Directed synthesis of a novel decamolybdate-encapsulated nanocage framework by mixed-ligands for light-driven hydrogen generation

doi:10.21203/rs.3.rs-1421528/v1

Download PDF

Research Article

Directed synthesis of a novel decamolybdate-encapsulated nanocage framework by mixed-ligands for light-driven hydrogen generation

https://doi.org/10.21203/rs.3.rs-1421528/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

A new compound [Cu^ICu^II₂(bpy)(pzta)₂][HMo₁₀O₃₄] {Cu^ICu^II₂-Mo₁₀, pzta = 3-(pyrid-4-yl) pyrazole and bpy = 4,4’-bipyridine} was obtained by a hydrothermal reaction method and characterized by various tests including XPS, IR, UV, PXRD, luminescent spectroscopy and single crystal X-ray diffraction. The Cu^ICu^II₂-Mo₁₀ shows an unusual decamolybdate-encapsulated nanocage framework structure. Beside the novel structure, the Cu^ICu^II₂-Mo₁₀ was father used for photocatalytic hydrogen generation and it possesses an improved catalytic H₂ productivity compared with its parent POM (NH₄)₆Mo₇O₂₄·4H₂O (Mo₇) under identical experimental conditions. Namely, the Cu^ICu^II₂-Mo₁₀ can reach 99.5 µmol^− 1 g^− 1 h^− 1, which is obviously higher than 42.35 µmol g^− 1 h^− 1 of Mo₇.

polyoxometalate

nanocage

mixed-ligands

hydrogen generation

Hydrogen (H₂) energy with various benefits such as high combustion value and clean pollution-free is expected to become one of the best suitable choices to meet the future global energy requirements (Zou et al., 2015). H₂ production from light-driven H₂O splitting is a sustainable and renewable route (Pang et al., 2021; Hou and Li, 2020). Nevertheless, the rate of H₂ generation is slow in most cases because of the high activation energy barrier of H-O bond in H₂O (Lewis et al., 2006). Hence, the development of photocatalysts with broad carrier separation rate and sensitive light-driven response is the key to realize the high conversion efficient of solar energy and to finally achieve effective H₂ production (Lv et al., 2014; Zhou et al., 2019; Han et al., 2017; Benjamin et al., 2013).

Polyoxometalate (POM)-encapsulated metal organic nano-cage framework (PEMONCF) is composed of POM guest and metal-organic nano-cage framework (MOF) host (Du et al., 2014). Such material possesses numerous advantages such as structural diversity, good environmental benignity as well as rapid and reversible electron transfer, makes it well suitable for application as photocatalysts (Zhang et al., 2019). However, it is difficult to purposefully construct PEMONCFs because this requires a control of the growing direction of the coordination bonds between the metal ions and the organic ligands (Li et al., 2018). The control of the assembly processes for constructing PEMONCFs is obviously a very interesting synthetic challenge.

It is well-known that organic ligands play significant roles in the construction of inorganic-organic hybrids with novel structures and improved properties (Wang et al., 2005; Wang et al., 2020). Compared with single ligands, mixed-ligands can provide more variability to build much more fascinating structures. Recent trends for the controlled synthesis of inorganic-organic hybrids, is a use of a new mixed-ligand system which aim to control the resulting structure and lead to further diversification of the frameworks with fascinating structures (Lisnard et al., 2005).

Herein, we study on a hydrothermal reaction system of pzta/bpy mix-ligands (Chart S1) as well as copper and polymolybdate salts in detail. Consequently, a new compound showing novel PEMONCF structure, [Cu^ICu^II₂(bpy)(pzta)₂][HMo₁₀O₃₄] {Cu^ICu^II₂-Mo₁₀, pzta = 3-(pyrid-4-yl) pyrazole and bpy = 4,4’-bipyridine}, was obtained. Also, the photocatalytic H₂ generation properties for the title compound were investigated.

The compound was obtained by a hydrothermal reaction process of (NH₄)₆Mo₇O₂₄·4H₂O, CuCl₂·2H₂O, bpy and pzta. The detailed experimental section is shown in ESI (Electronic supplementary information).

3.1. Structural Description of Cu^ICu^II₂-Mo₁₀

The structure of the compound Cu^ICu^II₂-Mo₁₀ is characterized by single-crystal X-ray diffraction. The Cu^ICu^II₂-Mo₁₀ crystallizes in the triclinic space group p-1 (please see the crystallographic data of the compound in Table 1).

Table 1

Crystallography data of Cu^ICu^II₂**-Mo**₁₀
Compound	Cu^ICu^II₂-Mo₁₀
Formula	C₂₂H₁₆Cu₅N₁₀O₃₄Mo₁₀
Formula weight	2241.55
Crystal system	Triclinic
Space group	P-1
a/Å	9.688(5)
b/Å	11.209(5)
c/Å	13.409(5)
α/^o	75.708(5)
β/^o	86.246(5)
γ/^o	87.415(5)
V/Å³	1407.4(11)
Z	2
D_calcd/g cm^− 3	3.106
T/K	293(2)
µ/mm^− 1	7.615
Refl. Measured	10369
Refl. Unique	7006
R_int	0.0180
F(000)	1209
GoF on F²	1.052
R₁/wR₂ [I ≥ 2σ(I)]	0.0366/0.1080
R₁ = ∑║F_o│─│F_c║/∑│F_o│. wR₂ = ∑[w(F_o²─F_c²)²]/∑[w(F_o²)²]^1/2

The Cu^ICu^II₂-Mo₁₀ is formed by bpy and pzta ligand molecules, Cu⁺/Cu²⁺ cations and [Mo₁₀O₃₂]^4- anion (Fig. 1a). For further details, there are three crystallographic independent Cu cations (Cu1, Cu2 and Cu3). Both Cu1 and Cu2 are five-coordination in a tetrahedral geometry (Fig. 1a), while the Cu3 is six-coordination in an elongated octahedral geometry. The bond lengths around Cu cations are in the range of 1.947 Å to 2.841 Å, which are in the normal ranges for the Cu-organic complexes. On the hand, the [Mo₁₀O₃₂]^4- (Mo₁₀) anion consists of a β-Mo₈ cluster and two additional {MoO₄} tetrahedra. The β-Mo₈ cluster shows the most compact structure of eight edge-sharing [MoO₆] octahedra with two [Mo₄O₁₃] subunits stacking together. Thus, the β isomer contains fourteen terminal, six doubly bridging, four triply bridging, and two five-fold bridging oxygen atoms. Further, the β-Mo₈ cluster and the two additional {MoO₄} tetrahedra are linked together by sharing two terminal oxygen atoms of the β-Mo₈ to construct the Mo₁₀ anion.

The most fascinating structural feature of 1 is its novel decamolybdate-encapsulated nanocage framework (Fig. 1c), and the specific formation of such framework can be understood in the following description. First, as shown in Fig. 1b, the Mo₁₀ is encapsulated in a metal-organic nanocage that is formed by four bpy and four pzta molecules as well as sixteen Cu cations (four Cu1, eight Cu2 and four Cu3). Each the nanocage shows a nearly cubic skeleton with volume of 13.4 × 9.6 × 11.2 Å³ (Fig. 2). Subsequently, the adjacent cages are fused together by the linkages of Cu-N bonds to finally achieve a 3D framework (Fig. 1c). After exhaustively reviewing previous references and the CSD database, only several examples of POM encapsulated nanocage frameworks were observed till now (Kuang et al., 2010; Hou et al., 2019) and most of these materials are based on Keggin polyoxoanions. Thus, the Cu^ICu^II₂-Mo₁₀ is an exceptional rare example based on decamolybdate polyoxoanion.

3.2. Analyses of XPS, IR and PXRD.

Firstly, the valence state of metal atoms (Cu and Mo) in the Cu^ICu^II₂-Mo₁₀ was confirmed by the high-resolution XPS spectra. Firstly, the analysis result of the Cu 2P spectrum indicates that partial Cu(II) atoms in the Cu^ICu^II₂-Mo₁₀ are reduced to Cu(I). Namely, as shown in Fig. 3a, the four main peaks at 952.19 eV, 932.60 eV, 995.16 eV and 935.12 eV of the Cu 2P spectrum can be temporarily corresponding attributed to Cu^I 2p_1/2, Cu^I 2p_3/2, Cu^II 2p_1/2 and Cu^II 2p_3/2, respectively. Also, from the Cu 2P spectrum, we can infer that the proportion of Cu(I) atoms to Cu(II) is about 1:2. Additionally, as shown in Fig. 3b, two characteristic peaks at 232.21 and 235.16 eV are attributed to Mo^Ⅵ3d_3/2 and Mo^Ⅵ3d_5/2 of Mo⁶⁺, respectively (Chai et al., 2019). Moreover, the IR and PXRD measurements were also done (see the corresponding description in ESI). In all, the results of XPS, IR and PXRD tests are consistent with the structural analysis of single-crystal X-ray diffraction.

3.3. Photocatalytic measurements

Before photocatalytic tests, several preliminary measurements were investigated to judge the catalytic performance of the Cu^ICu^II₂-Mo₁₀. As a result, by the measurements, we can deduce that the Cu^ICu^II₂-Mo₁₀ is expected to be an ideal photocatalyst for H₂ evolution reaction (HER). First, the optical properties of Cu^ICu^II₂-Mo₁₀ were conducted by the UV-Vis diffuse reflectance spectrum (DRS). The DRS shows two broad absorption peaks in the ranges of 200–450 nm and 550–800 nm, respectively (Fig. S4). The first peak is attributed to the Mo₁₀ polyanion while the other one is corresponding to d-d transition of Cu⁺/Cu²⁺ species and electron transfer among Cu ions and Mo₁₀ polyanions (Sun et al., 2019; Gawande et al., 2016). Furthermore, the joint tauc-plot method was used to obtain band gap width of the Cu^ICu^II₂-Mo₁₀ and the result shows that its band gap width is ca. 2.74 eV (Fig. S5). The recombination of photogenerated electrons and photogenerated holes is an exothermic process, and consequently the heat energy will be reflected in the form of fluorescence. Therefore, the fluorescent performance of the Cu^ICu^II₂-Mo₁₀ and its parent POM (NH₄)₆Mo₇O₂₄·4H₂O (Mo₇) were explored. As illustrated in Fig. S6, the Cu^ICu^II₂-Mo₁₀ and the Mo₇ exhibited a similar fluorescent peak around 360 nm, however, the fluorescent intensity of Cu^ICu^II₂-Mo₁₀ is much lower than that of Mo₇. Hence, the Cu^ICu^II₂-Mo₁₀ has a superior photo generated electron hole separation property. This result can be also verified by photocurrent tests measurements. Namely, the Cu^ICu^II₂-Mo₁₀ has higher transient photocurrent response than that of Mo₇ during many on-off cycles (Fig. 4 right).

Encouraged by the results above measurements, the photocatalytic HER tests of the Cu^ICu^II₂-Mo₁₀ were carried out (500W Xe light irradiation, sacrificial agent triethylamine, see the specific experimental process in ESI). Additionally, a comparison experiment of Mo₇ was investigated under identical experimental conditions. As a result, the photocatalytic H₂ productivity of the Cu^ICu^II₂-Mo₁₀ can reach 99.5 µmol^− 1 g^− 1 h^− 1, which is obviously higher than 42.35 µmol g^− 1 h^− 1 of Mo₇ (Fig. 4 right). A possible photocatalytic mechanism is listed in ESI.

In summary, a new POM-based metal–organic compound Cu^ICu^II₂-Mo₁₀ exhibiting a novel decamolybdate-encapsulated nanocage framework structure was prepared and it was father used for photocatalytic hydrogen generation. The Cu^ICu^II₂-Mo₁₀ as photocatalyst shows improved catalytic H₂ productivity compared with its parent POM Mo₇ under identical conditions. Namely, the Cu^ICu^II₂-Mo₁₀ can reach 99.5 µmol^− 1 g^− 1 h^− 1, which is obviously higher than 42.35 µmol g^− 1 h^− 1 of Mo₇. To some extent, this work shows a promising strategy for boosting the photocatalytic HER performance of POM materials.

Acknowledgments

All authors of this article have read the ‘‘Information For Authors’’ carefully and agree to the content contained therein. All the signed authors confirm that the article has not been submitted more than once and infringes others’ copyright. After publication, the copyright of the article belongs to the author, and the right of publication belongs to the journal of Environmental Engineering Science.

Authors’ Contributions

Chunjing Zhang, Xiao Liu and Shuang Ma: Data curation, Writing- Original draft preparation. Guannan Cui, and Liming Dong: Reviewing and Editing. Fengyang Sui and Haijun Pang: Conducted all the experiments.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was also financially supported by the Open Research Fund Program of Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry (CP2021YB08), the National Science Foundation of China (2217010844) and the National Science Foundation of Heilongjiang Province (LH2021B013).

Benjamin, M., Jennifer, F., JAMAL, M., Hani, A., Alexandre, P., Vincent, A., Guillaume, I., and Anna, P. (2013). Charge photo-accumulation and photocatalytic hydrogen evolution under visible light at an iridium(Ⅲ)-photosensitized polyoxotungstate. Energy Environ. Sci. 6, 1504.
Chai, D.F., Gómez-García, C.J., Li, B.N., Pang, H.J., Ma, H.Y., Wang X.M., and Tan L.C. (2019). Polyoxometalate-based metal-organic frameworks for boosting electrochemical capacitor performance, Chem. Eng. J. 373, 587.
Du, D.Y., Qin, J.S., and Li, S.L. (2014). Recent advances in porous polyoxometalate based metal-organic framework materials. Chem. Soc. Rev. 43, 4615.
Gawande, M.B., Goswami, A., Felpin, F.X., Asefa, T., Huang, X.X., Silva, R., Zou, X.X., Zboril, R., and Varma, R.S., (2016). Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis. Chem. Rev. 116, 3722.
Hou, Y., Pang, H.J., Xin. J.J., Ma, H.Y., Li, B.N., Wang. X.M., and Tan, L.C. (2020). Multi-Interface-Modulated CoS₂@MoS₂ Nanoarrays Derived by Predesigned Germanomolybdate Polymer Showing Ultrahighly Electrocatalytic Activity for Hydrogen Evolution Reaction in Wide pH Range. Adv. Mater. Interfaces. 2020, 2000780.
Han, X.B., Qin, C., Wang, X.L., Tan, Y,Z., and Zhan, X.J. Wang, E,B. (2017). Bio-inspired assembly of cubane-adjustable polyoxometalate-based high-nuclear nickel clusters for visible light-driven hydrogen evolution. Appl. Catal., B. 211, 349.
Hou, Y., Pang, H.J., Gómez-García, C.J., Ma, H.Y., Wang, X.M., and Tan, L.C. (2019). Polyoxometalate Metal−Organic Frameworks: Keggin Clusters Encapsulated into Silver-Triazole Nanocages and Open Frameworks with Supercapacitor Performance, Inorg. Chem. 58, 16028.
Kuang, X.F., Wu, X.Y., and Yu, R,M. (2010). Assembly of a metal-organic framework by sextuple intercatenation of discrete adamantane-like cages. Nat. Chem. 2, 461.
Li, B.N., Yu, X.J., Pang, H.J., Shen, Q.B., Hou, Y., Ma, H.Y., and Xin, J.J. (2020). Rational Regulation of Transition Metals in Polyoxometalate Hybrids without Noble-Metal Assistant for Efficient Light-Driven H₂ Production. Chem. Commun.56, 7199.
Lewis, N., and Nocera, D. (2006). Powering the planet: Chemical challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A. 43, 15729.
Lv, H.J., Guo, W.W., Wu, K.f., Chen, Z.Y., Bacsa, J., Musaev, D.G., Geletii, Y.V., Lauinger, S.M., Lian, T.Q., and Hill, C.L. (2014). A Noble-Metal-Free, Tetra-nickel Polyoxotungstate Catalyst for Efficient Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 136, 14015.
Li, S.B., Zhang, L., Lan Y.Q., O’Halloran, K.P., Ma, H.Y., and Pang, H.J. (2018). Polyoxometalate-encapsulated twenty-nuclear silver-tetrazole nanocage frameworks as highly active electrocatalysts for the hydrogen evolution reaction, Chem. Commun. 54, 1964.
Lisnard, L., Dolbecq, A., Mialane, P., Marrot, j., Riviere, E., Borshch, S., Petit, S., Robert, V., Dobuc, C., and Mccormac, T. (2006). Synthesis and characterizations of cyclic octanuclear mixed-valence vanadium(IV, V) clusters with polyoxometalate counterions. Dalton Trans. 43, 5141.
Pang, H.J., Ma, H.Y., Wang, X.M., and Tan, L.C. (2021). An irregular-octagonal-prism-shaped host-guest supramolecular network based on silicotungstate and manganese-complex for light-driven hydrogen evolution. New J. Chem. 45, 3954.
Sun, W.L., He, C., Liu, T., and Duan, C.Y. (2019). Synergistic catalysis for light-driven proton reduction using a polyoxometalate-based Cu–Ni heterometallic–organic framework. Chem. Commun. 55, 3805.
Wang, Y.T., Long, M.L., FAN, H.H., Wang, H.Z., and Chen, X.M. (2005) Homochiral crystallization of helical coordination chains bridged by achiral ligands:can it be controlled by the ligand structure?. Dalton Trans. 3, 424.
Wang, G.N., Chen, T.T., Gómez-García, C.J., Zhang, F., Zhang, M.Y., Ma, H.Y., Pang, H.J., Wang, X.M., and Tan, L.C. (2020). A High-Capacity Negative Electrode for Asymmetric Supercapacitors Based on a PMo₁₂ Coordination Polymer with Novel Water-Assisted Proton Channels. Small. 16, 2001626.
Zou, X.X., and Zhang, Y. (2015). Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev. 44, 5148.
Zhou, X.J., Yu, H., Zhao, D., Wang, X.C., and Zheng, S.T. (2019). Combination of polyoxotantalate and metal sulfide: A new-type noblemetal-free binary photocatalyst Na8Ta6O19/Cd-0.7 Zn0.3S for highly efficient visible-light-driven H-2 evolution. Appl. Catal., B. 248, 423.
Zhang, Y., Liu, J., Li, S.L., Su, Z.M., and Lan, Y.Q. (2019). Polyoxometalate-based Materials for Sustainable and Clean Energy Conversion and Storage. EnergyChem. 1, 100021.

Download PDF

Reviews received at journal
11 Mar, 2022
Reviewers invited by journal
08 Mar, 2022
Editor assigned by journal
07 Mar, 2022
First submitted to journal
04 Mar, 2022

You are reading this latest preprint version

Directed synthesis of a novel decamolybdate-encapsulated nanocage framework by mixed-ligands for light-driven hydrogen generation

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods

3. Results And Discussion

3.1. Structural Description of Cu^ICu^II₂-Mo₁₀

3.2. Analyses of XPS, IR and PXRD.

3.3. Photocatalytic measurements

4. Conclusion

Declarations

References

Status:

Version 1

Directed synthesis of a novel decamolybdate-encapsulated nanocage framework by mixed-ligands for light-driven hydrogen generation

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods

3. Results And Discussion

3.1. Structural Description of CuICuII2-Mo10

3.2. Analyses of XPS, IR and PXRD.

3.3. Photocatalytic measurements

4. Conclusion

Declarations

References

Status:

Version 1

3.1. Structural Description of Cu^ICu^II₂-Mo₁₀