Yang Xiaowei, Zhang Yuan, Li Feng, Guo Tiantian, Wu Yong, Jin Fengyan, Fang Min, Lan Yaqian, Li Yafei, Zhou Yong, Zou Zhigang
Department of Chemistry, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China.
Dalton Trans. 2017 Jun 27;46(25):8204-8218. doi: 10.1039/c7dt01560g.
Three water-stable isostructural metal-organic frameworks (MOFs) of the general formula [M(TCS)(BPY)] (M = Co(1), Ni(2) and Cu(3); HTCS = tetrakis(4-carboxyphenyl) silane, BPY = 4,4'-bipyridine) were synthesized and fully characterized. MOFs 1-3 are stable in pH = 5-11, 2-11, 3-11 aqueous solution respectively for at least 24 h at room temperature. Although HTCS absorbs only UV light, MOFs 1-3 absorb both UV and visible light in broad ranges (250-800 nm) and absorb more visible light than the ligand BPY. The rapid anodic photocurrent responses of MOFs 1-3 under UV and visible light illumination were observed. The photocurrent densities increase in the order of MOF 3 < 1 < 2 under visible light illumination (430 nm). The band gaps of MOFs 1-3 determined based on UV-Vis diffuse reflectance spectra and electrochemical (EC) analysis are 1.28, 1.35 and 0.67 eV, respectively. MOF 1 is able to photocatalyze the reduction of CO to CH under visible light, producing CH (1.44 μmol g in 8 h), which is unprecedented in MOFs. The catalytic activity of MOF 1 (0.75 μmol g after 4 h) under the irradiation of a 300 W xenon lamp is significantly better than those of MOFs 2 and 3 (0.14 μmol g after 4 h). The band structures, density of states and band gaps of MOFs 1-3 were calculated by the GGA-PBE and GGA-PBE+U method implemented in VASP code. The calculations show that all the three compounds can be viewed as bulk intermediate band (IB) materials. The density of states of the IB in MOF 1 is high, which could suppress the non-radiative recombination. The density of states of the IB levels in MOFs 2 and 3 are low, making these levels very effective recombination centres, thus jeopardizing the photocatalytic activities of MOFs 2 and 3. The calculated results are in good agreement with experimental results and explain the photocatalytic activity differences. This study is the first to successfully address the question of how the types of unpaired electron containing electron-rich metal ions (i.e. Cu(ii), Co(ii), Ni(ii)) affect the band gaps and band structures of MOFs and thus their photoelectronic properties.
合成并全面表征了三种通式为[M(TCS)(BPY)]（M = Co(1)、Ni(2)和Cu(3)；HTCS = 四(4 - 羧基苯基)硅烷，BPY = 4,4'-联吡啶）的水稳定同构金属有机框架材料（MOF）。MOF 1 - 3分别在pH = 5 - 11、2 - 11、3 - 11的水溶液中于室温下至少稳定24小时。尽管HTCS仅吸收紫外光，但MOF 1 - 3在较宽范围（250 - 800 nm）内同时吸收紫外光和可见光，且比配体BPY吸收更多可见光。观察到MOF 1 - 3在紫外光和可见光照射下有快速的阳极光电流响应。在可见光（430 nm）照射下，光电流密度按MOF 3 < 1 < 2的顺序增加。基于紫外 - 可见漫反射光谱和电化学（EC）分析确定的MOF 1 - 3的带隙分别为1.28、1.35和0.67 eV。MOF 1能够在可见光下光催化将CO还原为CH，产生CH（8小时内为1.44 μmol g），这在MOF中是前所未有的。在300 W氙灯照射下，MOF 1的催化活性（4小时后为0.75 μmol g）明显优于MOF 2和3（4小时后为0.14 μmol g）。采用VASP代码中实现的GGA - PBE和GGA - PBE + U方法计算了MOF 1 - 3的能带结构、态密度和带隙。计算结果表明，这三种化合物均可视为体中间带（IB）材料。MOF 1中IB的态密度较高，可抑制非辐射复合。MOF 2和MOF 3中IB能级的态密度较低，使这些能级成为非常有效的复合中心，从而损害了MOF 2和MOF 3的光催化活性。计算结果与实验结果吻合良好，并解释了光催化活性差异。本研究首次成功解决了含未成对电子的富电子金属离子类型（即Cu(ii)、Co(ii)、Ni(ii)）如何影响MOF的带隙和能带结构进而影响其光电性质这一问题。
