A Waugh type [CoMo9O32]6− cluster with atomically dispersed CoIV originates from Anderson type [CoMo6O24]3− for photocatalytic oxygen molecule activation

Yongdong Chen ab, Chaolei Zhang ab, Caoping Yang a, Jiangwei Zhang *b, Kai Zheng b, Qihua Fang b and Gao Li *b
aThe Center of New Energy Materials and Technology, College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, China
bGold Catalysis Research Center, State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China. E-mail: gaoli@dicp.ac.cn; jwzhang@dicp.ac.cn

Received 18th June 2017 , Accepted 10th July 2017

First published on 12th July 2017


An atomically dispersed Waugh type [CoMo9O32]6− cluster is obtained, employing the most flexible structure unit Anderson type [Co(OH)6Mo6O18]3− as a precursor. The structure of the [CoMo9O32]6− cluster is identified by single crystal X-ray diffraction and also well characterized by FT-IR, ESI-MS, UV-Vis, EA, and TGA spectroscopy. Its 3D framework forms a quasi 2D material and possesses curved edge triangle shape nanopores with a diameter of 8.9 Å. The CoIV and MoVI oxidation states and the related valence band and electronic state of Co are definitely confirmed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), and bond valence sum (BVS). The [CoMo9O32]6− cluster is a typical n-type inorganic semiconductor with a HOMO–LOMO gap of ca. 1.67 eV and exhibits reversible two-electron redox properties, evidenced by UPS, cyclic voltammetric (CV), and Mott–Schottky plot analyses. Furthermore, [CoMo9O32]6− can effectively generate 1O2 under laser (365 and 532 nm) and sunlight irradiation, detected using a water-soluble DAB probe. Such an n-type multielectron reservoir semiconductor anionic [CoMo9O32]6− cluster with thermal and electrochemical stability as an effective photosensitizer serves as a promising material in solar energy scavenging.


Introduction

Polyoxometalates (POMs) are an exceptional family of inorganic clusters consisting of early transition metal ions (e.g., Mo, W, V, etc.) in their highest oxidation states with structural versatility and a wide range of properties and applications.1–4 POMs can be divided into two main categories, i.e., isopolyanion and heteropolyanion. Isopolyanion is comprised of only one element bridging by O atoms. While, the heteropolyanion incorporates functional heteroatoms (e.g., transition metals, noble metals, and rare earth metals) to make the design of advanced functional materials more accessible and rational.5,6 Among these heteroatom candidates, the Co element has gained much attention due to its charming intrinsic catalytic applications in energy scavenging, such as water splitting and water–gas shift.7–10 Since Keggin first reported the X-ray crystal structure of 1[thin space (1/6-em)]:[thin space (1/6-em)]12 heteropolyanions in 1934, the basic POM topology structure units of Keggin, Anderson, Dawson, Waugh, Silverton, and Lindqvist have been sequentially discovered, forming six basic structural branches of the POM family.1

The discovery, exploration, and extension of the basic POM topology structure have always been the frontier and most urgent issue of POM chemistry. The Anderson type POM family stands out as one of the most adaptable and tunable subclasses benefitting from the diversity of the central heteroatom, which is accessible to many elements, such as transition and noble metals. And recently the direct triol functionalization of Anderson clusters11–15 has been intensively conducted and exhibited fascinating properties, such as catalysis,2 biology applications, and a few to name.16,17 The species of Waugh type [MnMo9O32]6− clusters isolated with only a single type of heteroatom MnIV was first reported in 1953.18 And the further related investigations have been ignored for a very long time until its intrinsic chiral topology and the corresponding spontaneous resolution were recently investigated.19,20 An obstacle to effectively extend heteroatoms into a Waugh type cluster should be conquered to enrich such an important and unique branch of the fundamental POM topology structure.

Two-dimensional (2D) materials (e.g., graphene, transition metal dichalcogenides, and metal oxides) have gained intensive attention, due to their versatile characteristics.21 The oxidative and photocatalytic nature of POMs has been prominently featured in the field of catalysis in both scientific investigations and industrial projects.1,2 However, the high lattice energy of POMs makes it difficult to design POM cluster based 2D functional materials. The extension of POM clusters into the field of low dimensional materials is still a major and challenging task. The key resides in the design of specific POM topology structures with suitable cations. Atomically precise heteropolyanion clusters containing transition and noble metals with their ionic nature serves as a promising precursor to form transition/noble metals with atomically dispersed single-atom 2D functional materials.

Herein, we report a new protocol to conveniently obtain an atomically dispersed Waugh type [CoMo9O32]6− cluster which is derived from Anderson type [Co(OH)6Mo6O18]3−. The 3D structure of [CoMo9O32]6− forms a quasi 2D material and possesses curved edge triangle shape nanopores with a diameter of 8.9 Å. Finally, its photocatalytic properties for oxygen molecule activation were further investigated.

Experimental

General methods and materials

The [Co(OH)6Mo6O18]3− cluster was synthesized according to a literature method.22 All syntheses and manipulations were performed in open air. All other chemicals, including solvents, were commercially available as reagent grade and used as received without further purification from Adamas-beta®. Fourier transform infrared spectroscopy (FT-IR) was performed on a PerkinElmer FT-IR spectrometer. UV-Vis spectra were measured in acetonitrile using an Agilent Cary 300 spectrophotometer. The mass spectra were obtained using an ion trap mass spectrometer (Thermofisher LTQ). Negative mode was chosen for the experiments (capillary voltage 33 V). Sample solution (in H2O) was infused into the ESI source at a flow rate of 300 μL min−1. Elemental analyses of C, H, and N were performed using an Elementar Analysensysteme GmbH (vario EL) while the elemental analyses of metallic elements were performed using an X-ray fluorescence (XRF) element analyzer PANalytical Epsilon 5. Brunauer–Emmett–Teller (BET) surface areas and pore volumes were evaluated on a sorption analyzer Micromeritics ASAP 2020 using nitrogen adsorption at 77 K. X-ray photoelectron spectroscopy (XPS) measurements were performed under ultrahigh vacuum (UHV) with 1.0 × 10−7 Torr, axis HS monochromatized Al Kα cathode source at 150 W, focused X-ray 100 μm beam, Pass energy: 55 eV with 0.1 eV step length, detect angle (take off): 45° on an X-ray microprobe (ULVAC-PHI Quantera SXM). The binding energy was calibrated with C 1s = 284.8 eV. Ultraviolet photoemission spectroscopy (UPS) was performed on an Omicron HA100 electron energy analyzer using a UVS 300 high current UV lamp with an excitation energy of He II (40.8 eV). Cyclic voltammetric (CV) analysis was carried out using a CHI660E electrochemical workstation. The saturated calomel electrode (SCE), platinum wire and glassy carbon (GC) were invoked as the reference, counter and working electrodes, respectively. The aqueous solution of 0.5 mM compound 1 and 0.5 M Na2SO4 was adjusted to pH = 1.0. For Mott–Schottky plot measurement, the aqueous solution of 0.5 mM compound 1 and 0.5 M Na2SO4 was adjusted to pH = 6.0. Thermal gravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA851 in flowing air of 50.0 ml min−1 at a heating rate of 20 °C min−1.

The synthesis of (NH4)6[CoMo9O32], compound 1

1.097 g [NH4]3[Co(OH)6Mo6O18] was dissolved in 10 mL H2O. And then the solution was acidified using 1 M HCl to pH 4.0 by refluxing for 0.5 h to isomerize to a β-isomer. Furthermore, 0.615 g Na2MoO4 and 1 ml H2O2 was added into the solution and the pH was adjusted to 2.0 with vigorous stirring and refluxing for 2 h. Then the solvents were removed by evaporation. The compound 1 was obtained as atrovirens crystalline products (80% yields based on Mo).

CoH24Mo9N6O32, Mr = 1542.60, H 1.74, N 5.56, Co 4.01, Mo 55.69, while calc. H 1.57, N 5.45, Co 3.82, Mo 55.97. IR (major absorbances, cm−1): 3952, 3741, 3502, 3186, 2544, 2360, 2136, 2024, 1609, 1404, 886, and 725. UV-Vis (H2O, nm): λLMCT = 248, λd–d = 604. ESI mass spectrometry (H2O): calc. m/z (z = 6) = 239.06 [CoMo9O32]6−; found 238.95.

The crystallization of compound 1: (NH4)6[CoMo9O32]·12H2O, CoH48Mo9N6O44, Mr = 1758.78.

1.54 g (NH4)6[CoMo9O32] was redissolved in 20 mL H2O. An additional 0.2 g NH4Cl was added into the solution to accelerate the crystallization process. Suitable single crystals for X-ray diffraction were grown by slow evaporation in air.

The synthesis of (NH4) [Co(OH)8Mo6O16]·8H2O, compound 2

1.097 g [NH4]3[Co(OH)6Mo6O18] was dissolved in 10 mL H2O. The solution was acidified using 1 M HCl to pH 2.0 with vigorous stirring and refluxing for 2 h. Suitable single crystals for X-ray diffraction were also grown by slow evaporation. After crystallization, compound 2 was obtained as green crystalline products. CoH28Mo6NO32, Mr = 1188.78.

X-ray crystallography

Suitable single crystals were selected. Data collections were performed using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Data reduction, cell refinement and experimental absorption correction were performed using the software package of Agilent Gemini Ultra CrysAlisPro (Ver 1.171.35.11). The structures were solved by direct methods and refined against F2 by full-matrix least-squares. All non-hydrogen atoms were refined anisotropically. All calculations were carried out using the program package of SHELXTL ver 5.1 and Olex2 ver 1.2.8.23,24
Crystal data and structure refinement for compounds 1 and 2. Crystal data for 1: (NH4)6[CoMo9O32]·12H2O, CoH48Mo9N6O44, Mr = 1758.78, trigonal, space group R32, a = b = 16.1035(7), c = 12.2637(6) Å, V = 2754.2(3) Å3, Z = 3, T = 293(2) K, 1598 reflections measured, R1 = 0.0387, wR2 = 0.0866.

Crystal data for 2: (NH4)[Co(OH)8Mo6O16]·8H2O, CoH28Mo6NO32, Mr = 1188.78, monoclinic, space group P21/c, a = 11.432(2) Å, b = 10.997(2) Å, c = 11.725(2) Å, β = 99.86(3)°, V = 1452.2(5) Å3, Z = 2, T = 293(2) K, 3293 reflections measured, R1 = 0.0588, wR2 = 0.1184.

CCDC 1550103 and 1550104 contain the supplementary crystallographic data for compounds 1 and 2 respectively in this paper.

Detection of 1O2

Water-soluble 3,3′-diaminobenzidine (DAB) probe detection method: 10 mg DAB and 2 mg compound 1 were dissolved in 5 mL aqueous solution. The solution was diluted 10 times. And then, the solution was irradiated under two types of light sources: a light-emitting diode (LED) at a wavelength of 365 nm (∼13 mW cm−2, 50 mW purple laser, TANK Co., Ltd, China) and 532 nm (∼34 mW cm−2, 50 mW green laser, Warnlaser, USA) and sunlight and convex lens focused sunlight. The adsorption spectra were recorded at different periods of light irradiation at 0, 1, 2, 4, 9, 15 and 20 min, respectively. A control experiment was also conducted under the same laser irradiated conditions without photosensitizer [CoMo9O32]6−. The comparison of 1O2 generation by the [CoMo9O32]6− cluster with a conventional dye photosensitizer (new methylene blue, NMB) was also conducted using the same protocol.

Results and discussion

Crystal structure and spectra characterization

The Waugh type cluster was obtained through two main steps, resulting from the Anderson type cluster. In the first step, the well-known flat hexagonal Anderson type α-isomer firstly isomerized to a butterfly-shaped β-isomer, Scheme 1. Such a process has been previously investigated in the [Pt(OH)4Mo6O20]4− Anderson cluster with high-valence noble metal heteroatoms.25 However, butterfly-shaped isomers with low-valence transition metals as central heteroatoms have not been achieved yet. Butterfly-shaped β-isomers of Anderson type POM clusters with low-valence transition metals are less stable than those of flat Anderson type POM clusters, and thus almost all Anderson clusters possessed a flat hexagonal topology. Further, the additional introduced [MoO4]2− building units are similarly capped on the position of the butterfly-shaped topology structure where the triol ligands covalently functionalized to stabilize such a butterfly-shape topology, and finally gave rise to the Waugh type topology structure.11–15
image file: c7nr04373b-s1.tif
Scheme 1 Synthesis of the Waugh type [CoMo9O32]6− cluster derived from Anderson type [Co(OH)6Mo6O18]3−.

To verify the feasibility of such a new protocol to obtain the Waugh type cluster, the parent Anderson cluster α-[Co(OH)6Mo6O18]3− was first dissolved and acidified using 1 M HCl to pH 4.0 with refluxing and stirring for 0.5 h. Then [MoO4]2− of a triple stoichiometric ratio and 1 ml H2O2 are added and the pH was adjusted to 2.0 with vigorous stirring and refluxing for additional 2 h. The green solution of α-[Co(OH)6Mo6O18]3− gradually turned to atrovirens, implying that the Waugh type (NH4)6[CoMo9O32] cluster (compound 1) was finally obtained (Fig. S1, for synthetic details see the Experimental section).

Suitable atrovirens block single crystals for X-ray diffraction were grown by solvent evaporation. Single crystal X-ray diffraction analysis revealed that compound 1 crystallizes in the trigonal chiral space group R32. However, the racemic twinning occurred as suggested by the Flack parameter of 0.49.26 As shown in Fig. 1, the [CoMo9O32]6− cluster is a typical Waugh type heteropolymolybdate with D3 symmetry, which consists of central CoO6 octahedron sharing edges with nine distorted MoO6 octahedra.19 There are four different coordination modes of oxygen atoms in the [CoMo9O32]6− cluster: (a) six μ3-O atoms coordinate with two molybdenum atoms and the central heteroatom is inherited from the Anderson β-isomer cluster; (b) two μ3-O atoms coordinate with three molybdenum atoms; (c) six μ2-O atoms coordinate with two molybdenum atoms; (d) 18 terminal O atoms coordinate with one molybdenum atom. The coordination geometry of the central Co ion in the [CoMo9O32]6− cluster presents a typically regular octahedron with identical Co–O distances of 1.894 Å. Of note, six NH4+ cations were found in the framework.


image file: c7nr04373b-f1.tif
Fig. 1 ORTEP drawings of cluster anions of compound 1. Thermal ellipsoids are drawn at the 30% probability level. Color code: Mo, blue; Co, green; O, red.

From the 3D framework of compound 1 projected in the direction of the 001 crystal plane (c-axis), the [CoMo9O32]6− packs one by one in the same direction, forming a quasi 2D material with atomically dispersed Co in the uniform distance of 10.1 Å and possessing approximate curved edge triangle shape nanopores with a diameter of 8.9 Å (Fig. 2 and S2a). The BET surface area (SBET) of compound 1 was calculated as 3.98 m2 g−1, and the pore volume was 0.06 cm3 g−1 with a pore diameter of 0.66 nm, which is due to the corresponding countercation NH4+ and water molecules occupied the pore space by a hydrogen-bonding interaction with the terminal O atoms of the [CoMo9O32]6− cluster (Fig. S2b).


image file: c7nr04373b-f2.tif
Fig. 2 3D framework of compound 1 packing along the c-axis. The counter-cation NH4+ and crystalline water molecules were omitted for clarity.

It is worth noting that the extra introduced molybdenum source of [MoO4]2− and oxidant H2O2 were essential to complete such topology transformation. A control experiment was conducted using a similar protocol, as shown in Scheme 1. Under identical conditions, the α-[Co(Oμ3H)6Mo6O18]3− cluster only can convert to the α-isomeric CoIII-Anderson cluster [Co(Oμ3H)6(Oμ2H)2Mo6O16]1− (compound 2, Fig. 3) with two of the μ2-O atoms being protonated as μ3-OH reactive sites in the absence of [MoO4]2− anions and oxidant H2O2, consistent with the reported literature.11


image file: c7nr04373b-f3.tif
Fig. 3 ORTEP drawings of cluster anions of compound 2. Thermal ellipsoids are drawn at the 30% probability level. Color code: Mo, blue; Co, green; O, red; H, grey.

Furthermore, compound 1 was characterized by FT-IR, ESI-MS, UV-Vis, XPS, UPS, CV, Mott–Schottky plot, TGA, and EA spectroscopy. In FT-IR analysis, the characteristic peak at 886 cm−1 was assigned to the vibrations of terminal Mo[double bond, length as m-dash]O units, and the peak at 725 cm−1 belonged to the vibrations of the Mo–O–Mo groups. Compared with the Anderson cluster precursor, no obvious peak split was observed (Fig. S3), which is due to the high symmetry (D3) of the [CoMo9O32]6− cluster. Electrospray ionization (ESI) mass spectrometry analysis showed only one ion peak at m/z = 238.95 with z = 6, which matches well with the anionic cluster of the [CoMo9O32]6− cluster (theoretical m/z: 239.06, Fig. S4). The UV-Vis spectrum of compound 1 in H2O was conducted to investigate the LMCT absorption and d–d transition absorption. The band located at 238 nm corresponds to the μ4-O π to metal-centered MoVI t2g* charge transfer transition (LMCT). The d–d transition absorption band located around 604 nm was assigned to the metal centered lowest energy electronic transition from the HOMO t2g* to the LUMO eg* transition of Co. The optical energy gap (Eg) was determined to be ca. 1.68 eV ascribed to the CoIV 3d-shell (3d5) state, and the d–d transition is directly allowed. Eg was located at a range of typical inorganic semiconductors, which may possess photocatalytic, electrocatalytic, and even photo-electro catalytic properties (Fig. 4).27


image file: c7nr04373b-f4.tif
Fig. 4 (a) UV/Vis spectrum of compound 1. (b) The optical energy gap of compound 1.

TGA measurement was applied to examine the thermal stability of compound 1. The compound 1 remained intact up to a temperature of 160 °C. First, water expulsion in the framework was found with 5.69% weight loss at the temperature of 100–160 °C. And then, the NH4+ counter-ion of 6.37% weight loss was found at 160–255 °C. Finally, the polyanion [CoMo9O32]6− cluster was completely decomposed into MoO3 and Co2O3 at 255–500 °C (Fig. S5).

To investigate the electrochemical properties, XPS was first utilized to characterize the oxidation state of Mo and Co in the [CoMo9O32]6− cluster. The existence of Mo and Co elements in [CoMo9O32]6− was evidenced by the full spectrum survey. The narrow element spectra of Mo and Co with raw intensity and peak fitting sum using the program XPSPEAK41 are presented (Fig. S6). In the XPS spectrum, the peak group at 780.82 and 785.31 eV was assigned to Co4+ (2p3/2) binding energy (BE), while the other peak group at 796.45 and 802.18 eV was assigned to Co4+ (2p1/2), Fig. 5. It is clearly indicated that the Co oxidation state was CoIV. Meanwhile, the peaks at 234.81 and 231.70 eV were assigned to BE of Mo6+ (3d3/2) and Mo6+ (3d5/2), implying that the molybdenum chemical state is the highest valence state of MoVI.28 These results match well with bond valence sum (BVS) calculation of Mo and Co atoms in [CoMo9O32]6− using the Bond_Str program.29 The BVS of Mo atoms varied from +5.915 to +6.178, and that of Co atoms was +3.996 (see BVS calculation details in Table S1). In all, the Co and Mo oxidation states in the complex 1 were determined to be CoIV and MoVI, respectively.


image file: c7nr04373b-f5.tif
Fig. 5 The narrow element XPS spectrum of Co (a) and Mo (b) with raw intensity and peak fitting sum.

Furthermore, UPS was applied to measure the HOMO–LUMO gap and electronic state of Co in the [CoMo9O32]6− cluster. A sharp peak was observed in the valence band maximum (VBM) and valence band region, indicating the entity of the CoIV 3d5 electron state. CoIV was octahedrally coordinated, and the oxygen crystal field splits the CoIV 3d orbital into t2g and eg levels. The filled Co t2g orbital constituted the highest full valence band, whereas the orbital eg formed the conduction band. Co was in a low spin state, while the VBM was assigned to CoIV eg orbitals with a minority spin.30 The valence band region shown on a magnified scale was also called the HOMO–LUMO gap region (Fig. 6, inset). The HOMO–LUMO gap is ascribed to the d–d transition and results from the crystal field splitting of the CoO6 octahedron; it was determined to be ca. 1.67 eV. It was highly consistent with the optical energy gap determined by the UV/Vis spectrum (1.68 eV, Fig. 4). Taking such a HOMO–LUMO gap into consideration, [CoMo9O32]6− is an n-type semiconductor that possesses electronic charge transport characteristics by energy transfer from light as a photosensitizer.


image file: c7nr04373b-f6.tif
Fig. 6 UPS spectrum of compound 1. The inset shows the valence band region; the HOMO−LUMO gap is ca. 1.67 eV.

Next, cyclic voltammetric (CV) and Mott–Schottky analyses were conducted to test the electrochemical properties of the [CoMo9O32]6− cluster and further to verify its semiconductor behavior. The CV curve of compound 1 consisted of two redox processes in 0.5 M Na2SO4 with a potential range from −0.8 to 0.6 V, Fig. 7a. The first process was ascribed to the CoIV/CoIII couple with obvious peak-to-peak separation. The ΔEp of 124.6 mV between the reduction Epc (−154 mV) and oxidation Epa (29.4 mV) with peak potentials I′ and I corresponded to the one-electron reversible redox process with potentials E1/2 of 91.7 mV. Coulometry showed that electron transfer per molecule for the CoIV/CoIII process in the [CoMo9O32]6− cluster is ca. 0.95, indicating that CoIV/CoIII of compound 1 involves one-electron charge-transfer steps.31 The second waves were attributed to the shell MoVI/MoV couple. The oxidation peaks were ill-defined with ΔEp of 85 mV between the reduction Epc (−312 mV) and oxidation Epa (−227 mV) with peak potentials II′ and II respectively, which also correspond to the one-electron reversible redox process with potentials E1/2 of 269.5 mV (Fig. 7a). The CV behavior of compound 1 showed a reversible two-electron process, while the CoIV/CoIII couple possessed an obvious defined peak. The electroactivity of CoIV with high valence state in the [CoMo9O32]6− cluster was the feature electrochemical property. Such a reversible two redox system in the [CoMo9O32]6− cluster may extend its application in photo-electronic catalysis reactions as a good multielectron reservoir and photosensitizer simultaneously with electrochemical stability.


image file: c7nr04373b-f7.tif
Fig. 7 (a) Cyclic voltammetry of compound 1 in Na2SO4 solution (0.5 M, pH = 1.0). (b) Mott–Schottky characteristic plot of compound 1 in Na2SO4 solution (0.5 M, pH = 6.0).

The Mott–Schottky characteristic plot was tested at a frequency of 1 kHz. The positive slope indicated an unambiguous evidence of an n-type semiconductor, well consistent with UPS evaluation. Polyoxometalates are usually regarded as multielectron reservoir anionic clusters.31 The potential Efb (−473 mV) is reliably obtained by extrapolating the straight line to zero potential, and the transition from the depletion layer to the accumulation zone occurred at 121 mV (Fig. 7b). As a result, the electrons of the [CoMo9O32]6− cluster can be excited by light, which can serve as an n-type multielectron semiconductor and a photosensitizer with energy transfer characteristics through electronic charge transport. Therefore, the [CoMo9O32]6− cluster can be deemed as a photo-electronic catalyst and photosensitizer candidate, which may transfer energy from light in oxygen molecule activation.

Photocatalytic 1O2 generation

Singlet oxygen (1O2) is a chemically reactive molecule and widely applied in photodynamic therapy (PDT) for cancer treatment, water purification, catalysis, etc.32–351O2 is usually yielded using organic dyes and noble nanoparticles as photosensitizers.32 However the dyes are very toxic and noble nanoparticles are too expensive, which limit their applications in the cancer treatment and water purification. Thus, these nontoxic and inexpensive POM clusters could be the alternatives. The inorganic semiconductor nature of the photosensitizer [CoMo9O32]6− cluster with the HOMO–LUMO gap of ca. 1.67 eV, larger than the activation energy of molecular oxygen (3O21O2, ΔE = 0.97 eV), guarantees such effective 1O2 generation. Taking both the multielectron redox property as n-type inorganic semiconductor and intrinsic photo-electronic catalytic property as photosensitizer of the [CoMo9O32]6− cluster, it was promising that such a cluster possessed photocatalytic properties. Hence, the light-excited 1O2 generation properties of [CoMo9O32]6− were further explored.

The 1O2 generation by [CoMo9O32]6− was firstly driven by two types of light sources: laser at 365 nm and sunlight, and further was evaluated by chemical probe detection. The criteria of these two types of light selections was that, for the 365 nm laser light source, it was monochrome and can generate high-energy light with low power, while for the sunlight source, it can be abundantly obtained on the earth but with low-energy compared to the laser since it was dispersive in time and space. 1O2 generation driven by a variety of light sources is an important feature of a good photosensitizer for broad application. [CoMo9O32]6− was able to generate 1O2 driven by both laser and sunlight sources. Under the irradiation of a 365 nm laser, the UV absorption spectrum of chemical probe DAB gradually attenuated to zero within 15 min (Fig. 8), indicating that DAB was converted to 3,3′-dinitrobenzidine in the presence of formed 1O2 species. Of note, DAB cannot react with 3O2. And UV also disappeared in 9 min using the sunlight source under identical conditions (Fig. S7a).


image file: c7nr04373b-f8.tif
Fig. 8 UV–vis spectra of DAB in an aqueous solution of the [CoMo9O32]6− cluster under (a) 365 nm laser and (b) sunlight irradiation in air.

Next, we also employ other varieties of light sources including the convex lens focused sunlight to generate higher-energy light and a 532 nm laser to investigate 1O2 generation under identical conditions. It is observed that the 1O2 generation rate was dependent on the energy density of light sources, and it decreases in the order: 365 nm laser > 532 nm laser > focused sunlight > sunlight (Fig. S8 and Table S2). Of note, the UV absorption spectrum of chemical probe DAB remained unchanged when ranged from 1 min to 20 min in the absence of the [CoMo9O32]6− cluster, indicating that it was essential for 1O2 generation in the presence of the inorganic semiconductor [CoMo9O32]6− as a photosensitizer, Fig. S9.

For completeness, we compared the [CoMo9O32]6− cluster with a conventional dye photosensitizer new methylene blue (NMB) for the 1O2 generation. Of note, the NMB photosensitizer is commonly employed as a standard substance for 1O2 investigation.33 The UV absorption spectrum of chemical probe DAB also gradually attenuated to zero within 15 min in the presence of NMB under 365 nm laser irradiation (Fig. S7). The initial 1O2 generation rate is related to the initial reaction rate (R) of a 1O2-trapping dye (e.g., DAB) with generated 1O2 and can be assumed to be equal to the initial slope (ΔAbs/Δtime), where Abs is the absorbance at 365 nm by DAB.33,34 The RCoMo9O32/RNMB value is estimated to be ca. 1.25 (Fig. S8 and Table S2), implying that the [CoMo9O32]6− cluster was more effective than the conventional NMB photosensitizer, regarding the 1O2 generation. Finally, we proposed the mechanism of 1O2 generation by employing the [CoMo9O32]6− cluster as a photosensitizer under light irradiation. Under light irradiation, the energy of photons was transferred to electrons of the [CoMo9O32]6− photosensitizer, the light-excited photoelectron can travel to the triplet state and eventually contribute to the 1O2 production via energy transfer from the n-type inorganic semiconductor [CoMo9O32]6− to 3O2, leading to the consumption of chemical probe DAB into 3,3′-dinitrobenzidine (Fig. 9).


image file: c7nr04373b-f9.tif
Fig. 9 Schematic illustration of the photo-oxidation of DAB into 3,3′-dinitrobenzidine by 1O2 species.

Conclusions

In conclusion, we have developed a novel protocol to conveniently obtain an atomically dispersed Co Waugh type [CoMo9O32]6− cluster, employing the most flexible structure unit Anderson type [Co(OH)6Mo6O18]3− cluster as a precursor. Considering the central heteroatom in Anderson cluster can be conveniently accessible to many elements including noble metals. This protocol may be extended to obtain a novel noble metal containing Waugh type cluster. The structures were definitely determined by the single crystal X-ray diffraction, as well as by XPS, UPS and BVS. The 3D framework structure of compound 1 formed as a quasi 2D material with atomically dispersed Co possessed curved edge triangle shape nanopores. The [CoMo9O32]6− cluster was a typical n-type inorganic semiconductor with a HOMO–LOMO gap of 1.67 eV and reversible two-electron redox properties, supported by UV-Vis, UPS, CV, and Mott–Schottky plot analyses. The [CoMo9O32]6− cluster can be applied as an efficient photosensitizer for the generation of singlet oxygen under visible (e.g., 365 nm and sunlight) irradiation.

Acknowledgements

We thank the financial support by the Liaoning Natural Science Foundation (No. 20170540897) (J. Z.) and the National Undergraduate Training Program for Innovation and Entrepreneurship (Grant 201610615011), the Yong Scholars Development Fund of SWPU (Grant 201131010047), and the Scientific Research Starting Project of SWPU (Grant 2014QHZ016) (Y. C.). We also thank Pro. Xu Zong and Dr Weiguang Ma for their help in CV and Mott–Schottky plot measurement and discussion.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr04373b

This journal is © The Royal Society of Chemistry 2017