Transition-Metal-Complex Cationic Dyes Photosensitive to Two Types

of Sciences, Fuzhou, Fujian 350002, P. R. China. Inorg. Chem. , 2016, 55 (23), pp 12193–12203. DOI: 10.1021/acs.inorgchem.6b01770. Publication D...
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Transition-Metal-Complex Cationic Dyes Photosensitive to Two Types of 2D Layered Silver Bromides with Visible-Light-Driven Photocatalytic Properties Cheng-Yang Yue,*,†,‡ Xiao-Wu Lei,*,† Yong-Fang Han,† Xin-Xiu Lu,† Ya-Wei Tian,† Jing Xu,† Xiao-Fan Liu,† and Xin Xu† †

Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of Chemistry and Chemical Engineering, Jining University, Qufu, Shandong 273155, P. R. China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

ABSTRACT: With mixed transition-metal (TM) complex, alkali-metal cations, or halogen anions as structure-directing agents, two types of two-dimensional (2D) layered inorganic− organic hybrid silver bromides were prepared and structurally characterized as K[TM(2,2-bipy)3]2Ag6Br11 (TM = Ni (1), Co (2), Zn (3), Fe (4)) and [TM(2,2-bipy)3]2Ag13Br17 (TM = Ni (5), Co (6), Zn (7), Fe (8)). Compounds 1−4 feature 2D microporous anionic [Ag6Br11]5− layers composed of [Ag3Br7] secondary building units based on AgBr4 tetrahedral units, and compounds 5−8 contain 2D [Ag13Br16]3− layers built from the one-dimensional complex [Ag8Br12] and [Ag5Br8] chains. The photosensitization of TM complex dyes led to the narrow semiconducting behaviors with tunable band gaps of 1.73−2.71 eV for the title compounds, which result in excellent and stable photocatalytic degradation activities over organic pollutants under visible-light irradiation. The studies of photocatalytic mechanism based on radical-trapping experiments and electronic band structural calculation show that the TM complex cations play important roles in the photocatalytical activities and photochemical stabilities due to their excellent separating abilities for photogenerated carriers. This technique affords one new type of visible-light-driven photocatalyst and facilitates the integration of 2D layered materials and semiconducting photocatalytic properties into one hybrid d10 TM halogenide.

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INTRODUCTION The photocatalytic decontamination of wastewater containing organic pollutants over the semiconductors has been intensively investigated as an effective economical technique with various advantages of high efficiency, simplicity, good reproducibility, etc., because the photocatalysts can directly utilize sunlight as the energy source.1−5 Unfortunately, the most widely studied photocatalyst of TiO2 only absorbs the UV light, which restricts its practical application. To better utilize the visible light of solar energy, a large amount of strategies including metals or nomentals doping, noble metal deposition, dye sensitization, or heterojunction composite have been adopted to modify TiO2 to be a visible-light-driven photocatalyst.6,7 Excepting the decorated TiO2 materials, a great deal of new metal oxides, chalcogenides, halogenides, or organic semiconductors have also been explored with excellent visible-light-driven photocatalytic activities, such as BiVO4, Ag3PO4, BiOX, Ag/AgX (X = Cl, Br, I), ZnIn2S4, gC3N4, etc.8,9 Among these new visible-light-driven photocatalysts, the Ag/ AgX plasmonic photocatalysts have been widely studied with the outstanding photocatalytic activities and stabilities due to the © XXXX American Chemical Society

surface plasma resonance effect of Ag nanoparticle and Schottky barrier formed between the Ag and AgX components. It is reported that the strong absorption ability of Ag nanoparticle for the photogenerated electron plays an important role in the photocatalytic activities and photochemical stabilities of Ag/AgX photocatalyst.10 In other words, the introduction of electron absorption components into silver halogenides is able to effectively accelerate the separation process of the photogenerated carriers and decrease the photogenerated electron density of AgX component, which will prevent the photolysis reaction of AgX and increase its photochemical stability. However, in these strategies, a big challenge is to understand the structure−property relationship because of the mixed compositions without clear crystal structures. For this reason, it is a greatly attractive and challenging goal to build a new structural mode based on silver halogenide as stable visible-lightdriven photocatalyst to investigate the electronic band structure Received: July 22, 2016

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DOI: 10.1021/acs.inorgchem.6b01770 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinements for Compounds 1−4 compound chemical formula fw space group a/Å b/Å c/Å V (Å3) Z Dcalcd (g·cm−3) temp (K) μ (mm−1) F(000) reflections collected unique reflections reflections (I > 2σ(I)) GOF on F2 R1,wR2 (I > 2σ(I))a R1,wR2 (all data) Δρmax (e/Å3) Δρmin (e/Å3) a

1 C60N12H48Ni2KAg6Br11 2619.85 R3̅ (No. 147) 14.195(4) 14.195(4) 30.752(9) 5366(3) 3 2.432 293(2) 8.369 3702 20 977 2734 2051 1.020 0.0317/0.0564 0.0526/0.0618 0.765 −0.925

2 C60N12H48Co2KAg6Br11 2620.29 R3̅ (No. 147) 14.1784(14) 14.1784(14) 30.746(3) 5352.7(9) 3 2.439 293(2) 8.328 3696 20 590 2718 2153 1.029 0.0288/0.0583 0.0435/0.0627 1.052 −0.944

3 C60N12H48Zn2KAg6Br11 2633.17 R3̅ (No. 147) 14.2015(11) 14.2015(11) 30.753(2) 5371.5(7) 3 2.442 293(2) 8.505 3714 21 057 2760 2182 1.037 0.0330/0.0731 0.0488/0.0788 0.754 −1.194

4 C60N12H48Fe2KAg6Br11 2653.23 R3̅ (No. 147) 14.1037(10) 14.1037(10) 31.380(2) 5405.7(7) 3 2.445 293(2) 8.247 3747 21 243 2804 2247 1.008 0.0632/0.1667 0.0784/0.1791 2.535 −3.023

R1 = ∑||F0| − |Fc||/∑|F0|, wR2 = {∑w[(F0)2 − (Fc)2]2/∑w[(F0)2]2}1/2.

containing Ag/AgX, and possible photocatalytic mechanisms are also not clear.13b,14b,15a,10 Intrigued by the rich coordination flexibilities and potential semiconducting behaviors of these hybrid materials, we undertook systematic studies in exploring new hybrid silver bromides via corporation of photosensitive [TM(2,2-bipy)3]2+ complex cationic dyes as a continuation of the pioneer work about hybrid cuprous and silver halogenides, which will afford the direct experimental understanding on the structure-dependent photocatalytic behaviors at the molecule scale.12b,13b,e,15a,17 Fortunately, we obtained two new types of hybrid phases of K[TM(2,2-bipy)3]2Ag6Br11 and [TM(2,2-bipy)3]2Ag13Br17 with 2D [Ag6Br11]5− and [Ag13Br16]3− anionic layers, respectively. The UV−vis diffuse reflection spectra revealed that the title compounds possess tunable band gaps of 1.73−2.71 eV, which lead to the highly efficient and stable photocatalytic activities under visible-light irradiation. In this paper, we report their syntheses, crystal structures, photocatalytic properties as well as the possible catalytic mechanism.

and possible electronic interactions based on single-crystal structure. Such expectation led us to prepare a new type of stable photocatalysts based on hybrid microporous silver halogenides via introducing photosensitive transition-metal (TM) complex cationic dyes as templates and electron absorption units on the basis of not only crystal engineering but also band structure decoration. On the one hand, the introduced TM complex cations as templates will lead to the various microporous structures, which is able to provide well-aligned pore structures with high specific surface areas and more active sites that shortens the diffusion paths of photogenerated carriers to active surface sites. On the other hand, the TM complex cations feature excellent electron absorptions and transfer capabilities to absorb the photogenerated electrons, which will effectively prevent the reduction of Ag+ ions and increase their photochemical stabilities. Furthermore, the diversiform TM complex cations are also able to obviously tune the band structures and optical properties of hybrid materials and finally regulate the photocatalytic properties. Until now, although a large amount of hybrid silver halogenides directed by organic templates have been reported and structurally characterized, there is little attention paid to the TM complex decorated silver halogenides (Table S1).11−15 Most of these TM complex templated hybrid phases feature zerodimensional (0D) oligmers including [Ag2I5]3−, [Ag3I6]3−, [Ag 4 I 8 ] 4− , one-dimensional (1D) chains of [Ag 3 I 5 ] 2− , [Ag5I7]2−, [Ag6I9]3−, [Ag10I12]2−, [Ag11I15]4−, [Ag10I14]4−, and little phases contain two-dimensional (2D) layers of [Ag3I6]3−, [Ag5I7]2−, [Ag6I11]5−, [Ag11I15]4−, and three-dimensional (3D) frameworks of [Ag2I4]2− and [Ag13I17]4−.11−15 As for the hybrid silver bromides, only a few oligmers, 1D chains, and one type of 2D layered [Ag2Br4]2− directed by organic cations were reported, and the TM complex templated hybrid phases have not been explored.16 On the basis of the structural analyses, stable UV− vis-light-driven photocatalytic properties were also found for partial hybrid iodoargentates, but the photocatalytic activities remain behind those of mixed plasmonic photocatalysts

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EXPERIMENTAL SECTION

Materials and Instruments. All the reagents were commercially available and directly used as received without further purifications. NDoped TiO2 (P25) was synthesized following the previously reported method, that is, by treating Degussa P25 in the NH3 (67%)/Ar atmosphere at 550 °C for 3 h.18 The N concentration was 0.99 atom % detected by X-ray photoelectron spectroscopy (XPS). Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 Advance powder X-ray diffractometer (Cu Kα, λ = 1.5418 Å) in the 2θ range of 5−80°. Elemental analyses of C, N, and H were performed on a PE2400 II elemental analyzer. The UV−vis absorption spectrum was performed on a PE Lambda 900 UV/vis spectro-photometer with wavelength of 200− 800 nm. The thermogravimetric analyses (TGA) of powder samples were performed by a Mettler TGA/SDTA 851 thermal analyzer under a N2 atmosphere from room temperature to 800 °C. XPS with monochromatized Al Kα X-rays (hν = 1486.6 eV) radiation (XPS, ThermoFisher Scientific Co. Escalab 250, USA) was used to investigate the surface properties. Syntheses of Compounds 1−4. A mixture of KBr (3 mmol), Ni(CH3COO)2·4H2O (1 mmol), 2,2-bipy (3 mmol), AgBr (3 mmol), B

DOI: 10.1021/acs.inorgchem.6b01770 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Crystal Data and Structure Refinements for Compounds 5−8 compound chemical formula fw space group a/Å b/Å c/Å V (Å3) Z Dcalcd (g·cm−3) temp (K) μ (mm−1) F(000) reflections collected unique reflections reflections (I > 2σ(I)) GOF on F2 R1,wR2 (I > 2σ(I))a R1,wR2 (all data) Δρmax (e/Å3) Δρmin (e/Å3) a

5 C60N12H48Ni2Ag13Br17 3815.30 Pcca (No. 54) 13.5580(10) 22.2467(17) 28.650(2) 8641.5(11) 4 2.933 293(2) 11.201 7016 96 524 9931 5411 1.018 0.0629/0.1341 0.1294/0.1619 1.617 −2.100

6 C60N12H48Co2Ag13Br17 3811.71 Pcca (No. 54) 13.5393(9) 22.1749(15) 28.5403(19) 8568.7(10) 4 2.955 293(2) 11.243 6992 97 099 9936 5868 1.032 0.0600/0.1572 0.1141/0.1893 2.889 −2.490

7 C60N12H48Zn2Ag13Br17 3828.62 Pcca (No. 54) 13.6525(6) 22.3249(9) 28.6791(12) 8741.1(6) 4 2.909 293(2) 11.191 7032 97 538 9995 5965 1.016 0.0577/0.1408 0.1076/0.1652 1.906 −2.388

8 C60N12H48Fe2Ag13Br17 3809.58 Pcca (No. 54) 13.4518(8) 22.1531(13) 28.5598(17) 8510.8(9) 4 2.973 293(2) 11.271 7000 97 451 9987 5852 1.029 0.0581/0.1442 0.1116/0.1714 1.667 −2.753

R1 = ∑||F0| − |Fc||/∑|F0|, wR2 = {∑w[(F0)2 − (Fc)2]2/∑w[(F0)2]2}1/2.

HBr aqueous solution (48%, 1 mL), and acetonitrile (5 mL) was sealed in a 15 mL Teflon-lined stainless container, which was heated at 140 °C for 5 d. With cooling rate of 5 °C·min−1 to room temperature, orangered block-shaped crystals of 1 in 51% yield based on AgBr were found and subsequently determined as K[Ni(2,2-bipy)3]2Ag6Br11 (1). The crystals were easily collected by hand and washed with ethanol. Anal. Calcd for C60N12H48Ni2KAg6Br11 (1): C, 27.51; H, 1.85; N, 6.41%; found: C, 27.59; H, 1.80; N, 6.34%. Compounds 2, 3, and 4 were also synthesized in the analogous manner to that of 1 with Co(CH3COO)2· 4H2O, Zn(CH3COO)2·4H2O, and FeSO4·7H2O instead of Ni(CH3COO)2·4H2O, respectively. The red crystals of 2, yellow crystals of 3, and dark red crystals of 4 were obtained in yields of 12%, 23%, and 35% based on AgBr, respectively. Anal. Calcd for C60N12H48Co2KAg6Br11 (2): C, 27.50; H, 1.85; N, 6.41%; found: C, 27.61; H, 1.90; N, 6.37%; C60N12H48Zn2KAg6Br11 (3): C, 27.37; H, 1.84; N, 6.38%; found: C, 27.45; H, 1.79; N, 6.40%; C60N12H48Fe2KAg6Br11 (4): C, 27.57; H, 1.85; N, 6.43%; found: C, 27.48; H, 1.79; N, 6.51%. Syntheses of Compounds 5−8. A mixture of Ni(CH3COO)2· 4H2O (0.5 mmol), 2,2-bipy (1.5 mmol), AgBr (4 mmol), KBr (2.0 mmol), acetonitrile (5.0 mL), and ammonia−water (2.0 mL) was sealed in a stainless steel reactor with a 15 mL Teflon liner and heated at 140 °C for 5 d and then slowly cooled to room temperature. A large amount of pink block-shaped crystals of 5 was found and subsequently determined as [Ni(2,2-bipy)3]2Ag13Br17. The compounds of 6−8 were prepared according to the same reaction condition with corresponding Co2+, Zn2+, and Fe2+ salts instead of Ni(CH3COO)2·4H2O, respectively. The crystals were easily collected by hand and washed by ethanol with yields of 32%, 20%, 22%, and 24% for compounds 5−8 based on AgBr, respectively. Anal. Calcd for C60N12H48Ni2Ag13Br17 (5): C, 18.89; H, 1.27; N, 4.40; found: C, 18.79; H, 1.35; N, 4.31%; C60N12H48Co2Ag13Br17 (6): C, 18.89; H, 1.27; N, 4.40; found: C, 18.95; H, 1.37; N, 4.29%; C60N12H48Zn2Ag13Br17 (7): C, 18.82; H, 1.26; N, 4.39; found: C, 18.75; H, 1.35; N, 4.22%; C60N12H48Fe2Ag13Br17 (8): C, 18.92; H, 1.27; N, 4.41; found: C, 18.84; H, 1.33; N, 4.35%. X-ray Crystallography. Single crystals of the compounds 1−8 were collected on a Bruker SMART CCD-based diffractometer (Mo Kα radiation, λ = 0.710 73 Å) at room temperature. The structures were solved by direct method and refined on F2 by full-matrix least-squares method using the SHELXS-97 program.19 All the non-hydrogen atoms were refined anisotropically, and the hydrogen atoms of 2,2-bipy ligands were generated theoretically and refined isotropically with fixed thermal

factors. It is found that the displacement parameters of K atoms feature slightly higher than other atoms in phases 1−4, which is maybe due to the weak bonding connections among the K+ ions, and similar phenomonen has also been observed in other hybrid iodocuprates and iodoargentates.15a,17 Note that the relatively large displacement parameters of Ag and Br atoms in compounds 5−8 may be related to the inferior crystal qualities, and similar phenomena have also been observed in many hybrid silver halogenides.12,13 The crystallographic data for all the compounds are listed in Tables 1 and 2, and the important bond lengths are listed in Tables S2−S6. Calculation Details. The crystallographic data of compound 1 were selected to calculate the electronic band structure by using density functional theory (DFT) with the CASTEP code based on gradientcorrected exchange-correlation function, which uses plane-wave basis set for the valence electrons and norm-conserving pseudopotential for the core electrons.20 Pseudoatomic calculations were performed for Ag4d105s1, Br-4s24p5, Ni-3d84s26s1, C-2s22p2, N-2s22p3, and H-1s1. The other calculating parameters and convergence criteria were set using the default values of the CASTEP code. Photocatalytic Activity Measurements. The common used organic pollutant of Rhodamine B (RhB) was selected to evaluate the photocatalytic activities of as-prepared samples under visible-light irradiation from a 50 W Xe lamp, which was attached by a cutoff filter to remove all ultraviolet and infrared light (400 < λ < 780 nm). Note that the samples 4 and 8 easily dissolved in water to form the red solutions, which led to great difficulties for analyzing and recollecting the samples. So we did not investigate the photocatalytic activities of samples 4 and 8. In a photocatalytic activity measurement, 15 mg of each powder sample was added to a 30 mL solution (1× 10−5 mol·L−1) of RhB in a quartz tube, and the above suspension was continually stirred in the dark for 10 h until the absorption/desorption equilibrium between the photocatalyst powder and RhB molecule. After that, the solution was exposed to visible-light irradiation with distance of ∼10 cm from Xe lamp, and 4 mL of the mixture was continually taken from the reaction cell after a given irradiation time. The samples were separated from the suspensions by centrifugation, and the resulting supernatant fluid was analyzed by recording the maximum absorption band (554 nm for RhB) on a GBC Cintra 2020 UV/vis spectro-photometer. The leaking Ag+ content of supernatant fluid was analyzed via inductively coupled plasma (ICP) optical emission spectrometry (Ultima2). For collecting the adequate sample in recycling experiment, two or even more photocatalytic processes were performed under the same condition, and then the C

DOI: 10.1021/acs.inorgchem.6b01770 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Detailed view of the [Ag3Br7] SBU (a), 2D [Ag6Br11]5− layer (b), and the packing structure of compound 1 along the b-axis (c). The K, Ag, and Br atoms are drawn as yellow, green, and red spheres, respectively (symmetry codes: (i) x, 1 + y, z; (ii) −y, x − y, z; (iii) 1 − x + y, 1 − x, z; (v) −y, x − y, z; (iv) −y, x − y, z). samples were separated through centrifugation. All the precipitates from the different processes were collected, combined, and dried in an oven at 80 °C for 12 h. After that, 15 mg of dried sample was performed for the second photocatalytic experiment according to the same method as that of first study. Another recycling experiment was also performed with the same method. The radical-trapping experiments were performed by adding a little of ammonium oxalate (AO), tert-butyl alcohol (TBA), and

benzoquinone (BQ) in the photodegradation reaction of RhB, respectively.

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RESULTS AND DISCUSSION Syntheses Discussion. All the title compounds 1−8 were prepared by adopting the same materials (AgBr, TM2+ salts, 2,2-

D

DOI: 10.1021/acs.inorgchem.6b01770 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Forming schema of 2D [Ag13Br16]3− layer on the ab plane (a) and the packing structure of compound 5 along the a-axis (b). The Ag and Br atoms are drawn as green and red spheres, respectively (symmetry codes: (i) −0.5 + x, 1 − y, 0.5 − z; (ii) x, 1 − y, 0.5 + z; (iii) 1.5 − x, y, 0.5 + z, (iv) 1 − x, y, 0.5 − z; (v) 1.5 − x, y, 0.5 + z; (vi) −0.5 + x, −y, 0.5 − z; (vii) 2 − x, y, 0.5 − z).

in situ generated [TM(2,2-bipy)3]2+ cations was also able to obtain the same products on the same reaction condition. The KBr added in the solution not only was used as the source of Br− but also effectively increased the solubility of the AgBr. Hence, the concentration of Br− ions plays an important role in the control of hybrid material syntheses, especially for the dimension of the structures. Higher concentration of Br− ions will lead to the low-dimensional oligmers or chains, while the low one results in the high-dimensional structures. In the syntheses of the title compounds, we controlled the ratio of AgBr/KBr to be ∼1:1 for 1−4 and 2:1 for 5−8. Crystal Structures. Single-crystal X-ray diffraction analyses revealed that 1−4 belong to isostructural phases; hence,

bipy, and KBr) and reaction temperature condition. However, acidic HBr solution led to the first type of K[TM(2,2bipy)3]2Ag6Br11, and alkaline solution regulated by ammonia− water resulted in another type of [TM(2,2-bipy)3]2Ag13Br17. Detailed experimental studies indicated that the acidic environment only led to the compounds 1−4, and alkaline solution also solely resulted in compounds 5−8, which shows that the pH value played the critical role in the preparations of two types of hybrid silver bromides. In the high-temperature reaction, the TM2+ ions were in situ coordinated by 2,2-bipy ligands into saturated [TM(2,2-bipy)3]2+ cations, which has been reported in many inorganic−organic hybrid materials.21 Further studies showed that the use of prepared [TM(2,2-bipy)3]Cl2 instead of E

DOI: 10.1021/acs.inorgchem.6b01770 Inorg. Chem. XXXX, XXX, XXX−XXX

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hydrogen-bonding interactions (C···Br distances: 3.544(1)− 3.773(8) Å; Figure 2b). Under the templated or directed actions of organic or metal complex cations, the inorganic−organic hybrid cuprous and silver halogenides feature various 2D (MmXn)(n−m)− anionic layers with diversiform condensation modes based on MX4 tetrahedron, and it is very interesting to compare their structural characterizations according to their basic units or SBUs (Figure S4). The [Ag2I3]− layer is built by the interconnection of two parallel ladderlike [Ag2I2] chains via sharing I atoms.25 The [Ag3I4]− layer features two distinct structural types; the α[Ag3I4]− layer is composed of [Ag6I12] units, and the β-[Ag3I4]− layer is constituted by trimeric [Ag3I8] units.26 The [Ag5I7]2− layer is built from the [Ag3I7] and [Ag7I13] SBUs, and [Ag5I8]3− layer is constituted by [Ag5I10] SBUs as four-connected nodes.12b,c The 2D [Ag6I11]5− and [Ag6I10]4− layers are composed of the same trimeric [Ag3I7] SBUs. Differently, the [Ag3I7] trimers are directly interconnected via corner-sharing to form the 2D [Ag6I11]5− layer, whereas two neighboring [Ag3I7] SBUs are initially condensed into a hexameric [Ag6I12] ternary building unit as a new node, which further self-assembles leading to the 2D [Ag6I10]4− layer.13b The [Ag7I11]4− layer is formed by alternating interconnection of two different types of [Ag7I12] double chains via corner-sharing, and [Ag11I15]4− layer is built from the three-connected [Ag6I10] and [Ag3I7] building units with (6,3) topology.12b,13e The hybrid cuprous halogenides also show diversiform 2D anionic layers including [Cu3I4]−, [Cu6I11]5−, [Cu9I11]2−, [CuI7CuII2I14]3−, [Cu11I17]6−, etc.27 As for hybrid silver bromides, only one type of 2D [Ag2Br4]2− layer based on [Ag 2 Br6 ] dimers as four-connected nodes is characterized.16 Hence, the 2D [Ag6Br11]5− is isostructural with [Ag6I11]5− and [Cu6I11]5−, and the [Ag13Br16]3− reported here represents one new type of anionic layer in hybrid silver bromides, which further indicates the unique structural directing effects of TM complex cations. Thermal Stabilities. The thermal stabilities of compounds 1−8 were investigated by TGA under N2 atmosphere in the range of 30−800 °C as shown in Figure S5. The results show that sample 1 first loses one KBr in the temperature range of 30−210 °C with a mass loss of 4.47%, which is close to the theoretical value of 4.54%. In the range of 210−500 °C, all the organic ligands are removed with a total observed weight loss of 34.90% in accordance with the theoretical value of 35.77%. After the major weight loss, compound 1 continues to slowly lose weight until 800 °C. Sample 2 features two-step weight loss of one KBr and all organic molecules per formula in the ranges of 170−320 and 320−610 °C with a observed total weight loss of 40.43%, which is close to the theoretical value of 40.10%. Compound 3 starts to slowly compose at ∼180 °C and does not achieve the balance until 800 °C. Compound 4 begins to lose weight at ∼230 °C with a final weight loss of 40.2% (calcd 39.8%) when the temperature reaches 450 °C. This can be attributed to the removal of all the organic ligands as well as one KBr molecule. Compounds 5 and 6 feature successive weight losses of all 2,2bipy ligands and two NiBr2 or CoBr2 molecules in the ranges of ∼200−470 and 200−360 °C, and the observed weight losses of 37.0% and 36.9% are close to the theoretical values of 36.0% and 36.1%, respectively. The compounds 7 and 8 feature slow decomposition and start to lose weight at ∼140 and 240 °C, respectively. Note that the isostructural compounds 1−4 and 5− 8 feature distinct thermogravimetric behaviors, which may be due to the great differences of particle size, humidity, and crystallization state as well as bonding interaction strength and

compound 1 is taken as an example to depict their crystal structures. In the asymmetric unit of compound 1, there is 1/6 crystallographically independent K, 1/3 Ni, 1 Ag, 11/6 Br atoms and one 2,2-bipy ligand. As shown in Figure 1a, the Ag(1) atom adopts the tetrahedral coordination environment with one μ3-Br and three μ2-Br atoms. The Ag−Br bond distances of 2.6107(7)−2.8437(8) Å and Br−Ag−Br bond angles of 101.851(18)−124.635(19)° are comparable with those of other hybrid silver bromides, such as [Et4N]3Ag6(CN)3.39Br5.61, [BMIE]Ag2Br4, [BMIB]Ag2Br4, etc.22,16 Three neighboring AgBr4 tetrahedra are self-condensed via sharing Br(2) and Br(3) atoms into a triangular [Ag3Br7] trimer with three outside Br(1) atoms. Within the [Ag3Br7] trimer, three Ag(1) atoms form a [Ag3] triangle with weak Ag···Ag distance of 3.0699(11) Å, which is evidently shorter than the sum of the van der Waals radii of AgI (3.44 Å), indicating the presence of weak argentophilic interactions.23 Similar trimerical [Ag3I7] and [Cu3I7] units have also been reported in K[TM(2,2-bipy)]2M6I11 (M = Cu, Ag) and [De-DABCO]2[Me-DABCO]Cu11I17, respectively.15,17,24 Each [Ag3Br7] trimer as a new structural building unit (SBU) is further interconnected to three adjacent ones via sharing Br(1) atoms to form a 2D [Ag6Br11]5− layer along the ab-plane with a (6,3) topological network (Figure 1b). Simultaneously, one type of [Ag12Br12] 24-membered ring is formed with a hexagonal cross-section of 12.418 × 12.418 Å2 presented along the c-axis. The 2D [Ag6I11]5− layers feature parallel stacking and are further interconnected by the K+ cations via weak K−Br bonds (3.629−3.630 Å) to form a 3D K@[Ag6Br11] framework with 1D large tunnels along the a- or b-axis, which are occupied by the [Ni(2,2-bipy)3]2+ with C−H··· Br hydrogen-bonding interactions (C···Br distances: 3.715(5)− 3.743(5) Å; Figure 1c). Compounds 5−8 crystallize in the same orthorhombic space group Pcca (No. 54) and feature the 2D [Ag13Br16]3− anionic layers built from 1D [Ag8Br12] and [Ag5Br8] chains interconnected via corner-sharing (Figure 2a). The asymmetric unit of 5 consists of 1/2 nickel, 13/4 silver, 17/4 bromine atoms and 3/2 coordinated 2,2-bipy ligands. All the Ag atoms are tetrahedrally coordinated by four bromine atoms, except that Ag(6) and Ag(7) atoms are surrounded by three bromine atoms with triangle environments. The Ag−Br bond distances in [AgBr4] tetrahedral and [AgBr3] triangle units are in the ranges of 2.6133(16)−2.8518(17) and 2.5459(16)−2.8761(17) Å, respectively, which are comparable with those of compounds 1−4. As shown in Figure 2a, each [Ag(1)Br4], [Ag(4)Br4], and [Ag(5)Br4] tetrahedron is successively condensed via edgesharing to form a 1D [Ag3Br6] chain along the a-axis, and such two neighboring [Ag3Br6] chains are further interlinked by a pair of [Ag2Br4] units composed of two Ag(6)Br3 triangles via sharing corner Br atoms to form a [Ag8Br12] double chain. As for [Ag5Br8] chain, the [Ag2Br6] dimers composed of two Ag(2)Br4 tetrahedral units are initially interlinked by [Ag(3)Br4] tetrahedra via edge-sharing to form a 1D [Ag3Br6] chain, which is further attached by [Ag(7)Br3] triangles into the 1D [Ag5Br8] along the a-axis. The above 1D [Ag8Br12] and [Ag5Br8] chains are alternately connected via sharing bromine atoms to form the 2D [Ag13Br16]3− anionic layers in the ab plane. Within the 2D [Ag13Br16]3− layers, there are abundant weak Ag···Ag distances of 3.1618(18)−3.3288(18) Å, which are comparable with those of compounds 1−4 indicating the presence of weak argentophilic interactions.23a The neighboring 2D [Ag13Br16]3− layers feature parallel packing along the c-axis, which are linked by [Ni(2,2bipy)3]2+ complex cations and Br− anions via C−H···Br F

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slightly larger than those of compounds 1−4. Obviously, all the title compounds have smaller band gaps and exhibit red shifts of the absorption edge compared with the bulk AgBr (2.81 eV). Furthermore, the different electronic configurations of TMs lead to the evidently different band gaps for all compounds, which demonstrates the opportunity to tune the band structures and optical properties of hybrid materials by adopting the different TM complexes. Photocatalytic Properties. The narrow band gaps and semiconducting characterizations of these phases encourage us to investigate their visible-light-driven photocatalytic properties, which have not been studied for the hybrid silver bromides until now. Their photocatalytic activities are evaluated via photodecomposition of organic pollutant (such as RhB) over the powder samples 1−3 and 5−7 as examples under the visible-light irradiation. The blank experiments without any photocatalyst or the catalytic experiment without light irradiation did not show any observable decreases in RhB concentration with time. Subsequently, the samples 1, 2, and 3 were singly added to the RhB solution, which was kept in the dark for 10 h to ensure absorption/desorption equilibrium state prior to the irradiation. Under the photodegradation action of compound 1, the purplish-red color of the RhB solution quickly changed to colorless, and the concentration of organic molecules featured continuous decrease under the visible-light irradiation (Figure S7). As shown in Figure 4a, the degradation ratio of RhB reaches 98% upon exposure to visible light for 10 min, and then reaches

melting points of different constituents. Similar phenomena have also been observed for some hybrid materials.13b,17 Optical Properties. The solid-state optical absorption spectra of compounds 1−8 were measured at room temperature (Figure 3). As shown in Figure S6, the optical band gaps obtained

Figure 3. Solid-state optical absorption spectra of compounds 1−8.

by extrapolation of the linear portion of the absorption edges are estimated as 2.10, 1.81, 2.61, and 1.73 eV for compounds 1−4, respectively, which is accordance with their colors of orange-red, red, yellow, and dark red. The band gaps of compounds 5−8 are estimated as 2.18. 2.02, 2.71, and 1.88 eV, respectively, which is

Figure 4. Photocatalytic degradation of RhB over compounds 1−3 and 5−7 under visible-light irradiation (a), the calculated degradation rate of RhB over compounds 1−3 and 5−7 (b), the cycling degradation rate of RhB for compound 1 (c), and the radical-trapping experiments over compound 1 (d). G

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(Figures S8−S10). Hence, the sample 1 belongs to a stable photocatalyst under visible-light irradiation relative to the primary AgBr. Possible Photocatalytic Mechanism. As well as we know, primary AgBr is seldom independently used as the photocalyst due to the instability under sunlight. As a stable photocatalyst, the hybrid silver bromide requires an effective way of separating the photogenerated electron−hole pairs to prevent the reduction of Ag+ ions. To get insight into the photocatalytic activity and stability of sample 1 and further study the possible photocatalytic mechanism, we first perform the radical trapping experiments to detect the main active species during the photocatalytic process by adding several scavengers, that is, tert-butyl alcohol (TBA, an · OH radical scavenger), ammonium oxalate (AO, a hole scavenger), and benzoquinone (BQ, an ·O2− radical scavenger), respectively. As shown in Figure 4d, the additions of TBA and AO have slight effects on the photocatalytic activities of sample 1, while the photodegradation reaction is greatly suppressed when BQ is introduced. This indicates that ·O2− as the dominant reactive species plays the important role for degradation of organic dyes over sample 1. Subsequently, we examine the electronic band structure of compound 1 by performing DFT calculation to explore the relationship of crystal structure and optical property (Figure 5). On the basis of the total and partial

nearly 100% after 12 min, resulting in complete decolorization. Sample 3 features similar photodegradation behavior to that of 1, while compound 2 shows slightly slow photodegradation speed, and ∼80% of RhB is decomposed after irradiation for 12 min. Hence, the photocatalytic activities of compounds 1−3 follow the order of 1 > 3 > 2, and the sample 1 features the highest reaction activity. At the same reaction condition, we also study the visible-light-driven photocatalytic properties of samples 5−7, and the results show that all the samples also exhibit excellent photodegradation abilities and feature the same photocatalytic activity order of 1 > 3 > 2. Note that the photocatalytic activities of all the title compounds follow the order of Ni phase > Zn phase > Co phase. Generally speaking, wider light-absorption range will lead to the higher photocatalytic activity. But the sequence of photocatalytic activities for the title compounds is not directly related to their band gaps, which shows that the optical absorption is not the sole affecting factor for the photocatalytic activity. After all, many structural factors also play influence on the photocatalytic reaction, such as the sample morphology, activated site, defects inside the catalyst or on the surface, etc. The reason why the discrepancy in the photocatalytic activities occurs between these phases may be also related to the slight structural difference, such as the electronaccepting abilities of TM complexes, but it is difficult to explain at current stage. For comparing, we also studied the photocatalytic property of N-dotted TiO2 as benchmark under the completely identical condition, and the result shows that more than 96% of RhB is still alive after visible-light irradiation. Although the primary AgBr also shows photodegradation effect, the Ag + ions are simultaneously reduced to Ag nanoclusters leading to the photolysis of AgBr. Hence, the samples 1−3 and 5−7 feature more excellent and stable degradation abilities than the N-doped TiO2 and primary AgBr. Given the degradation process follows a pseudo-first-order reaction, the experimental ln(C0/C) values over reaction time were performed through linear fitted method according to the equation of ln(C0/C) = kt, where C0 and C represent the initial and remnant concentrations of organic dyes at time t, respectively, and k is the rate constant (Figure 4b). The calculated k values of degradation reactions over the samples 1−3 and 5−7 are of 0.337, 0.201, 0.255, 0.301, 0.119, and 0.239 min−1, respectively, which are approximately 24, 14, 18, 21, 8, and 17 times higher than that of the N-doped TiO2 (0.014 min−1). Such photocatalytic activities are also greatly higher than those of hybrid iodoargentates, which is mainly due to the more positive valence band energies and strong oxidation abilities of Br 4p orbitals than those of I 3p electrons as discussed in ref 18. This also indicates the possibility of regulating the photocatalytic activities via adopting different inorganic halogenide networks or proper doping strategy. In addition to the photocatalytic activity, photochemical stability also plays an important role in determining the practical application, especially for those Ag-based photosensitive materials. Therefore, we further study the cyclic photodegradation experiments for RhB dye over sample 1 as represented under the identical conditions. It is clearly found that sample 1 is stable under the five repeated applications, with no obvious decreases of photodecomposition rates and efficiencies (Figure 4c). The ICP results show that insignificant amount of the sample dissolves in the solution indicating the stabilities of the title compounds in the photocatalytic reaction. The XRD, XPS, and IR patterns at the end of repeated experiments are almost identical to those of as-prepared samples

Figure 5. Total and partial DOS of compound 1.

density of state (DOS) diagrams, the top of the valence band just below the Fermi level is mainly contributed by the 4p state of Br and the 4d state of Ag mixed with 3d orbital of Ni, and the lowest conduction bands are mainly composed of C-2p and N-2p as well as Ni-3d states. Hence, the photoinduced charge transition of the compound 1 mainly occurs between the anionic 2D [Ag6Br11]5− layers and [Ni(2,2-bipy)3]2+ cations. On the basis of these radical-trapping experiments and calculation results as well as the relative reports, the possible photocatalytic mechanism about band structure and charge transfer is proposed in Figure 6.12,13,15,28 In the photocatalytic process over sample 1, the photogenerated electrons are able to easily transfer from the 2D [Ag6Br11]5− layers (VB) to the [Ni(2,2-bipy)3]2+ complex cations (CB), and then the photogenerated electrons are further trapped by O2 in the solution to form superoxide ion ·O2−, which contributes to the decomposition of organic dyes. Simutaneously, the remaining photogenerated holes can be transferred to oxide Br− ions into Br0, which oxidize RhB molecule and become reduced to Br− ions again. H

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Figure 6. Photocatalytic degradation mechanism for organic dyes over compound 1 under visible-light irradiation.

Hence, the TM complex cations play an important role for the photocatalytic activities and stabilities of inorganic−organic hybrid silver bromides, and we attribute this to the following three factors. First, the templated effects of TM complex cations lead to the microporous anionic [Ag6Br11]5− layer, which not only greatly improves the absorption properties of organic dyes but also provides more abundant reactive sites for degradation of dye molecules. Second, the photosensitive TM complex cations greatly contribute to the CBs leading to the narrow band gaps, which obviously increase the absorption of visible light for hybrid materials. Finally, the photogenerated electrons easily transfer to the TM complex cations rather than being located on the Ag+ ions, which can effectively prevent the reduction of Ag+ ions. The above factors involve microporous structure decoration, visiblelight absorption, and the separation of photogenerated carriers in the photocatalytic degradation process. Hence, the introduction of photosensitive TM complex cations as templates not only adjust the crystal and electronic band structures but also effectively tune the characterization of photogenerated carriers, which is indeed effective in acquiring highly efficient and stable visible-light-driven photocatalysts.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (C.-Y.Y.) *E-mail: [email protected]. (X.-W.L.) ORCID

Xiao-Wu Lei: 0000-0003-4603-9093 Notes

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The authors declare no competing financial interest.

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CONCLUSIONS Two new types of 2D layered silver bromides directed by TM complex cations have been solvothermally synthesized and structurally and optically characterized. Our materials show excellent visible-light-driven photodegradative abilities than Ndoped TiO2 and AgBr due to the multielectronic effects of TM complex cations. The successful syntheses and visible-lightdriven photocatalytic properties of the title compounds further illustrate the possibilities of constructing new hybrid silver halogenides with tunable abilities of band structures and physical properties by introducing different TM complex cationic dyes. Further studies on the structural regulation and relationships of structure−photocatalytic property are in progress in our group.

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Crystallographic data in CIF format (CCDC Nos. 1478144 for 1, 1478145 for 2, 1478146 for 3, 1478147 for 4, 1494346 for 5, 1494347 for 6, 1494348 for 7 and 1494345 for 8) (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF)

ACKNOWLEDGMENTS We are thankful for the financial support from the National Nature Science Foundation of China (Nos. 21571081 and 21671080) and Fund of State Key Laboratory of Structural Chemistry (No. 20150005).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01770. Tables of selected bond distances, structural figures containing hydrogen bonds, thermogravimetric curves, absorption spectra of photodegradative RhB, XRD powder patterns, XPS and IR data. (PDF) I

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b01770 Inorg. Chem. XXXX, XXX, XXX−XXX