an adsorption energy of −0.40 eV/molecule, PBE-D3 gives an adsorption energy of −0.52 eV/molecule Examining the electronic structure of MOF-901 with 11.4 wt% methanol in Fig. 3b, Eg is observed to reduce to 1.63 eV From the PDOS plot, we observe contribution of both methanol and 4-aminobenzoate to the formation of HOMO, while the lowest-unoccupied state is constructed by 4-aminobenzoate like the previous case It should be noticed, however, the contribution of methanol to the in-gap state herein is much more dominant than that in the 6-methanol case In terms of mechanical stability, we observe that the resultant elastic moduli for MOF-901 with 11.4 wt% methanol are further stabilized in comparison with the previous 5.7 wt% methanol case More specifically, Cij values given by lattice distortion increase by 4–26% In general, Cij given by lattice distortion are larger than those values obtained for pure MOF-901 and MOF-901 with 5.7 wt% methanol As ionic response is taken into account, it should be noticed that C33 is -157 kBar, which again implies the 2D characteristics the framework Equation (4) is satisfied, which implies that the MOF structure containing methanol still possesses hexagonal symmetry In the third case, we hypothetically push up the concentration of solvent to 13.5 wt% (14 methanol molecules) In this hypothesized structure, we assume that methanol can further find settlement in the small pore ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 www.nature.com/scientificreports/ Figure 3. (a) Band structure of MOF-901 containing guest methanol molecules (5.7 wt%) and PDOS of Ti, O(carboxylate), methoxy, methanol, and 4-aminobenzoate (remaining), (b) band structure of MOF-901 containing 12 guest methanol molecules (11.4 wt%) and PDOS of Ti, O(carboxylate), methoxy, methanol, and 4-aminobenzoate (remaining) given by PBE calculations Symmetrized elastic modulus (without ionic contribution) C11 C12 C13 C33 C44 C66 MOF-901 with 5.7 wt% MeOH 2604 542 469 1031 617 1031 MOF-901 with 11.4 wt% MeOH 2608 543 497 1152 665 1032 MOF-901 with 13.3 wt% MeOH 2971 600 484 1244 650 1158 MOF-901 with 5.7 wt% MeOH 526 139 −119 −155 −70 193 535 272 −47 −157 −25 131 313 15 −186 −203 −46 135 Total elastic modulus (with MOF-901 with 11.4 wt% MeOH ionic contribution) MOF-901 with 13.3 wt% MeOH Table 4.  Elastic tensor moduli (kBar) of MOF-901 encapsulating guest methanol molecules arising from PBE calculations of MOF-901, which might not be observed in experiment spectrometers Recall that in the previous adsorption cases, it is demonstrated in experiment that methanol only adsorbs in the large pore In this case, 14 methanol molecules are placed in both large and small pores randomly Due to complexity of methanol arrangement, we carry out a molecular dynamic simulation using the PBE functional with a fixed step size of 0.5 fs and fixed unit cell parameters at 300 K This dynamic process enables us to search for local minimum configurations We select three configurations of methanol arrangements (blue arrows in the time-energy diagram in Fig. 4) at three different time frames to get variant arrangement of the methanol molecules The total time period for molecular dynamic simulation is 0.25 ps Then, the chosen structures are optimized with variant unit cell, and the adsorption energy is calculated as -0.33 eV/molecule without D3 correction or -0.61 eV/molecule with D3 correction In this case, both PBE and PBE-D3 calculations show that 14-MeOH-molecule encapsulation is even more attractive to the network of MOF-901, which means that MeOH also prefers to occupy small pores The next goal is to validate mechanical stability of MOF-901 with 14 methanol molecules encapsulation ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 www.nature.com/scientificreports/ Figure 4. (a) MOF-901 with a load of 14 methanol molecules and (a) time-energy diagram Three blue arrows in the energy diagram indicate three chosen configurations where we believe the local minima reside Color code: C, black; O, red; N, green; Ti, blue polyhedra H atoms are omitted for clarity The calculated elastic tensors are reported in Table 4 The Cij values given by unit-cell differentiating increases at least 14% in comparison with the value of pure MOF-901 More specifically, C33 (1244 kbar) increases tremendously by 35% in comparison with that of pure MOF-901 (921 kbar) Of course, these values survive the tests from inequalities (1)–(3) As we consider ionic relaxation contribution into total elastic tensors, C33 and C13 are more negative than those reported for the previous pure MOF-901 case At this point, our hypothesis has been resolved The elastic tensor data clearly indicates that solvent (methanol) actually fills the vacuum porosity in MOF-901, thus enhance the mechanical stability of the structure Elastic tensor data in Table 4 also implies the 2D characteristics of MOF-901 In terms of electronic structure, the appearance of guest molecule (MeOH) in MOF-901 pores causes geometry distortion to phenyl rings, thereby narrows the HOMO-LUMO gap Dehalogenation of ethyl α-bromophenylacetate at the electronically excited Ti site of [Ti 6O 6(O-CH 3) (4-aminobenzoate)6].  Experimentally15, the photocatalytic activity of MOF-901 was demonstrated in the polymerization of methyl methacrylate with ethyl α-bromophenylacetate as a co-initiator as shown in Fig. 5 With the motivation from the experiment, we theoretically demonstrate in this section that Br can be removed from ethyl α-bromophenylacetate (dehalogenation) under the influence of active Ti(III) sites According to the electronic DOS (Fig. 2), the 3d electrons of Ti contribute partially to build up the HOMO-2 and LUMO+1 level, which corresponds to a hopping energy of 2.93 eV (423 nm) The oxidation state hopping energy was investigated in various theoretical and experimental works For example, magnetic exchange-coupling constant was investigated for di- and trinuclear transition metal complexes by employing various ab initio levels of theory For dicobalt complexes, the spin-orbit coupling effect had to be taken into account to obtain qualitative results23 Investigating the mechanism of SBU in a chromium-based MOF, Cantu et al24 suggested the highest barrier of ~35 kcal/mol for the formation of a dimetal linker and high-low spin hopping The flip of Ti oxidation state from IV to III has been shown to occur under the influence of visible light in the literature In a previous experiment, Dan-Hardi and co-workers25 synthesized a MOF structure involving titanium-oxo-hydroxo and dicarboxylate linkers (namely MIL-125) Interestingly, an optically-activated electronic hopping was observed by the evidence of electron paramagnetic resonance spectroscopy, and the oxidation state of Ti was reported to switch from IV to III26 Under the photo-activation of UV-A light, the Ti(IV) sites in COK-69 was reported to successfully convert to Ti(III)27 MIL-101, on the other hand, was constructed solely by Ti(III), and possessed a capability to capture O2 at the Ti site (then, the Ti oxidation state became IV after bonding with O2)28 For our case, it is of importance to verify the possibility of switching oxidation state of Ti to further study the mechanism of polymerization reaction under visible light irradiation catalyzed by MOF-901 An isolated model of Ti6O6(OCH3)6(4-aminobenzoate)6 is considered, which consists of 138 atoms and possesses C3v symmetry ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 www.nature.com/scientificreports/ Figure 5.  Mechanism for UV-vis irradiation of a polymerization reaction promoted by MOF-901 in the presence of ethyl α-bromophenylacetate as a co-initiator In the singlet ground state of Ti6O6(OCH3)6(4-aminobenzoate)6, when Ti switches the oxidation state from IV to III, the methoxy (•OCH3) radical group should be released as a consequence Moreover, there is another lone-pair electron at the Ti site Therefore, the overall multiplicity should be triplet Indeed, achieving self-consistency in total energy calculation for this triplet state is hardly done; therefore, for such a highly complex molecular system, we perform two doublet calculations instead: one for •OCH3 and another for the remaining cluster The sum of total energies of two doublet configurations is 4.19 eV higher than the singlet energy of Ti6O6(O-CH3)6(4-aminobenzoate)6 according to PBE/6–31 G calculations or 3.97 eV lower according to PBE/gen calculations These predicted energy differences correspond to UV-A wavelengths of 296 nm or 312 nm, respectively To get a more realistic picture of the singlet-triplet hopping energy, we consider a simplified cluster, in which the 4-aminobenzoate moiety is omitted and replaced by a dummy H atom, while all -OCH3 groups still remain in the complex (66 atoms in total) It is still very challenging to achieve self-consistency for triplet-state calculations, and we have to employ a quadratically-convergent method for solving Hartree-Fock SCF29 For this model, upon using PBE/6–31 G, the singlet-triplet hopping energy is 2.82 eV (440 nm, in the visible region), while PBE/gen suggests a hopping energy of 2.80 eV (443 nm) Those two hopping energies indicate good correspondence with the earlier plane-wave calculations and more importantly, possess good agreement with the actual condition for polymerization, where the reaction was catalyzed under visible light15 At this stage, we look forward to validating the removal of Br from ethyl α-bromophenylacetate at the Ti active site (without •OCH3) of the simplified model In the reaction, Br approaches Ti(III) from a far distance Ti(III) then captures Br to establish a Ti-Br ionic bond; then, the C-Br bond is broken as a consequence To achieve a transition state for this proposed mechanism, we first optimize an initial state, where ethyl α-bromophenylacetate is set to interact with the core cluster from a far distance (to be specific, more than 6 Å), and a final state, where Br is successfully captured by Ti and radical ethyl phenylacetate is left behind While the final state can be successfully optimized, it is interesting to observe from both PBE/6–31 G and PBE/gen calculations that an initial state cannot be found in both sets of calculations; in fact, the initial structure is dragged to converge toward the final state Hence, dehalogenation under the activation of Ti(III) is a barrierless and thermodynamically spontaneous process because Ti(III) is very active Thereafter, radical ethyl phenylacetate is produced We then carry out the key step of polymerization using the gas-phase calculation by allowing the radical C site in ethyl phenylacetate to approach the double bond in methyl methacrylate, as shown in Fig. 6b In this optimization, the C(radical)-C(double bond) distance is narrowed by a step size of 0.1 Å, while other coordinates are relaxed As the transition state is located, the radical C atom from ethyl phenylacetate approaches the sp2 C atom in methyl methacrylate with a distance of 2.18 Å, and the potential barrier is 0.30 eV The Gibbs free energy of this polymerization is 0.37 eV, thus the reaction rate constant at room temperature can be estimated as 3.43·106 s−1 using the Eyring equation30 We believe such a reaction barrier is low enough to be activated in ambient condition Nevertheless, such reaction free energy and rate constant are estimated in the gas phase Within the context of this report, we not carry out a demonstration of polymerization in the condensed phase within the pores of MOF-901; still, we believe that the reaction rate would be slower Conclusion In this study, we carry out a theoretical investigation of MOF-901 to clarify the difference in mechanical stability of solvent-free and solvent-interacting systems Structural optimizations predict the a lattice parameter to be in good accordance with the experiment, while the c lattice is compressed due to the tilting behavior of −OCH3 The adsorption of methanol is proved to stabilize the framework of MOF-901 The analysis of elastic tensors Cij for solvent-free MOF-901 and MOF-901 with solvent indicates that both systems are mechanically stable and possess 2D characteristics Moreover, it also unveils the role of solvent in stabilizing the porous material Upon introducing methanol into MOF-901, all elastic moduli Cij derived from a stress-strain relationship increase This is an indication that the framework has attained better structural stability We then examine the band structure of ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 www.nature.com/scientificreports/ Figure 6. (a) Singlet-state structure of [Ti6O6(OMe)6(AB)6] cluster (AB = 4-aminobenzoate) and the simplified model in which AB groups are replaced by formate functionalities (HCO2) to save computational effort Color code: C, black; O, red; N, green; Ti, blue polyhedra H atoms are omitted for clarity (b) Energy profile of the polymerization of methyl methacrylate under the activation of radical phenylacetate showing a barrier of 0.30 eV MOF-901 given by PBE calculations, and observe a direct HOMO-LUMO gap of 2.07 eV at the Γ point From the PDOS analysis, both HOMO and LUMO are mainly constituted by the hybdrized orbitals of 4-aminobenzoate groups, while the contributions from Ti, O(carboxylate), and methoxy are insignificant The 5.7 wt% encapsulation of methanol within MOF-901 pores (6 methanol molecules) narrows the energy gap to 1.84 eV (decreased by ~12%) By further increasing methanol concentration to 11.4 wt% (12 methanol molecules), the HOMO-LUMO gap suffers more narrowing (1.63 eV) as a result of distorting the equilibrium geometry of 4-aminobenzoate linkers It should be noted that those two methanol-incorporating structures are obtained from a previous experimental data Guest methanol itself contributes an in-gap state, which is also responsible for Eg narrowing This finding opens up a new direction to MOF energy gap tailoring by control solvent concentration Finally, the role of Ti(III) in dehalogenation of ethyl α-bromophenylacetate is verified Our localized atomic-orbital-basis calculations suggest that a Ti(IV) site can undergo electronic hopping to become Ti(III) under the effect of visible light (~440–443 nm) Subsequently, Ti(III) is capable of grabbing Br from a far distance and break the C-Br bond in a spontaneous process without a reaction barrier Finally, the polymerization of methyl methacrylate can be activated by radical ethyl phenylacetate with a low reaction barrier of 0.30 eV Methods Condensed-phase calculations.  Plane-wave DFT calculations are performed for the investigation of MOF-901 With a hexagonal structure, MOF-901 consists of 336 atoms within one primary unit cell (12 Ti, 12 N, 144 C, 120 H, and 48 O atoms) as shown in Fig. 1 The total charge of the system of interest vanishes because all atoms are chemically guaranteed Electronic structure calculations for periodic MOF-901 are executed using the well-established Vienna Ab Initio Simulation Package (VASP)31–34 To be specific, DFT calculations are performed by employing the Perdew-Burke-Ernzerhof35,36 (PBE) parameterization for exchange-correlation description within generalized-gradient approximations (GGA) The electronic wave-function is constituted using the well-developed projector-augmented wave (PAW) method implemented within VASP37,38 The cut-off energy for plane-wave expansions is tested at various values from 300 eV to 425 eV, and we observe that total energy is lowest at 400 eV Therefore, we choose the cut-off energy of 400 eV, which is most suitable and affords computational feasibility for such a large periodic system The convergence threshold in total energy self-consistency is 10−5 eV Each optimization process has two steps because of very large and porous structure In the first step, the unit cell is fixed to retain structural symmetry and ionic position is optimized until total energy difference ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 www.nature.com/scientificreports/ between self-consistent-field (SCF) loops falls within an allowable limit of 10−3 eV Subsequently, the unit cell and ionic positions are relaxed simultaneously In the second step, a convergence criterion of 10−3 eVÅ−1 is applied to gradient convergence For illustration of structures, crystallographic pictures are made with the Visualization for Electronic and Structural Analysis software39 We also study methanol encapsulation inside MOF-901 in a later section To survey the geometric arrangement of multiple methanol molecules, we first introduce 14 CH3OH molecules into MOF-901 randomly; then, a molecular dynamic trajectory is employed to shuffle CH3OH The constant temperature of 300 K is acquired by utilizing the Andersen thermostat technique40,41, and the total time of this simulation is 0.25 ps (500 molecular dynamic steps) Localized atomic-orbital-basis calculations.  The localized atomic-orbital-basis calculations are exe- cuted using unrestricted PBE calculations implemented in the Gaussian 16 suite of program42 Two independent calculation sets are performed for validation purposes In the first set, we employ the simple split-valence Pople basis set of 6-31 G43,44 In the second calculation set, an effective core potential45 with the LANL2dz basis set46–48 (for convenience, we denote the combination of basis sets and effective core potential as gen) is employed The long-range van der Waals interaction is accounted in those optimizations by activating the empirical Grimme D3 correction terms49 References Moghadam, P Z.; et al Development of a Cambridge Structural Database Subset: A Collection of Metal–Organic Frameworks for Past, Present, and Future Chem Mater 29, 2618-2625 (2017) Nguyen, B T., Nguyen, H L., Nguyen, T C., Cordova, K E & Furukawa, H High Methanol Uptake Capacity in Two New Series of Metal–Organic Frameworks: Promising Materials for Adsorption-Driven Heat Pump Applications Chem Mater 28, 6243–6249 (2016) Trickett, C A et al The Chemistry of Metal–Organic Frameworks for CO2 Capture Regeneration and Conversion 2, 17045 (2017) Furukawa, H., Cordova, K E., O’Keeffe, M & Yaghi, O M The Chemistry and Applications of Metal-Organic Frameworks Science 341, 1230444 (2013) Zhang, L et al Adv Sci 3, 1500243 (2016) Tran, T V et al A Five Coordination Cu(ii) Cluster-Based MOF and Its Application in the Synthesis of Pharmaceuticals via sp3 C-H/N-H Oxidative Coupling Catal Sci Technol 7, 3453–3458 (2017) Kitagawa, S & Kitaura, R Noro, S.-i Functional Porous Coordination Polymers Angew Chem Int Ed 43, 2334–2375 (2004) Nasalevich, M A et al Co@NH2-MIL-125(Ti): Cobaloxime-Derived Metal-Organic Framework-Based Composite for LightDriven H2 Production Energ Environ Sci 8, 364–375 (2015) Nguyen, H L The Chemistry of Titanium-Based Metal-Organic Frameworks New J Chem 41, 14030–14043 (2017) 10 Hendon, C H et al Engineering the Optical Response of the Titanium-MIL-125 Metal–Organic Framework through Ligand Functionalization J Am Chem Soc 135, 10942–10945 (2013) 11 Pham, H Q et al Engineering of Band Gap in Metal–Organic Frameworks by Functionalizing Organic Linker: A Systematic Density Functional Theory Investigation J Phys Chem C 118, 4567–4577 (2014) 12 Hohenberg, P & Kohn, W Inhomogeneous Electron Gas Phys Rev 136, B864–B871 (1964) 13 Kohn, W & Sham, L J Self-Consistent Equations Including Exchange and Correlation Effects Phys Rev 140, A1133–A1138 (1965) 14 Becke, A D Perspective: Fifty Years of Density-Functional Theory in Chemical Physics J Chem Phys 140, 18A301 (2014) 15 Nguyen, H L et al A Titanium–Organic Framework as an Exemplar of Combining the Chemistry of Metal– and Covalent–Organic Frameworks J Am Chem Soc 138, 4330–4333 (2016) 16 Nguyen, H L et al A Titanium–Organic Framework: Engineering of the Band-Gap Energy for Photocatalytic Property Enhancement ACS Catal 7, 338–342 (2017) 17 Hong, K & Chun, H Nonporous Titanium–Oxo Molecular Clusters That Reversibly and Selectively Adsorb Carbon Dioxide Inorg Chem 52, 9705–9707 (2013) 18 Chan, M K Y & Ceder, G Efficient Band Gap Prediction for Solids Phys Rev Lett 105, 196403 (2010) 19 Zhang, R F et al Crystal Field Splitting is Limiting the Stability and Strength of Ultra-incompressible Orthorhombic Transition Metal Tetraborides Sci Rep 6, 23088 (2016) 20 Mouhat, F & Coudert, F.-X Necessary and Sufficient Elastic Stability Conditions in Various Crystal Systems Phys Rev B 90, 224104 (2014) 21 Hill, R The Elastic Behaviour of a Crystalline Aggregate Proceed Phys Soc A 65, 349 (1952) 22 Le Page, Y & Saxe, P Symmetry-general least-squares extraction of elastic data for strained materials from ab initio calculations of stress Phys Rev B 65, 104104 (2002) 23 Vogiatzis, K D., Klopper, P W., Mavrandonakis, D A & Karin Fink, D Magnetic Properties of Paddlewheels and Trinuclear Clusters with Exposed Metal Sites ChemPhysChem 12(17), 3307–19 (2011) 24 Cantu, D C., McGrail, B P & Glezakou, V.-A Formation Mechanism of the Secondary Building Unit in a Chromium Terephthalate Metal–Organic Framework Chem Mater 26(22), 6401–6409 (2014) 25 Dan-Hardi, M et al A New Photoactive Crystalline Highly Porous Titanium(IV) Dicarboxylate J Am Chem Soc 131, 10857–10859 (2009) 26 Bueken, B et al A Flexible Photoactive Titanium Metal–Organic Framework Based on a [TiIV3(μ3-O)(O)2(COO)6] Cluster Angew Chem Int Ed 54, 13912–13917 (2015) 27 Mason, J A., Darago, L E., Lukens, W W & Long, J R Synthesis and O2 Reactivity of a Titanium(III) Metal–Organic Framework Inorg Chem 54, 10096–10104 (2015) 28 Frisch, M J., et al Gaussian 16, Wallingford, CT, (2016) 29 Bacskay, G B A Quadratically Convergent Hartree-Fock (QC-SCF) Method Application to Closed Shell Systems Chem Phys 61, 385–404 (1981) 30 Eyring, H The Activated Complex in Chemical Reactions The Journal of Chemical Physics 3, 107 (1935) 31 Kresse, G & Hafner, J Ab Initio Molecular Dynamics for Liquid Metals Phys Rev B 47, 558–561 (1993) 32 Kresse, G & Hafner, J Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium Phys Rev B 49, 14251–14269 (1994) 33 Kresse, G & Furthmüller, J Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set Comput Mater Sci 6, 15–50 (1996) 34 Kresse, G & Furthmüller, J Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set Phys Rev B 54, 11169–11186 (1996) 35 Perdew, J P., Burke, K & Ernzerhof, M Generalized Gradient Approximation Made Simple Phys Rev Lett 77, 3865–3868 (1996) ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 10 www.nature.com/scientificreports/ 36 Perdew, J P.; Burke, K.; Ernzerhof, M Generalized Gradient Approximation Made Simple [Phys Rev Lett 77, 3865 (1996)] Phys Rev Lett 78, 1396–1396 (1997) 37 Blöchl, P E Projector Augmented-Wave Method Phys Rev B 50, 17953–17979 (1994) 38 Kresse, G & Joubert, D From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method Phys Rev B 59, 1758–1775 (1999) 39 Momma, K & Izumi, F VESTA for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data J Appl Cryst 44, 1272–1276 (2011) 40 Andersen, H C Molecular Dynamics Simulations at Constant Pressure and/or Temperature J Chem Phys 72, 2384–2393 (1980) 41 Tanaka, H., Nakanishi, K & Watanabe, N Constant Temperature Molecular Dynamics Calculation on Lennard-Jones Fluid and Its Application to Water J Chem Phys 78, 2626–2634 (1983) 42 Ditchfield, R., Hehre, W J., Pople, J A & Self-Consistent Molecular-Orbital Methods., I X An Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules J Chem Phys 54, 724–728 (1971) 43 Hehre, W J., Ditchfield, R & Pople, J A Self-Consistent Molecular Orbital Methods XII Further Extensions of Gaussian-Type Basis Sets for Use in Molecular Orbital Studies of Organic Molecules J Chem Phys 56, 2257–2261 (1972) 44 Francl, M M et al Self-Consistent Molecular Orbital Methods XXIII A Polarization-Type Basis Set for Second-Row Elements J Chem Phys 77, 3654–3665 (1982) 45 Xu, X & Truhlar, D G Accuracy of Effective Core Potentials and Basis Sets for Density Functional Calculations, Including Relativistic Effects, As Illustrated by Calculations on Arsenic Compounds J Chem Theory Comput 7, 2766–2779 (2011) 46 Hay, P J & Wadt, W R Ab Initio Effective Core Potentials for Molecular Calculations Potentials for the Transition Metal Atoms Sc to Hg J Chem Phys 82, 270–283 (1985) 47 Hay, P J & Wadt, W R Ab Initio Effective Core Potentials for Molecular Calculations Potentials for K to Au Including the Outermost Core Orbitals J Chem Phys 82, 299–310 (1985) 48 Wadt, W R & Hay, P J Ab Initio Effective Core Potentials for Molecular Calculations Potentials for Main Group Elements Na to Bi J Chem Phys 82, 284–298 (1985) 49 Grimme, S., Antony, J., Ehrlich, S & Krieg, H A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu J Chem Phys 132, 154104 (2010) Acknowledgements We are grateful for computational support from the High-Performance Computing Laboratory, Faculty of Computer Science & Engineering, University of Technology, Vietnam National University and the Institute for Material Research, Tohoku University D.N.-M acknowledges the support from the High-Performance Computer Center at the CINECA Headquarters in Bologna (Italy) for its provision of supercomputer resources on the Marconi machine Finally, we acknowledge a financial support from Vietnam National University under grant TX-2018-50-01 Author Contributions H.C.D performed condensed-matter calculations and made Figs and H.L.N retrieved the experimental data, wrote part of the paper, and made Figs and 4–6 H.M.L performed localized atomic-orbital-basis calculations and wrote most part of the paper T.N and Y.K contributed technical support for all calculations D.N.-M discussed technical details and contributed to finalize the manuscript Additional Information Competing Interests: The authors declare no competing interests Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ © The Author(s) 2018 ScientiFic REPOrTs | (2018) 8:16651 | DOI:10.1038/s41598-018-35117-9 11 LÝ LỊCH TRÍCH NGANG Họ tên: ĐỒNG CAO HIẾU Ngày, tháng, năm sinh: 01/01/1994 Nơi sinh: Quảng Ngãi Địa liên lạc: Thôn Đông Trúc Lâm, Xã Hành Nhân, Huyện Nghĩa Hành, Tỉnh Quảng Ngãi QUÁ TRÌNH ĐÀO TẠO (Bắt đầu từ Đại học đến nay) 8/2012 – 10/2016: Sinh viên trường đại học Tự nhiên Tp HCM 8/2017 – đến nay: Học viên cao học đại học Bách Khoa Tp HCM Q TRÌNH CƠNG TÁC (Bắt đầu từ làm đến nay) 8/2017 – đến nay: Trung tâm Nghiên cứu Vật liệu Cấu trúc Nano Phân tử (INOMAR, http://inomar.edu.vn/) ĐHQG-HCM ... KHẢO SÁT CẤU TRÚC HÌNH HỌC, ĐIỆN TỬ VÀ ĐỘ BỀN CƠ HỌC CỦA MOF- 901 DƯỚI SỰ ẢNH HƯỞNG CỦA PHÂN TỬ HẤP PHỤ METHANOL II NHIỆM VỤ VÀ NỘI DUNG - Tối ưu hóa cấu trúc MOF- 901 MOF- 901 hấp phụ 12 phân tử. ..ĐẠI HỌC QUỐC GIA TP HỒ CHÍ MINH TRƯỜNG ĐẠI HỌC BÁCH KHOA THÀNH PHỐ HỒ CHÍ MINH ĐỒNG CAO HIẾU KHẢO SÁT CẤU TRÚC HÌNH HỌC, ĐIỆN TỬ VÀ ĐỘ BỀN CƠ HỌC CỦA MOF- 901 DƯỚI SỰ ẢNH HƯỞNG CỦA PHÂN TỬ HẤP PHỤ...  EMOF -901: lượng tổng MOF- 901  Emethanol: lượng tổng phân tử methanol cô lập  En? ?methanol +MOF- 901: lượng tổng MOF- 901 với n phân tử methanol Nếu lượng hấp phụ âm, MOF- 901 có khả hấp phụ phân