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A perfectly aligned 63 helical tubular cuprous bromide single crystal for selective photo-catalysis, luminescence and sensing of nitro-explosives† Ru-Xin Yao,a Reshalaiti Hailili,b Xin Cui,a Li Wang*b and Xian-Ming Zhang*a A perfectly aligned 63 helical tubular cuprous bromide single crystal has been synthesized and characterized, which can selectively decompose negatively charged dyes of Methyl Orange (MO) and Kermes Red (KR), and the photocatalytic efficiency is higher than that of nanosized (∼25 nm) TiO2 and ZnO. The direction and magnitude of the dipole moments as well as the band structure were calculated to reveal high photocatalytic efficiency. Moreover, luminescence studies indicate that the CuBr tube materials show very strong yellowish green emissions in the solid state and emulsion even at room temperature, and
Received 28th November 2014, Accepted 23rd December 2014
exhibit extremely high detection sensitivity towards nitro-explosives via fluorescence quenching. Detectable luminescence responses were observed at a very low concentration of 20 ppm with a high quench-
DOI: 10.1039/c4dt03657c
ing efficiency of 94.90%. The results suggest that they may be promising multifunctional materials for
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photo-catalysis, luminescence and sensing of nitro-explosives.
Introduction Carbon nanotubes, discovered in 1991 by Iijima,1 show unusual properties and wide potential applications as catalysts, sensors, electrodes, and biological models resulting from their unique tubular structures, which have stimulated intensive exploration to nanotube structures containing carbon and non-carbon elements.2 To date, the known classes of tubular structures are overwhelmingly dominated by carbon nanotubes. Examples of non-carbon nanotubes are numbered,3 which include metal chalcogenides such as WS2, MoS2, VOx and TiO2, metal halide NiCl2, group III nitrides such as GaN and AlN, and newly developed metal–organic nanotubes,4 and helical non-carbon nanotubes are even less. Furthermore, it should be noted that the majority of tubular structures are electrically neutral, which can exist in the absence of templates or counterions. Charged tubular structures may show new or superior properties related to electrostatic interactions, but need to be stabilized by templates or counterions.
a Department of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, P. R. China. E-mail: [email protected] b College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi 830054, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Crystallographic studies, crystallographic data, calculation details, PXRD pattern, TGA/DTA plots, and photocatalytic properties. CCDC 945278. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03657c
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Light based multifunctional materials are important in energetic, environmental and secure fields. For example, photo-catalysis can split water to produce clean hydrogen energy and decompose organic contaminants that are difficult to be removed by conventional adsorption, filtration and even bio-degradation methods.5 Luminescent materials have potential applications in chemical sensors, light-emitting devices, medical diagnostics, and cell imaging.6 To date, photocatalytic materials have focused mainly on the metal oxides, sulfides, oxynitrides, and heterojunctions, especially recently reported metal–ion doped oxides such as In1−xNixTaO4.7 However, photocatalytic reactions based on most semiconductors are often accompanied by the formation of highly reactive radical species (e.g. •OH) that are typically active and nonselective, resulting in nearly total degradation of organic compounds. Moreover, the yield of active species is very low, and nonselective degradation is wasteful due to the destruction of innoxious organic components or even useful bio-enzymes. Thus it is still a great challenge to develop a new photocatalytic system with desirable selectivity. It is believed that the breaking of highly symmetric M–O configuration in general metal oxide semiconductors may create some photocatalytic selectivity. For example, famous “star” semiconductor TiO2 has a closed octahedral coordinated TiO6 unit, which is thought to be less active than low symmetric open tetrahedral Ti4+ sites that could be thought as defects stabilized via coordination with substrates.8 However, the existence of these defects always leads to low charge-separation
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efficiency. This may face a dilemma between the photocatalytic efficiency and the selectivity. Thus the search for new and available semiconductor materials, e.g. the photocatalytically active distorted polyhedron with a large dipole moment, may be crucial since the internal fields will promote the charge separation and obtain reactive selectivity.9 The Cu(I) ions commonly exhibit flexible coordination in cuprous halides, which contribute to luminescence, ionic conduction and high reactivity in materials and biochemistry,10 which encouraged us to synthesize cuprous halide materials for multifunction applications. In this article, we report a perfectly aligned 63 helical tubular cuprous bromide single crystal. It selectively decomposes anionic organic dyes MO and KR, due to their surface properties, large dipole moment afforded by the distorted CuBr4 tetrahedron structure, and the typical composition of the valence band. Moreover, it could also be used as a green luminescent material and a sensor for detecting nitro-explosives at the ppm level.
Experimental section Materials and physical measurements The FT-IR spectra were recorded from KBr pellets in the range of 400–4000 cm−1 on a Nicolet 5DX spectrometer. Elemental analysis was performed on a Vario EL III elemental analyzer. PXRD data were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, λ = 1.5418 Å). Thermal analysis (TGA) was carried out under an air atmosphere using SETARAM LABSYS equipment with a heating rate of 10 °C min−1. UV-vis absorption was monitored using a U-3310 spectrophotometer. The morphology was examined using transmission electron microscopy (TEM, JEOL, JEM-2100F, 200 kV) and field-emission scanning electron microscopy (SEM, JSM-7500F, 5 kV). Photoluminescence analysis was performed on an Edinburgh FLS920 luminescence spectrometer. Crystallographic studies X-ray single-crystal diffraction data of 1 were collected on an Agilent Technologies Gemini EOS diffractometer at 298 K using Mo Kα radiation (λ = 0.71073 Å). The program SAINT was
Table 1
used for integration of diffraction profiles, and the program SADABS was used for absorption correction. The structure was solved with the XS structure solution program by direct methods and refined by the full-matrix least-squares technique using Olex2. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of organic cations were generated theoretically onto the specific carbon atoms, and refined isotropically with fixed thermal factors. Further details for structural data are summarized in Table 1 and Table S1.† Calculation details Time-dependent density functional theory (TD-DFT) calculations were performed using the B3LYP functional. The LANL2DZ basis sets were employed for Cu, and 6-31+G basis sets were employed for Br. The initial ground-state geometry of [Cu5Br6]− directly obtained from the X-ray crystal structure has been used for calculations with the Gaussian 09 software package.11 The dimensional plots of molecular configurations and orbitals were generated with the Gauss View program.12 The electronic band structure along with density of states (DOS) for the anionic Cu5Br83− tube was calculated by DFT using the crystallographic data with the CASTEP code, which uses a plane wave basis set for the valence electrons and normconserving pseudopotential for the core electrons. The number of plane waves included in the basis was determined by a cutoff energy Ec of 550 eV. Pseudoatomic calculations were performed for Br-4s24p5 and Cu-3d104s1. The parameters used in the calculations and convergence criteria were set by the default values of the CASTEP code, for example, the eigen energy convergence tolerance 1.0 × 10−5 eV. The calculation method of the dipole moment The well-known Debye equation, μ = neR (μ is the net dipole moment in Debye (10−18 esu cm), n is the total number of electrons, e is the charge on an electron, −4.8 × 10−10 esu, and R is the difference in cm (between the “centroids” of positive and negative charge)) has been used to calculate the dipole moment.13 Distribution of the electrons on the M/O/N atoms was estimated using the bond valence theory (Si = exp[(Ro −
Crystal data for compound 1
Compound 1 Formula Fw Crystal system Space group a (Å) b (Å) c (Å) γ (°) V (Å3) Z ρcalc. (g cm−3) a
Cu15Br25C40H90N10 3662 Hexagonal P63/m 18.2711(8) 18.2711(8) 13.4431(12) 120 3886.5(5) 2 2.643
μ (mm−1) F(000) Crystal size (mm) Reflections Rint Tmax/Tmin Data/parameters S R1 a, wR2 b [I > 2σ(I)] R1, wR2 (all data) Δρmax/Δρmin (e Å−3)
16.871 2780 0.32 × 0.24 × 0.20 25 374/3341 0.0851 0.1334/0.0746 3341/81 1.014 0.0671, 0.1653 0.1273, 0.1883 1.65, −2.07
R1 = Fo − Fc/Fo. b wR2 = [w(Fo2 − Fc2)2/w(Fo2)2]1/2.
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Ri)/B]; where Ro is an empirical constant, Ri is the length of the bond “i” in Å, and B = 0.37).
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Synthesis of [(Me2DABCO)5(Cu15Br24)Br] (1) A mixture of CuBr2 (0.110 g, 0.5 mmol), DABCO (0.065 g, 0.30 mmol), MeOH (5 mL) and H2O (1 mL) was stirred and the pH value was adjusted with 1 drop HClO4 (71% w/w), and then it was sealed in a 15 mL Teflon-lined stainless autoclave and heated to 100 °C for 5 days. After it was cooled to room temperature and subjected to filtration, colorless block crystals of 1 were recovered in 73.5% yield based on CuBr2. Anal. calcd (%) for Cu15Br25C40H90N10 (1): C 13.12, H 2.48, N 3.82; found: C 13.26, H 2.67, N 3.74. IR data (KBr, cm−1): ν = 3424(vs), 2921(m), 2835(w), 1634(s), 1463(s), 1320(w), 1122(w), 1047(m), 842(m), 799(m), 697(w). Photocatalytic experiments For photocatalytic decomposition activity measurement, 50 mg of 1 was added to 100 mL of 1 × 10−5 mol L−1 aqueous solution of KR, MO, Methylene Blue (MB) and Rhodamine B (RhB), respectively. The suspensions were magnetically stirred in the dark for 30 min to ensure adsorption equilibrium and uniform dispersity. The solution was then exposed to UV irradiation from a 125 W high-pressure Hg lamp with the strongest emission at 365 nm (3.40 eV) at room temperature. After a given irradiation time, 5 mL of the mixture was withdrawn, and the catalysts were separated from the suspensions by centrifugation. The degradation process was monitored through a wavelength scan on U-3310 spectrophotometer.
Results and discussion Colorless columnar crystals of 1 in 73.5% yield were synthesized by the solvothermal treatment of a mixture of CuBr2, DABCO, MeOH and H2O at 100 °C for five days. FT-IR spectra, elemental and TGA confirmed the formula of 1. The phase purity of the bulky samples was confirmed by a PXRD study (Fig. S1†). TGA/DTA analysis shows that 1 is thermally stable up to 190 °C under an air atmosphere, and the methylated Me2DABCO cations are removed in the range of 190–340 °C (Fig. S2†). Single crystal X-ray analysis reveals that 1 consists of perfectly aligned 63 helical cuprous bromide tubes stabilized by in situ alkylated Me2DABCO templates. Compound 1 crystallizes in the hexagonal space group P63/m, and the asymmetric unit consists of three Cu(I) atoms, six bromides and one [Me2DABCO]2+ cation. All Cu(I) atoms show tetrahedral coordination geometry (Fig. S3†). The Cu(1) and Cu(3) atoms are coordinated by two μ2-Br atoms and two μ3-Br atoms with Cu–Br distances of 2.372(2)–2.929(1) Å and Br–Cu–Br angles of 98.745 (2)–128.12(10)°. The Cu(2) is coordinated by four μ3-Br atoms with Cu–Br distances of 2.463(3)–2.539(3) Å and Br–Cu–Br angles of 104.652(10)–115.44(6)°. It should be noted that five of the six Br atoms are bonded to Cu atoms in μ2 or μ3 modes, while the sixth Br(6) atom is free with symmetry of −6
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Fig. 1 View of the 63 helical Cu5Br83− tube (a) along the c-axis, (b) packing array of tubes with organic templates in 1.
(Wychoff letter 2c). The N(1) and C(2) atoms of Me2DABCO lie at threefold axis positions with a site occupancy of 1/3. The crystallographic mirror plane passes through Cu(2), Br(1), and Br(4) atoms. The Cu(3) tetrahedron is edge-shared with two Cu(1) and two Cu(2) tetrahedra via μ3-Br(3) atoms, thus forming a tortoiseshell-like Cu5Br10 unit. The Cu⋯Cu distances are in the range of 2.955(3)–3.1151(1) Å, which are longer than the sum of van der Waals radii of two Cu atoms (2.80 Å), indicative of the presence of very weak Cu⋯Cu interactions. The Cu5Br10 units are further linked together via sharing four peripheral μ2-Br(5) atoms to furnish an anionic cuprous bromide tube formulated as [Cu5Br8]n3n− with 4.1 nm interior diameter and 2.1 nm exterior diameter (Fig. 1a). Interestingly, the [Cu5Br8]n3n− tube is 63 helical with the pitch of 1.34 nm. Several helical tubes including single-, double- and hexahelical structures have been reported,14 and to the best of our knowledge, compound 1 is of the first cuprous bromide helical tube-like structure unit. The anionic [Cu5Br8]n3n− helical tubes are stabilized by [Me2DABCO]2+ cations that come from in situ methylation of the DABCO group in the presence of bromides (Fig. 1b). The charge balance requires that there are five [Me2DABCO]2+ cations per formula but only one ordered [Me2DABCO]2+ cation could be located from the Fourier map. The existence of additional four [Me2DABCO]2+ cations is confirmed by elemental and thermal analyses. A calculation by PLATON shows that there are 1400 Å3 voids per unit cell volume of 3886.5 Å3, which is approximately in agreement with the existence of additional four [Me2DABCO]2+ cations. Existence of not located guest molecules is a commonly observed phenomenon in porous structures, especially those crystallizing in high symmetric space groups. In spite of not being located from the X-ray Fourier map, the detailed analyses on the structure and charge density may give some clues on the possible locations of additional Me2DABCO groups. As can be seen along the c-axis direction, bromides project the tube both inward and outward, while
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copper atoms are sandwiched by bromides. Thus, both the interior and exterior walls of the [Cu5Br8]n3n− tube are negatively charged, which need to be neutralized by positively charged species. Otherwise, highly negative charge will result in the instability of the structure. After considering the van der Waals radii of bromide, the interior channel diameter of the [Cu5Br8]n3n− tube is 4.1 Å, which indicates that the tube provides substantial interior space for accommodating molecules. In addition, one may find that there are windows sized 6.1 Å × 6.1 Å (9.79 Å × 9.79 Å without considering van der Waals radii of Br atoms) along the propagating direction of the tube, which also provides space for Me2DABCO cations. The [Cu5Br8]n3n− tubes are stabilized by [Me2DABCO]2+ cations via C–H⋯Br hydrogen bonds to form a 3D supramolecular array. Metal-bound halogens (M–X) and halide ions (X−) have been shown to be very good hydrogen bond acceptors, and the donor atoms can be C, N and O atoms. Based on the analyses of thousands of crystal structures in the CCDC database, Brammer pointed geometric preferences for C–H⋯X–M interactions in metal-bound halogens.15 The C and H atoms from DABCO and Br(3) involve C–H⋯Br hydrogen bonds with H⋯Br distances of 2.98 Å, and the RHBr value of 0.977 in 1 is a little shorter than the statistical RHBr value of 0.982, where RHBr = d(H⋯Br)/rH + rBr. Luminescence properties Emission spectra of powdered 1 are measured at different temperatures. It exhibits very strong temperature-dependent yellowish green emission (λem = 528–544 nm) upon excitation at 360 nm. As shown in Fig. 2, the emission intensity increases with the decrease of temperature, which may be attributed to the decrease of radiationless transition at low temperature.16 The lifetime of the emission band is 6.59 μs and the quantum yield (ϕPL) is 20.5% in aqueous solution at room temperature. In order to understand the emission mechanism, TD-DFT
Fig. 2 (a) Solid-state luminescence spectra of 1 at different temperatures; (b) photos of crystals 1 (left) under UV light (365 nm) and (right) natural light.
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calculations by using the model [Cu5Br6]− complex adapted from the X-ray data have been performed at the PBE1PBE level, which shows that the lowest singlet excitation is dominated by the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) transition. As shown in Fig. S4,† the electron densities of the HOMO are composed of 4p orbitals of Br− and 3d orbitals of Cu(I), while those of the LUMO are dominated by 4s orbitals of Cu(I). The energy gap between HOMO and LUMO is 3.49 eV (355 nm), which is close to the experimental value of 360 nm. Accordingly, the origin of the luminescence of 1 may be assigned as bromine-to-copper transition (XMCT) with a mixture of Cu [3d→4s] transition (MC). Due to the strong emission and high surface area coming from the tubular structure that can interact with guest molecules, one may expect that it could be a highly sensitive sensor. To examine the potential luminescence sensing behavior, the finely ground sample 1 (3 mg) was immersed in different aromatic solvents (4 mL), treated by ultrasonication for 30 min and then aged for 1 h to form a stable emulsion before the fluorescence study. The selected solvents were focused on two different categories of aromatic compounds containing electron-withdrawing groups and electron-donating groups, namely, anisole, phemethylol, toluene, benzene, chlorobenzene to nitrobenzene (NB). As shown in Fig. S5,† the fluorescence spectra of the emulsion exhibited a solventdependent behavior with a maximum peak at around 530 nm upon excitation at 358 nm. Interestingly, fluorescence enhancement or quenching for 1 as the substituent group of aromatic solutions was modified from electron-donating to electron-withdrawing, which may be explained by the donor–acceptor electron transfer mechanism.17 The strong influence of solvents on the emitting position and the intensity of fluorescent materials can be attributed to the solvent effect. The strongest and weakest emissions are exhibited in anisole and NB with intensity difference of at least one hundred times. Detection of nitro-explosives is important for security screening and environmental monitoring. Compared with the traditional detection methods, such as sophisticated instruments, fluorescence sensing is a much simple and very sensitive technique.18,19 To examine sensing sensitivity toward NB in more detail, a batch of emulsions of 1 dispersed in anisole with gradually increased NB concentration were prepared to monitor their emissive response. As shown in Fig. 3, the luminescence intensity of the emulsions significantly decreased with increase of nitrobenzene, and emission spectra were completely quenched at a concentration of 200 ppm for 1. Detectable luminescence responses were observed at a very low concentration of 20 ppm with a very high quenching efficiency of 94.90%, exhibiting extremely high detection sensitivity of 1 towards NB. Such solvent-dependent luminescence makes 1 to be a potential sensor for efficient detecting trace NB explosive. From the crystal structure of 1, we can find that the small pore size of this complex might exclude their encapsulation of the analytes, which indicates that the sensing mechanism of
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Fig. 3 Emission intensity of 1 in anisole solution upon excitation at 358 nm with different concentrations of NB. Insert: (a) photos of the emulsion of 1 in anisole solution without NB and (b) with 20 ppm NB.
our experiments might be different from most cases of guestinduced luminescence response reported.20,21 Therefore, this fluorescence quenching mechanism might be attributed to the photoinduced electron transfer from the excited state electrondonating cuprous bromide framework to the high electronwithdrawing NB adsorbed on the surface of the cuprous bromide tube materials.
Fig. 4 Calculated band gap, total density of states and the partial orbital density for 1.
Photocatalytic properties The UV-Vis diffuse reflectance spectrum shows that 1 can adsorb energy with a wavelength shorter than 365 nm. The band gap is determined to be 3.40 eV (Fig. S6†) when it is converted to the Kubelka–Munk function [hνF(R∞)]1/2 = A(hν − Eg). DFT calculation of the electronic band structure of [Cu5Br8]n3n− in 1 and density of states has been carried out with the CASTEP code.22 Results indicate that the top of valence bands (VBs) are mostly formed by Cu-3d states mixing with bromine4p states, while the bottom of the conduction bands (CBs) are almost a contribution from Cu-4s and Br-4s states (Fig. 4). The calculated energy band gap for the [Cu5Br8]n3n− tube (3.09 eV) is smaller than the experimental value (3.40 eV). The wellknown band gap problem based on the Kohn–Sham densityfunctional theory within local density or generalized gradient approximations contributes to this disagreement.23 Comparing the CB and VB energy levels of 1 with the various redox couples in water, suggests that the cuprous bromide material is a suitable photocatalyst, owing to its band edges located at energetically favorable positions (Fig. S7†). The band gap of 1 encouraged us to investigate photocatalytic degradation of organic dyes (anionic dyes MO, KR and cationic dyes MB, RhB, Fig. S8–S11†). The experimental results indicate that the residual concentration (C/C0) of the negatively charged MO (0.038) and KR (0.012) is far below the positively charged MB (0.283) and RhB (0.421), as shown in Fig. 5. For further comparison with the degradation efficiency of different dyes,24 the anionic dye MO and the cationic dye MB were equimolar mixed with 1 as the photocatalyst; after photocatalytic degradation, the residual amount of MB (0.296) was found to be much higher than that of MO (0.042) after 150 min (Fig. 6). This clearly indicated that the complete
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Fig. 5
Photodegradation of four organic dyes in the presence of 1.
Fig. 6 UV-vis spectra of an equimolar MO and MB mixture in the presence of 1, monitored with time.
decomposition of anionic dye MO by 1 is rarely influenced by the presence of cationic dye MB. In addition, compound 1 is still stable after six cycles of photocatalytic degradation of MO
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(15 hours, Fig. S12†), which has been verified by the PXRD patterns (Fig. S1†). In comparison, UV-driven photocatalysts TiO2 (P25, 25 nm, Eg = 3.2 eV, BET = 35.7 m2 g−1) and ZnO (nanorods, Eg = 3.4 eV, BET = 32.4 m2 g−1) were synthesized according to the literature.25 Photocatalytic degradation effectiveness of MO, KR, MB and RhB by TiO2 and ZnO nanorods is less than that of single crystal 1 (size in the 100–500 μm range, average size of ∼250 μm crystal bulk sample was used herein) under identical conditions. These results also proved that the photocatalysis reaction by single crystal 1 is selective degradation, while TiO2 and ZnO are not. More impressively, after the photocatalysis reaction, 1 can be easily filtered and collected without undergoing difficult procedures of separating the suspension of nanoscaled catalysts from the multi-phase reaction system (Fig. S13†). Several important factors may significantly contribute to the highly selective photocatalytic activity of the tubular cuprous bromide bulk crystal. (i) Exposed (002) facet is beneficial for the surface selective adsorption of anionic pollutants such as MO. Furthermore, the oxidation degradation of anionic pollutants is realized by the photo-induced hole on the valence bands. The interplanar spacing of the exposed (002) polar faces is 0.672 nm (confirmed by high resolution TEM, Fig. 7). Cationic Cu(I) sites of (002)
polar faces can attract anionic species such as MO due to the least spatial masking of terminated Br. As a result, anionic dyes can be directly oxidized by the photo-induced hole. For TiO2 or ZnO photocatalysis, the valence band exclusively consisted of the O-2p state which is unfavorable for anionic substrates approaching the cationic ion or molecule substrates. Thus the photo-induced hole for TiO2 or ZnO generally directly oxidizes H2O via the H-bond linked to the •OH radical, which diffuses and attacks to MO. Such an indirect oxidation obviously exhibits no selectivity for target pollutants with low efficiency owing to the easy capturing •OH radical by other co-existing substrates. (ii) More importantly, it is believed that the internal fields can promote the electron–hole separation. A bond-valence method is used to calculate the direction and magnitude of the dipole moments of the asymmetric unit of Cu(1)Br4, Cu(2) Br4 and Cu(3)Br4 tetrahedra of 1. The asymmetric unit of 6-coordinated TiO6 octahedron of anatase TiO2 and rutile TiO2, 4-coordinated ZnO4 tetrahedron unit of ZnO are also calculated for comparison, and the calculation results are shown in Table 2. The local dipole moments of the distorted Cu(1)Br4, Cu(2)Br4, Cu(3)Br4 tetrahedra (1.43 D, 1.07 D and 1.71 D) are larger than that of the ZnO4 tetrahedron of ZnO (0.78 D) and TiO6 octahedron of rutile TiO2 (0 D). Although the local dipole moment of the distorted TiO6 octahedron is large (7.86 D), the closed M–O octahedron structure will restrict the carrier separation and transfer.26 Therefore, the bulk photocatalyst 1 had much more reactivity than the nano-particles TiO2 or ZnO for selective degradation of anionic dye pollutants.
Conclusions
Fig. 7 (a) SEM image, (b) HRTEM image of single crystal 1, (c) hexagonal crystal system model, (d) Br-terminated (002) polar faces of 1.
Table 2
In conclusion, a perfectly aligned 63 helical tubular cuprous bromide single crystal has been prepared and characterized, which can selectively decompose negatively charged dyes of MO and KR, better than that of nanosized TiO2 and ZnO. Moreover, it shows very strong yellowish green emissions in the solid state and in emulsion, which result in extremely high detection sensitivity towards nitro-explosive via fluorescence quenching. The results suggest that it may be promising a
Magnitude dipole moment of the asymmetric unit of compound 1, ZnO, and TiO2
Dipole moment Polyhedron Tetrahedron Tetrahedron Octahedron
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Symmetric code Compound 1 Cu(1)Br4 (x,y,z) Cu(2)Br4 (x,y,z) Cu(3)Br4 (x,y,z) ZnO (Wurtzite) ZnO4 TiO2 (Anatase) Ti(1)O6 TiO2 (Rutile) Ti(1)O6
x
y
z
Magnitude (D)
−0.0379 −0.4330 1.4848
0.8056 0.7879 1.4105
1.1735 0 0.9129
1.4344 1.0722 1.7127
0.0001
0.0005
0.7800
0.7800
0
7.8591
0
0
0 0
−7.8591 0
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multifunctional material for photocatalysis, luminescence and sensing of nitro-explosives.
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Acknowledgements Financial support from the 973 Program (2012CB821701), the Ministry of Education of China (grant IRT1156), NSFC (21371147, 21365021 and 21401119), XJOYF 2013711009, XJGRI2013132 and ZR1201 is greatly appreciated.
Notes and references 1 S. Iijima, Nature, 1991, 354, 56. 2 (a) K. H. Kim, Y. Oh and M. F. Islam, Adv. Funct. Mater., 2013, 23, 377; (b) Q. Li, X.-F. Lu, H. Xu, Y.-X. Tong and G.-R. Li, ACS Appl. Mater. Interfaces, 2014, 6, 2726; (c) F. H. Su, M. H. Miao, H. T. Niu and Z. X. Wei, ACS Appl. Mater. Interfaces, 2014, 6, 2553. 3 (a) H. Zhang, H. Wang, Y. Xu, S. Zhuo, Y. Yu and B. Zhang, Angew. Chem., Int. Ed., 2012, 51, 1459; (b) G. X. Tong, J. G. Guan and Q. J. Zhang, Adv. Funct. Mater., 2013, 23, 2406. 4 (a) K. Otsubo, Y. Wakabayashi, J. Ohara, S. Yamamoto, H. Matsuzaki, H. Okamoto, K. Nitta, T. Uruga and H. Kitagawa, Nat. Mater., 2011, 10, 291; (b) G.-Q. Kong, S. Ou, C. Zou and C.-D. Wu, J. Am. Chem. Soc., 2012, 134, 19851; (c) F. Wang, Z.-S. Liu, H. Yang, Y.-X. Tan and J. Zhang, Angew. Chem., Int. Ed., 2011, 50, 450. 5 (a) X. X. Xu, C. Randorn, P. Efstathiou and J. T. S. Irvine, Nat. Mater., 2012, 11, 595; (b) C. Lavorato, A. Primo, R. Molinari and H. Garcia, Chem. – Eur. J., 2014, 20, 187; (c) M. Latorre-Sánchez, A. Primo and H. García, Angew. Chem., Int. Ed., 2013, 52, 11813. 6 (a) Y. J. Cui, Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev., 2012, 112, 1126; (b) H. Li and L. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 10502. 7 (a) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269; (b) U. M. K. Shahed, S. A. Mofareh and B. I. J. William, Science, 2002, 297, 2243. 8 G. H. Li, M. N. Dimitrijevic, L. Chen, M. J. Nichols, T. J. Rajh and K. A. Gray, J. Am. Chem. Soc., 2008, 130, 5402. 9 J. Sato, H. Kobayashi and Y. Inoue, J. Phys. Chem. B, 2003, 107, 7970. 10 (a) A. Neuba, R. Haase, W. Meyer-Klaucke, U. Florke and G. Henkel, Angew. Chem., Int. Ed., 2012, 51, 1714; (b) K. Ariga, Q. Ji, M. J. McShane, Y. M. Lvov, A. Vinu and J. P. Hill, Chem. Mater., 2011, 24, 728. 11 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb and J. R. Cheeseman, Gaussian 03, Revision C.01, Gaussian, Inc, Wallingford, CT, 2004.
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12 R. Dennington, K. Todd, J. Millam, K. Eppinnett, W. L. Hovell and R. Gilliland, Gauss View, version 3.09, Semichem, Inc, Shawnee Mission, KS, 2003. 13 H. Zhang, H. Wang, Y. Xu, S. Zhuo, Y. Yu and B. Zhang, Angew. Chem., Int. Ed., 2012, 51, 1459. 14 (a) T.-T. Luo, H.-C. Wu, Y.-C. Jao, S.-M. Huang, T.-W. Tseng, Y.-S. Wen, G.-H. Lee, S.-M. Peng and K.-L. Lu, Angew. Chem., Int. Ed., 2009, 48, 9461; (b) Y.-G. Huang, B. Mu, P. M. Schoenecker, C. G. Carson, J. R. Karra, Y. Cai and K. S. Walton, Angew. Chem., Int. Ed., 2011, 50, 436. 15 L. Brammer, E. A. Bruton and P. Sherwood, Cryst. Growth Des., 2001, 1, 277. 16 (a) M. Knorr, A. Pam, A. Khatyr, C. Strohmann, M. M. Kubicki, Y. Rousselin, S. M. Aly, D. Fortin and P. D. Harvey, Inorg. Chem., 2010, 49, 5834; (b) S. Perruchas, X. F. L. Goff, S. Maron, I. Maurin, F. Guillen, A. Garcia, T. Gacoin and J.-P. Boilot, J. Am. Chem. Soc., 2010, 132, 10967. 17 (a) S. Pramanik, C. Zheng, X. Zhang, T. J. Emge and J. Li, J. Am. Chem. Soc., 2011, 133, 4153; (b) H. Wang, W. T. Yang and Z.-M. Sun, Chem. – Asian J., 2013, 8, 982. 18 (a) M. Guo and Z.-M. Sun, J. Mater. Chem., 2012, 22, 15939; (b) Q. S. Zheng, F. L. Yang, M. L. Deng, Y. Ling, X. F. Liu, Z. X. Chen, Y. H. Wang, L. H. Weng and Y. M. Zhou, Inorg. Chem., 2013, 52, 10368. 19 (a) B. Gole, A. K. Bar and P. S. Mukherjee, Chem. Commun., 2011, 47, 12137; (b) S. Zhang, L. Han, L. Li, J. Cheng, D. Yuan and J. Luo, Cryst. Growth Des., 2013, 13, 5466. 20 (a) Y. Takashima, V. M. Martínez, S. Furukawa, M. Kondo, S. Shimomura, H. Uehara, M. Nakahama, K. Sugimoto and S. Kitagawa, Nat. Commun., 2011, 2, 168; (b) O. S. Wenger, Chem. Rev., 2013, 113, 3686. 21 (a) T. K. Kim, J. H. Lee, D. Moon and H. R. Moon, Inorg. Chem., 2013, 52, 589; (b) A. J. Lan, K. H. Li, H. H. Wu, D. H. Olson, T. J. Emge, W. Ki, M. C. Hong and J. Li, Angew. Chem., Int. Ed., 2009, 48, 2334. 22 (a) M. Segall, P. Linda, M. Probert, C. Pickard, P. Hasnip, S. Clark and M. Payne, Materials Studio CASTEP, version 2.2, Accelrys, San Diego, CA, 2002; (b) M. D. Segall, P. J. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark and M. C. Payne, J. Phys.: Condens. Matter, 2002, 14, 2717. 23 J. Yang and M. Dolg, J. Phys. Chem. B, 2006, 110, 19254. 24 (a) X. Zhao, X. H. Bu, T. Wu, S.-T. Zheng, L. Wang and P. Y. Feng, Nat. Commun., 2013, 4, 2344; (b) T. Wen, D.-X. Zhang and J. Zhang, Inorg. Chem., 2013, 52, 12. 25 L. Wang, L. X. Chang, L. Q. Wei, S. Z. Xu, M. H. Zeng and S. L. Pan, J. Mater. Chem., 2011, 21, 15732. 26 Q. Liu, Y. Zhou, J. Kou, X. Chen, Z. Tian, J. Gao, S. Yan and Z. Zou, J. Am. Chem. Soc., 2010, 132, 14385.
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Cite this: DOI: 10.1039/c4dt03657c
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A perfectly aligned 63 helical tubular cuprous bromide single crystal for selective photo-catalysis, luminescence and sensing of nitro-explosives† Ru-Xin Yao,a Reshalaiti Hailili,b Xin Cui,a Li Wang*b and Xian-Ming Zhang*a A perfectly aligned 63 helical tubular cuprous bromide single crystal has been synthesized and characterized, which can selectively decompose negatively charged dyes of Methyl Orange (MO) and Kermes Red (KR), and the photocatalytic efficiency is higher than that of nanosized (∼25 nm) TiO2 and ZnO. The direction and magnitude of the dipole moments as well as the band structure were calculated to reveal high photocatalytic efficiency. Moreover, luminescence studies indicate that the CuBr tube materials show very strong yellowish green emissions in the solid state and emulsion even at room temperature, and
Received 28th November 2014, Accepted 23rd December 2014
exhibit extremely high detection sensitivity towards nitro-explosives via fluorescence quenching. Detectable luminescence responses were observed at a very low concentration of 20 ppm with a high quench-
DOI: 10.1039/c4dt03657c
ing efficiency of 94.90%. The results suggest that they may be promising multifunctional materials for
www.rsc.org/dalton
photo-catalysis, luminescence and sensing of nitro-explosives.
Introduction Carbon nanotubes, discovered in 1991 by Iijima,1 show unusual properties and wide potential applications as catalysts, sensors, electrodes, and biological models resulting from their unique tubular structures, which have stimulated intensive exploration to nanotube structures containing carbon and non-carbon elements.2 To date, the known classes of tubular structures are overwhelmingly dominated by carbon nanotubes. Examples of non-carbon nanotubes are numbered,3 which include metal chalcogenides such as WS2, MoS2, VOx and TiO2, metal halide NiCl2, group III nitrides such as GaN and AlN, and newly developed metal–organic nanotubes,4 and helical non-carbon nanotubes are even less. Furthermore, it should be noted that the majority of tubular structures are electrically neutral, which can exist in the absence of templates or counterions. Charged tubular structures may show new or superior properties related to electrostatic interactions, but need to be stabilized by templates or counterions.
a Department of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, P. R. China. E-mail: [email protected] b College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi 830054, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Crystallographic studies, crystallographic data, calculation details, PXRD pattern, TGA/DTA plots, and photocatalytic properties. CCDC 945278. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03657c
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Light based multifunctional materials are important in energetic, environmental and secure fields. For example, photo-catalysis can split water to produce clean hydrogen energy and decompose organic contaminants that are difficult to be removed by conventional adsorption, filtration and even bio-degradation methods.5 Luminescent materials have potential applications in chemical sensors, light-emitting devices, medical diagnostics, and cell imaging.6 To date, photocatalytic materials have focused mainly on the metal oxides, sulfides, oxynitrides, and heterojunctions, especially recently reported metal–ion doped oxides such as In1−xNixTaO4.7 However, photocatalytic reactions based on most semiconductors are often accompanied by the formation of highly reactive radical species (e.g. •OH) that are typically active and nonselective, resulting in nearly total degradation of organic compounds. Moreover, the yield of active species is very low, and nonselective degradation is wasteful due to the destruction of innoxious organic components or even useful bio-enzymes. Thus it is still a great challenge to develop a new photocatalytic system with desirable selectivity. It is believed that the breaking of highly symmetric M–O configuration in general metal oxide semiconductors may create some photocatalytic selectivity. For example, famous “star” semiconductor TiO2 has a closed octahedral coordinated TiO6 unit, which is thought to be less active than low symmetric open tetrahedral Ti4+ sites that could be thought as defects stabilized via coordination with substrates.8 However, the existence of these defects always leads to low charge-separation
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efficiency. This may face a dilemma between the photocatalytic efficiency and the selectivity. Thus the search for new and available semiconductor materials, e.g. the photocatalytically active distorted polyhedron with a large dipole moment, may be crucial since the internal fields will promote the charge separation and obtain reactive selectivity.9 The Cu(I) ions commonly exhibit flexible coordination in cuprous halides, which contribute to luminescence, ionic conduction and high reactivity in materials and biochemistry,10 which encouraged us to synthesize cuprous halide materials for multifunction applications. In this article, we report a perfectly aligned 63 helical tubular cuprous bromide single crystal. It selectively decomposes anionic organic dyes MO and KR, due to their surface properties, large dipole moment afforded by the distorted CuBr4 tetrahedron structure, and the typical composition of the valence band. Moreover, it could also be used as a green luminescent material and a sensor for detecting nitro-explosives at the ppm level.
Experimental section Materials and physical measurements The FT-IR spectra were recorded from KBr pellets in the range of 400–4000 cm−1 on a Nicolet 5DX spectrometer. Elemental analysis was performed on a Vario EL III elemental analyzer. PXRD data were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, λ = 1.5418 Å). Thermal analysis (TGA) was carried out under an air atmosphere using SETARAM LABSYS equipment with a heating rate of 10 °C min−1. UV-vis absorption was monitored using a U-3310 spectrophotometer. The morphology was examined using transmission electron microscopy (TEM, JEOL, JEM-2100F, 200 kV) and field-emission scanning electron microscopy (SEM, JSM-7500F, 5 kV). Photoluminescence analysis was performed on an Edinburgh FLS920 luminescence spectrometer. Crystallographic studies X-ray single-crystal diffraction data of 1 were collected on an Agilent Technologies Gemini EOS diffractometer at 298 K using Mo Kα radiation (λ = 0.71073 Å). The program SAINT was
Table 1
used for integration of diffraction profiles, and the program SADABS was used for absorption correction. The structure was solved with the XS structure solution program by direct methods and refined by the full-matrix least-squares technique using Olex2. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of organic cations were generated theoretically onto the specific carbon atoms, and refined isotropically with fixed thermal factors. Further details for structural data are summarized in Table 1 and Table S1.† Calculation details Time-dependent density functional theory (TD-DFT) calculations were performed using the B3LYP functional. The LANL2DZ basis sets were employed for Cu, and 6-31+G basis sets were employed for Br. The initial ground-state geometry of [Cu5Br6]− directly obtained from the X-ray crystal structure has been used for calculations with the Gaussian 09 software package.11 The dimensional plots of molecular configurations and orbitals were generated with the Gauss View program.12 The electronic band structure along with density of states (DOS) for the anionic Cu5Br83− tube was calculated by DFT using the crystallographic data with the CASTEP code, which uses a plane wave basis set for the valence electrons and normconserving pseudopotential for the core electrons. The number of plane waves included in the basis was determined by a cutoff energy Ec of 550 eV. Pseudoatomic calculations were performed for Br-4s24p5 and Cu-3d104s1. The parameters used in the calculations and convergence criteria were set by the default values of the CASTEP code, for example, the eigen energy convergence tolerance 1.0 × 10−5 eV. The calculation method of the dipole moment The well-known Debye equation, μ = neR (μ is the net dipole moment in Debye (10−18 esu cm), n is the total number of electrons, e is the charge on an electron, −4.8 × 10−10 esu, and R is the difference in cm (between the “centroids” of positive and negative charge)) has been used to calculate the dipole moment.13 Distribution of the electrons on the M/O/N atoms was estimated using the bond valence theory (Si = exp[(Ro −
Crystal data for compound 1
Compound 1 Formula Fw Crystal system Space group a (Å) b (Å) c (Å) γ (°) V (Å3) Z ρcalc. (g cm−3) a
Cu15Br25C40H90N10 3662 Hexagonal P63/m 18.2711(8) 18.2711(8) 13.4431(12) 120 3886.5(5) 2 2.643
μ (mm−1) F(000) Crystal size (mm) Reflections Rint Tmax/Tmin Data/parameters S R1 a, wR2 b [I > 2σ(I)] R1, wR2 (all data) Δρmax/Δρmin (e Å−3)
16.871 2780 0.32 × 0.24 × 0.20 25 374/3341 0.0851 0.1334/0.0746 3341/81 1.014 0.0671, 0.1653 0.1273, 0.1883 1.65, −2.07
R1 = Fo − Fc/Fo. b wR2 = [w(Fo2 − Fc2)2/w(Fo2)2]1/2.
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Ri)/B]; where Ro is an empirical constant, Ri is the length of the bond “i” in Å, and B = 0.37).
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Synthesis of [(Me2DABCO)5(Cu15Br24)Br] (1) A mixture of CuBr2 (0.110 g, 0.5 mmol), DABCO (0.065 g, 0.30 mmol), MeOH (5 mL) and H2O (1 mL) was stirred and the pH value was adjusted with 1 drop HClO4 (71% w/w), and then it was sealed in a 15 mL Teflon-lined stainless autoclave and heated to 100 °C for 5 days. After it was cooled to room temperature and subjected to filtration, colorless block crystals of 1 were recovered in 73.5% yield based on CuBr2. Anal. calcd (%) for Cu15Br25C40H90N10 (1): C 13.12, H 2.48, N 3.82; found: C 13.26, H 2.67, N 3.74. IR data (KBr, cm−1): ν = 3424(vs), 2921(m), 2835(w), 1634(s), 1463(s), 1320(w), 1122(w), 1047(m), 842(m), 799(m), 697(w). Photocatalytic experiments For photocatalytic decomposition activity measurement, 50 mg of 1 was added to 100 mL of 1 × 10−5 mol L−1 aqueous solution of KR, MO, Methylene Blue (MB) and Rhodamine B (RhB), respectively. The suspensions were magnetically stirred in the dark for 30 min to ensure adsorption equilibrium and uniform dispersity. The solution was then exposed to UV irradiation from a 125 W high-pressure Hg lamp with the strongest emission at 365 nm (3.40 eV) at room temperature. After a given irradiation time, 5 mL of the mixture was withdrawn, and the catalysts were separated from the suspensions by centrifugation. The degradation process was monitored through a wavelength scan on U-3310 spectrophotometer.
Results and discussion Colorless columnar crystals of 1 in 73.5% yield were synthesized by the solvothermal treatment of a mixture of CuBr2, DABCO, MeOH and H2O at 100 °C for five days. FT-IR spectra, elemental and TGA confirmed the formula of 1. The phase purity of the bulky samples was confirmed by a PXRD study (Fig. S1†). TGA/DTA analysis shows that 1 is thermally stable up to 190 °C under an air atmosphere, and the methylated Me2DABCO cations are removed in the range of 190–340 °C (Fig. S2†). Single crystal X-ray analysis reveals that 1 consists of perfectly aligned 63 helical cuprous bromide tubes stabilized by in situ alkylated Me2DABCO templates. Compound 1 crystallizes in the hexagonal space group P63/m, and the asymmetric unit consists of three Cu(I) atoms, six bromides and one [Me2DABCO]2+ cation. All Cu(I) atoms show tetrahedral coordination geometry (Fig. S3†). The Cu(1) and Cu(3) atoms are coordinated by two μ2-Br atoms and two μ3-Br atoms with Cu–Br distances of 2.372(2)–2.929(1) Å and Br–Cu–Br angles of 98.745 (2)–128.12(10)°. The Cu(2) is coordinated by four μ3-Br atoms with Cu–Br distances of 2.463(3)–2.539(3) Å and Br–Cu–Br angles of 104.652(10)–115.44(6)°. It should be noted that five of the six Br atoms are bonded to Cu atoms in μ2 or μ3 modes, while the sixth Br(6) atom is free with symmetry of −6
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Fig. 1 View of the 63 helical Cu5Br83− tube (a) along the c-axis, (b) packing array of tubes with organic templates in 1.
(Wychoff letter 2c). The N(1) and C(2) atoms of Me2DABCO lie at threefold axis positions with a site occupancy of 1/3. The crystallographic mirror plane passes through Cu(2), Br(1), and Br(4) atoms. The Cu(3) tetrahedron is edge-shared with two Cu(1) and two Cu(2) tetrahedra via μ3-Br(3) atoms, thus forming a tortoiseshell-like Cu5Br10 unit. The Cu⋯Cu distances are in the range of 2.955(3)–3.1151(1) Å, which are longer than the sum of van der Waals radii of two Cu atoms (2.80 Å), indicative of the presence of very weak Cu⋯Cu interactions. The Cu5Br10 units are further linked together via sharing four peripheral μ2-Br(5) atoms to furnish an anionic cuprous bromide tube formulated as [Cu5Br8]n3n− with 4.1 nm interior diameter and 2.1 nm exterior diameter (Fig. 1a). Interestingly, the [Cu5Br8]n3n− tube is 63 helical with the pitch of 1.34 nm. Several helical tubes including single-, double- and hexahelical structures have been reported,14 and to the best of our knowledge, compound 1 is of the first cuprous bromide helical tube-like structure unit. The anionic [Cu5Br8]n3n− helical tubes are stabilized by [Me2DABCO]2+ cations that come from in situ methylation of the DABCO group in the presence of bromides (Fig. 1b). The charge balance requires that there are five [Me2DABCO]2+ cations per formula but only one ordered [Me2DABCO]2+ cation could be located from the Fourier map. The existence of additional four [Me2DABCO]2+ cations is confirmed by elemental and thermal analyses. A calculation by PLATON shows that there are 1400 Å3 voids per unit cell volume of 3886.5 Å3, which is approximately in agreement with the existence of additional four [Me2DABCO]2+ cations. Existence of not located guest molecules is a commonly observed phenomenon in porous structures, especially those crystallizing in high symmetric space groups. In spite of not being located from the X-ray Fourier map, the detailed analyses on the structure and charge density may give some clues on the possible locations of additional Me2DABCO groups. As can be seen along the c-axis direction, bromides project the tube both inward and outward, while
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copper atoms are sandwiched by bromides. Thus, both the interior and exterior walls of the [Cu5Br8]n3n− tube are negatively charged, which need to be neutralized by positively charged species. Otherwise, highly negative charge will result in the instability of the structure. After considering the van der Waals radii of bromide, the interior channel diameter of the [Cu5Br8]n3n− tube is 4.1 Å, which indicates that the tube provides substantial interior space for accommodating molecules. In addition, one may find that there are windows sized 6.1 Å × 6.1 Å (9.79 Å × 9.79 Å without considering van der Waals radii of Br atoms) along the propagating direction of the tube, which also provides space for Me2DABCO cations. The [Cu5Br8]n3n− tubes are stabilized by [Me2DABCO]2+ cations via C–H⋯Br hydrogen bonds to form a 3D supramolecular array. Metal-bound halogens (M–X) and halide ions (X−) have been shown to be very good hydrogen bond acceptors, and the donor atoms can be C, N and O atoms. Based on the analyses of thousands of crystal structures in the CCDC database, Brammer pointed geometric preferences for C–H⋯X–M interactions in metal-bound halogens.15 The C and H atoms from DABCO and Br(3) involve C–H⋯Br hydrogen bonds with H⋯Br distances of 2.98 Å, and the RHBr value of 0.977 in 1 is a little shorter than the statistical RHBr value of 0.982, where RHBr = d(H⋯Br)/rH + rBr. Luminescence properties Emission spectra of powdered 1 are measured at different temperatures. It exhibits very strong temperature-dependent yellowish green emission (λem = 528–544 nm) upon excitation at 360 nm. As shown in Fig. 2, the emission intensity increases with the decrease of temperature, which may be attributed to the decrease of radiationless transition at low temperature.16 The lifetime of the emission band is 6.59 μs and the quantum yield (ϕPL) is 20.5% in aqueous solution at room temperature. In order to understand the emission mechanism, TD-DFT
Fig. 2 (a) Solid-state luminescence spectra of 1 at different temperatures; (b) photos of crystals 1 (left) under UV light (365 nm) and (right) natural light.
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calculations by using the model [Cu5Br6]− complex adapted from the X-ray data have been performed at the PBE1PBE level, which shows that the lowest singlet excitation is dominated by the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) transition. As shown in Fig. S4,† the electron densities of the HOMO are composed of 4p orbitals of Br− and 3d orbitals of Cu(I), while those of the LUMO are dominated by 4s orbitals of Cu(I). The energy gap between HOMO and LUMO is 3.49 eV (355 nm), which is close to the experimental value of 360 nm. Accordingly, the origin of the luminescence of 1 may be assigned as bromine-to-copper transition (XMCT) with a mixture of Cu [3d→4s] transition (MC). Due to the strong emission and high surface area coming from the tubular structure that can interact with guest molecules, one may expect that it could be a highly sensitive sensor. To examine the potential luminescence sensing behavior, the finely ground sample 1 (3 mg) was immersed in different aromatic solvents (4 mL), treated by ultrasonication for 30 min and then aged for 1 h to form a stable emulsion before the fluorescence study. The selected solvents were focused on two different categories of aromatic compounds containing electron-withdrawing groups and electron-donating groups, namely, anisole, phemethylol, toluene, benzene, chlorobenzene to nitrobenzene (NB). As shown in Fig. S5,† the fluorescence spectra of the emulsion exhibited a solventdependent behavior with a maximum peak at around 530 nm upon excitation at 358 nm. Interestingly, fluorescence enhancement or quenching for 1 as the substituent group of aromatic solutions was modified from electron-donating to electron-withdrawing, which may be explained by the donor–acceptor electron transfer mechanism.17 The strong influence of solvents on the emitting position and the intensity of fluorescent materials can be attributed to the solvent effect. The strongest and weakest emissions are exhibited in anisole and NB with intensity difference of at least one hundred times. Detection of nitro-explosives is important for security screening and environmental monitoring. Compared with the traditional detection methods, such as sophisticated instruments, fluorescence sensing is a much simple and very sensitive technique.18,19 To examine sensing sensitivity toward NB in more detail, a batch of emulsions of 1 dispersed in anisole with gradually increased NB concentration were prepared to monitor their emissive response. As shown in Fig. 3, the luminescence intensity of the emulsions significantly decreased with increase of nitrobenzene, and emission spectra were completely quenched at a concentration of 200 ppm for 1. Detectable luminescence responses were observed at a very low concentration of 20 ppm with a very high quenching efficiency of 94.90%, exhibiting extremely high detection sensitivity of 1 towards NB. Such solvent-dependent luminescence makes 1 to be a potential sensor for efficient detecting trace NB explosive. From the crystal structure of 1, we can find that the small pore size of this complex might exclude their encapsulation of the analytes, which indicates that the sensing mechanism of
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Fig. 3 Emission intensity of 1 in anisole solution upon excitation at 358 nm with different concentrations of NB. Insert: (a) photos of the emulsion of 1 in anisole solution without NB and (b) with 20 ppm NB.
our experiments might be different from most cases of guestinduced luminescence response reported.20,21 Therefore, this fluorescence quenching mechanism might be attributed to the photoinduced electron transfer from the excited state electrondonating cuprous bromide framework to the high electronwithdrawing NB adsorbed on the surface of the cuprous bromide tube materials.
Fig. 4 Calculated band gap, total density of states and the partial orbital density for 1.
Photocatalytic properties The UV-Vis diffuse reflectance spectrum shows that 1 can adsorb energy with a wavelength shorter than 365 nm. The band gap is determined to be 3.40 eV (Fig. S6†) when it is converted to the Kubelka–Munk function [hνF(R∞)]1/2 = A(hν − Eg). DFT calculation of the electronic band structure of [Cu5Br8]n3n− in 1 and density of states has been carried out with the CASTEP code.22 Results indicate that the top of valence bands (VBs) are mostly formed by Cu-3d states mixing with bromine4p states, while the bottom of the conduction bands (CBs) are almost a contribution from Cu-4s and Br-4s states (Fig. 4). The calculated energy band gap for the [Cu5Br8]n3n− tube (3.09 eV) is smaller than the experimental value (3.40 eV). The wellknown band gap problem based on the Kohn–Sham densityfunctional theory within local density or generalized gradient approximations contributes to this disagreement.23 Comparing the CB and VB energy levels of 1 with the various redox couples in water, suggests that the cuprous bromide material is a suitable photocatalyst, owing to its band edges located at energetically favorable positions (Fig. S7†). The band gap of 1 encouraged us to investigate photocatalytic degradation of organic dyes (anionic dyes MO, KR and cationic dyes MB, RhB, Fig. S8–S11†). The experimental results indicate that the residual concentration (C/C0) of the negatively charged MO (0.038) and KR (0.012) is far below the positively charged MB (0.283) and RhB (0.421), as shown in Fig. 5. For further comparison with the degradation efficiency of different dyes,24 the anionic dye MO and the cationic dye MB were equimolar mixed with 1 as the photocatalyst; after photocatalytic degradation, the residual amount of MB (0.296) was found to be much higher than that of MO (0.042) after 150 min (Fig. 6). This clearly indicated that the complete
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Fig. 5
Photodegradation of four organic dyes in the presence of 1.
Fig. 6 UV-vis spectra of an equimolar MO and MB mixture in the presence of 1, monitored with time.
decomposition of anionic dye MO by 1 is rarely influenced by the presence of cationic dye MB. In addition, compound 1 is still stable after six cycles of photocatalytic degradation of MO
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(15 hours, Fig. S12†), which has been verified by the PXRD patterns (Fig. S1†). In comparison, UV-driven photocatalysts TiO2 (P25, 25 nm, Eg = 3.2 eV, BET = 35.7 m2 g−1) and ZnO (nanorods, Eg = 3.4 eV, BET = 32.4 m2 g−1) were synthesized according to the literature.25 Photocatalytic degradation effectiveness of MO, KR, MB and RhB by TiO2 and ZnO nanorods is less than that of single crystal 1 (size in the 100–500 μm range, average size of ∼250 μm crystal bulk sample was used herein) under identical conditions. These results also proved that the photocatalysis reaction by single crystal 1 is selective degradation, while TiO2 and ZnO are not. More impressively, after the photocatalysis reaction, 1 can be easily filtered and collected without undergoing difficult procedures of separating the suspension of nanoscaled catalysts from the multi-phase reaction system (Fig. S13†). Several important factors may significantly contribute to the highly selective photocatalytic activity of the tubular cuprous bromide bulk crystal. (i) Exposed (002) facet is beneficial for the surface selective adsorption of anionic pollutants such as MO. Furthermore, the oxidation degradation of anionic pollutants is realized by the photo-induced hole on the valence bands. The interplanar spacing of the exposed (002) polar faces is 0.672 nm (confirmed by high resolution TEM, Fig. 7). Cationic Cu(I) sites of (002)
polar faces can attract anionic species such as MO due to the least spatial masking of terminated Br. As a result, anionic dyes can be directly oxidized by the photo-induced hole. For TiO2 or ZnO photocatalysis, the valence band exclusively consisted of the O-2p state which is unfavorable for anionic substrates approaching the cationic ion or molecule substrates. Thus the photo-induced hole for TiO2 or ZnO generally directly oxidizes H2O via the H-bond linked to the •OH radical, which diffuses and attacks to MO. Such an indirect oxidation obviously exhibits no selectivity for target pollutants with low efficiency owing to the easy capturing •OH radical by other co-existing substrates. (ii) More importantly, it is believed that the internal fields can promote the electron–hole separation. A bond-valence method is used to calculate the direction and magnitude of the dipole moments of the asymmetric unit of Cu(1)Br4, Cu(2) Br4 and Cu(3)Br4 tetrahedra of 1. The asymmetric unit of 6-coordinated TiO6 octahedron of anatase TiO2 and rutile TiO2, 4-coordinated ZnO4 tetrahedron unit of ZnO are also calculated for comparison, and the calculation results are shown in Table 2. The local dipole moments of the distorted Cu(1)Br4, Cu(2)Br4, Cu(3)Br4 tetrahedra (1.43 D, 1.07 D and 1.71 D) are larger than that of the ZnO4 tetrahedron of ZnO (0.78 D) and TiO6 octahedron of rutile TiO2 (0 D). Although the local dipole moment of the distorted TiO6 octahedron is large (7.86 D), the closed M–O octahedron structure will restrict the carrier separation and transfer.26 Therefore, the bulk photocatalyst 1 had much more reactivity than the nano-particles TiO2 or ZnO for selective degradation of anionic dye pollutants.
Conclusions
Fig. 7 (a) SEM image, (b) HRTEM image of single crystal 1, (c) hexagonal crystal system model, (d) Br-terminated (002) polar faces of 1.
Table 2
In conclusion, a perfectly aligned 63 helical tubular cuprous bromide single crystal has been prepared and characterized, which can selectively decompose negatively charged dyes of MO and KR, better than that of nanosized TiO2 and ZnO. Moreover, it shows very strong yellowish green emissions in the solid state and in emulsion, which result in extremely high detection sensitivity towards nitro-explosive via fluorescence quenching. The results suggest that it may be promising a
Magnitude dipole moment of the asymmetric unit of compound 1, ZnO, and TiO2
Dipole moment Polyhedron Tetrahedron Tetrahedron Octahedron
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Symmetric code Compound 1 Cu(1)Br4 (x,y,z) Cu(2)Br4 (x,y,z) Cu(3)Br4 (x,y,z) ZnO (Wurtzite) ZnO4 TiO2 (Anatase) Ti(1)O6 TiO2 (Rutile) Ti(1)O6
x
y
z
Magnitude (D)
−0.0379 −0.4330 1.4848
0.8056 0.7879 1.4105
1.1735 0 0.9129
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multifunctional material for photocatalysis, luminescence and sensing of nitro-explosives.
Published on 20 January 2015. Downloaded by New Mexico State University on 21/01/2015 08:39:37.
Acknowledgements Financial support from the 973 Program (2012CB821701), the Ministry of Education of China (grant IRT1156), NSFC (21371147, 21365021 and 21401119), XJOYF 2013711009, XJGRI2013132 and ZR1201 is greatly appreciated.
Notes and references 1 S. Iijima, Nature, 1991, 354, 56. 2 (a) K. H. Kim, Y. Oh and M. F. Islam, Adv. Funct. Mater., 2013, 23, 377; (b) Q. Li, X.-F. Lu, H. Xu, Y.-X. Tong and G.-R. Li, ACS Appl. Mater. Interfaces, 2014, 6, 2726; (c) F. H. Su, M. H. Miao, H. T. Niu and Z. X. Wei, ACS Appl. Mater. Interfaces, 2014, 6, 2553. 3 (a) H. Zhang, H. Wang, Y. Xu, S. Zhuo, Y. Yu and B. Zhang, Angew. Chem., Int. Ed., 2012, 51, 1459; (b) G. X. Tong, J. G. Guan and Q. J. Zhang, Adv. Funct. Mater., 2013, 23, 2406. 4 (a) K. Otsubo, Y. Wakabayashi, J. Ohara, S. Yamamoto, H. Matsuzaki, H. Okamoto, K. Nitta, T. Uruga and H. Kitagawa, Nat. Mater., 2011, 10, 291; (b) G.-Q. Kong, S. Ou, C. Zou and C.-D. Wu, J. Am. Chem. Soc., 2012, 134, 19851; (c) F. Wang, Z.-S. Liu, H. Yang, Y.-X. Tan and J. Zhang, Angew. Chem., Int. Ed., 2011, 50, 450. 5 (a) X. X. Xu, C. Randorn, P. Efstathiou and J. T. S. Irvine, Nat. Mater., 2012, 11, 595; (b) C. Lavorato, A. Primo, R. Molinari and H. Garcia, Chem. – Eur. J., 2014, 20, 187; (c) M. Latorre-Sánchez, A. Primo and H. García, Angew. Chem., Int. Ed., 2013, 52, 11813. 6 (a) Y. J. Cui, Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev., 2012, 112, 1126; (b) H. Li and L. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 10502. 7 (a) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269; (b) U. M. K. Shahed, S. A. Mofareh and B. I. J. William, Science, 2002, 297, 2243. 8 G. H. Li, M. N. Dimitrijevic, L. Chen, M. J. Nichols, T. J. Rajh and K. A. Gray, J. Am. Chem. Soc., 2008, 130, 5402. 9 J. Sato, H. Kobayashi and Y. Inoue, J. Phys. Chem. B, 2003, 107, 7970. 10 (a) A. Neuba, R. Haase, W. Meyer-Klaucke, U. Florke and G. Henkel, Angew. Chem., Int. Ed., 2012, 51, 1714; (b) K. Ariga, Q. Ji, M. J. McShane, Y. M. Lvov, A. Vinu and J. P. Hill, Chem. Mater., 2011, 24, 728. 11 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb and J. R. Cheeseman, Gaussian 03, Revision C.01, Gaussian, Inc, Wallingford, CT, 2004.
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Paper
12 R. Dennington, K. Todd, J. Millam, K. Eppinnett, W. L. Hovell and R. Gilliland, Gauss View, version 3.09, Semichem, Inc, Shawnee Mission, KS, 2003. 13 H. Zhang, H. Wang, Y. Xu, S. Zhuo, Y. Yu and B. Zhang, Angew. Chem., Int. Ed., 2012, 51, 1459. 14 (a) T.-T. Luo, H.-C. Wu, Y.-C. Jao, S.-M. Huang, T.-W. Tseng, Y.-S. Wen, G.-H. Lee, S.-M. Peng and K.-L. Lu, Angew. Chem., Int. Ed., 2009, 48, 9461; (b) Y.-G. Huang, B. Mu, P. M. Schoenecker, C. G. Carson, J. R. Karra, Y. Cai and K. S. Walton, Angew. Chem., Int. Ed., 2011, 50, 436. 15 L. Brammer, E. A. Bruton and P. Sherwood, Cryst. Growth Des., 2001, 1, 277. 16 (a) M. Knorr, A. Pam, A. Khatyr, C. Strohmann, M. M. Kubicki, Y. Rousselin, S. M. Aly, D. Fortin and P. D. Harvey, Inorg. Chem., 2010, 49, 5834; (b) S. Perruchas, X. F. L. Goff, S. Maron, I. Maurin, F. Guillen, A. Garcia, T. Gacoin and J.-P. Boilot, J. Am. Chem. Soc., 2010, 132, 10967. 17 (a) S. Pramanik, C. Zheng, X. Zhang, T. J. Emge and J. Li, J. Am. Chem. Soc., 2011, 133, 4153; (b) H. Wang, W. T. Yang and Z.-M. Sun, Chem. – Asian J., 2013, 8, 982. 18 (a) M. Guo and Z.-M. Sun, J. Mater. Chem., 2012, 22, 15939; (b) Q. S. Zheng, F. L. Yang, M. L. Deng, Y. Ling, X. F. Liu, Z. X. Chen, Y. H. Wang, L. H. Weng and Y. M. Zhou, Inorg. Chem., 2013, 52, 10368. 19 (a) B. Gole, A. K. Bar and P. S. Mukherjee, Chem. Commun., 2011, 47, 12137; (b) S. Zhang, L. Han, L. Li, J. Cheng, D. Yuan and J. Luo, Cryst. Growth Des., 2013, 13, 5466. 20 (a) Y. Takashima, V. M. Martínez, S. Furukawa, M. Kondo, S. Shimomura, H. Uehara, M. Nakahama, K. Sugimoto and S. Kitagawa, Nat. Commun., 2011, 2, 168; (b) O. S. Wenger, Chem. Rev., 2013, 113, 3686. 21 (a) T. K. Kim, J. H. Lee, D. Moon and H. R. Moon, Inorg. Chem., 2013, 52, 589; (b) A. J. Lan, K. H. Li, H. H. Wu, D. H. Olson, T. J. Emge, W. Ki, M. C. Hong and J. Li, Angew. Chem., Int. Ed., 2009, 48, 2334. 22 (a) M. Segall, P. Linda, M. Probert, C. Pickard, P. Hasnip, S. Clark and M. Payne, Materials Studio CASTEP, version 2.2, Accelrys, San Diego, CA, 2002; (b) M. D. Segall, P. J. D. Lindan, M. J. Probert, C. J. Pickard, P. J. Hasnip, S. J. Clark and M. C. Payne, J. Phys.: Condens. Matter, 2002, 14, 2717. 23 J. Yang and M. Dolg, J. Phys. Chem. B, 2006, 110, 19254. 24 (a) X. Zhao, X. H. Bu, T. Wu, S.-T. Zheng, L. Wang and P. Y. Feng, Nat. Commun., 2013, 4, 2344; (b) T. Wen, D.-X. Zhang and J. Zhang, Inorg. Chem., 2013, 52, 12. 25 L. Wang, L. X. Chang, L. Q. Wei, S. Z. Xu, M. H. Zeng and S. L. Pan, J. Mater. Chem., 2011, 21, 15732. 26 Q. Liu, Y. Zhou, J. Kou, X. Chen, Z. Tian, J. Gao, S. Yan and Z. Zou, J. Am. Chem. Soc., 2010, 132, 14385.
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