Syntheses, crystal structures and visible light driven photocatalytic properties of organic-inorganic hybrid cuprous halides

Journal Pre-proof Syntheses, crystal structures and visible light driven photocatalytic properties of organic-inorganic hybrid cuprous halides Yun-Di Yue, Chen Sun, Wei-Feng Zhang, Xing-Yu Sun, Yan Du, Hong-Mei Pan, YueYu Ma, Ting-Jiang Yan, Zhi-Hong Jing PII:

S0022-4596(20)30042-6

DOI:

https://doi.org/10.1016/j.jssc.2020.121212

Reference:

YJSSC 121212

To appear in:

Journal of Solid State Chemistry

Received Date: 16 November 2019 Revised Date:

7 January 2020

Accepted Date: 19 January 2020

Please cite this article as: Y.-D. Yue, C. Sun, W.-F. Zhang, X.-Y. Sun, Y. Du, H.-M. Pan, Y.-Y. Ma, T.-J. Yan, Z.-H. Jing, Syntheses, crystal structures and visible light driven photocatalytic properties of organicinorganic hybrid cuprous halides, Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/ j.jssc.2020.121212. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Inc.

Graphic Abstract

By using photosensitivity conjugated organic cations as structural template, we prepared two types of cuprous halides, namely as, [(Me)2-2,2-bipy]Cu3I5 and [(Me)2-bipy)]2Cu7Br9I2, with one-dimensional anionic [Cu3I5]2- and [Cu7Br9I2]2anionic chains charge balanced by [(Me)2-2,2-bipy]2+ cations. The photosensitization of conjugated organic cations leads to narrow band gaps of 1.95 eV-2.13 eV, which result in stable photocatalytic degradation activity over organic pollutant under visible light irradiation.

Syntheses, Crystal Structures and Visible Light Driven Photocatalytic Properties of Organic-Inorganic Hybrid Cuprous Halides

Yun-Di Yue, Chen Sun, Wei-Feng Zhang, Xing-Yu Sun, Yan Du, Hong-Mei Pan, Yue-Yu Ma, Ting-Jiang Yan, Zhi-Hong Jing*

College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu, Shandong, 273165, P. R. China

*Corresponding author: Zhi-Hong Jing

E-mail address: [email protected]

Abstract By using in situ N-alkylating [(Me)2-2,2-bipy]2+ organic cation as structural-directing agent (SDA), two new types of organic-inorganic hybrid cuprous halides, namely, [(Me)2-2,2-bipy]Cu3I5 (1) and [(Me)2-bipy)]2Cu7Br9I2 (2), have been solvothermally synthesized and structurally characterized. The compound 1 crystallizes in the monoclinic space group P21/n (No. 147) and features one-dimensional (1D) anionic [Cu3I5]2- chains separated by [(Me)2-2,2-bipy]2+ cations. The structure of compound 2 contains a 1D [Cu7Br9I2]2- anionic chain, which belongs to the first hybrid cuprous halide with mixed halogen elements. The UV-vis absorption optical measurement reveals that the compounds possesses narrow band gaps of 1.95-2.14 eV, which leads to stable photocatalytic degradation activity over organic pollutant under visible light irradiation. The possible photocatalytic mechanism and reason of photochemical stability is also proposed based on experimental results and electronic band structural calculation.

Keywords: Inorganic-organic hybrid cuprous halides; crystal structure; visible light driven photocatalytst.

Introduction

In the past several decades, inorganic-organic hybrid metal halide materials have been investigated intensively because of their fascinating structural diversities and distinctive properties inheriting from inorganic and organic components, such as luminescence, photoelectric, photochromic and thermochromic effects, etc [1-10]. Among these hybrid materials, the organic cations template d10 transition metal (TM = Ag+ and Cu+) halides received special attention for their abundant structural types originating from the strong affinities of TM+ for X- (X = Br and I) anions as well as myriad coordination geometries including linear TMX2, TMX3 triangle and TMX4 tetrahedron [11-17]. More importantly, the tetrahedral TMX4 units always feature diversiform self-assembly condensation modes including corner-, edge- and face-sharing as well as short Ag···Ag and Cu···Cu interactions.

Up to now, a large amount of organic cations templated hybrid cuprous halides have been reported and characterized with various structural types from zero-dimensional (0D) clusters of [Cu4X6]2-, [Cu4X9]5-, [Cu5X7]2-, [Cu6I10]4-, 1D chains of [Cu2X3]-, [Cu2X4]2-, [Cu3X4]-, [Cu4X6]2-, to two-dimensional (2D) layers of [Cu3I4]-, [Cu4I5]-, [Cu6I11]5-, etc [18-25]. In addition to the rich structural chemistry, these hybrid cuprous halide phases also feature interesting photoelectric behaviors. For example, the (Bu2DABCO)1.5Cu3I6 shows significant temperature-dependent structural

distortion

and

thermochromic

luminescence

behavior

[26];

[deDABCO]2[meDABCO]Cu11I17 feature polymorphism phenomenon, green photoluminescence and highly efficient heterogeneous photocatalytic property under UV light irradiation [27]; [Fe(2.2-bipy)3]Cu4Br6 feature narrow band gap and stable visible light driven photocatalytic properties including water reduction to provide H2 and photodegradation of organic pollutants [28];

[N-Bz-Py]2[Cu6I8] exhibit interesting reversible thermochromism based on the intermolecular charge transfer [29].

In the construction of hybrid cuprous halides, there is no doubt that the SDA cations play important role in the structural types and electronic band regulation, which belong to the essential factor for their semiconducting photoelectric properties [30-32]. As a result, it is greatly attractive to design and prepare novel hybrid cuprous halides via introducing new type of organic cations as SDAs. Intrigued by the diverse structural topologies and potential photoelectric behaviors of these hybrid materials, we continue systematic studies in the hybrid cuprous halides by introducing the synchronous synthesized [(Me)2-2,2-bipy]2+ as template. Fortunately, we obtained two new types of [(Me)2-2,2-bipy]Cu3I5 and [(Me)2-bipy)]2Cu7Br9I2 with narrow band gaps and efficient visible light driven photocatalytical activities.

Experimental section

General: All the chemical reagents were commercially available and directly used as received without further purifications: KI, (99% pure, Aladdin), KBr (99% Aladdin); CuI (99.5%, Aladdin); CuBr (99.0%, Aladdin); 2,2-bipyridine (99%, Aladdin); methyl alcohol (99.8% Aladdin); acetonitrile (99%, Aladdin); HI aqueous solution (48%, Aladdin); HI aqueous solution (48%, Aladdin). The powder X-ray diffraction (PXRD) data was collected on the Bruker D8 ADVANCE powder X-ray diffractometer (Cu Kα, λ = 1.5418 Å) in the 2θ range of 5–80º. Elemental analyses of C, N and H were performed on a PE2400 II elemental analyzer. Elemental analyses for Cu, Br, and I were

performed on a JSM-6700F scanning electron microscope equipped with an energy dispersive X-ray spectroscope (EDS, Oxford INCA). The thermal behavior (TGA) was studied by a Mettler TGA/SDTA 851 thermal analyzer under a N2 atmosphere with a heating rate of 10 °C min-1. The UV-vis absorption spectrum was recorded on a PE Lambda 900 UV/vis spectrophotometer with wavelength in the range of 200–800 nm. Preparation of compound 1. A mixture of KI (4 mmol), 2,2-bipy (1 mmol), CuI (4 mmol), acetonitrile (4 mL), HI aqueous solution (48%, 1 mL) and methyl alcohol (2 mL) was sealed in a 25-mL Teflon-lined stainless container. The mixture was heated at 160 °C for 5 days and slowly cooled to room temperature leading to a large amount of dark red crystals of compound 1 with yield of 35% based on CuI. The block crystals were easily collected by hand under optical microscopeb, washed with ethanol and distilled water. Elem. Anal. Calcd for C12N2H14Cu3I5 (1): C, 14.25; H, 1.39; N, 2.77 %; found: C, 14.40; H, 1.42; N, 2.65 %. Preparation of compound 2. A mixture of KBr (2 mmol), 2,2-bipy (dpp, 2 mmol), CuBr (8 mmol), HI aqueous solution (48%, 1 mL), acetonitrile (5 mL) and methyl alcohol (2 mL) was sealed in a 25-mL Teflon-lined stainless container, which was then heated at 140 °C for 5 days. With cooling rate of 5 °C·min-1 to room temperature, red block shaped crystals were found and subsequently determined as [(Me)2-bipy)]2Cu7Br9I2 (2) with yield of 14% based on CuBr. The block crystals were easily collected by hand, washed with ethanol and distilled water. Microprobe elemental analyses on several single crystals indicated the presence of Cu, Br, and I elements in a molar ratio of 3.5(8): 4.85(1): 1.0(0), which was in agreement with the results from single crystal

XRD studies (Figure S5). Elem. Anal. Calcd for C24N4H28Cu7Br9I2 (2): C, 16.10; H, 1.57; N, 3.13 %; found: C, 16.18; H, 1.62; N, 3.25 %.

Crystal structure determination. Single crystal data of the compounds 1 and 2 were collected on Bruker SMART CCD-based diffractometer (Mo Kα radiation, λ = 0.71073 Å) at room temperature. The structures were solved by direct method and refined on F2 by full-matrix least-squares method using the SHELXS-97 program [33]. All the non-hydrogen atoms were anisotropically refined and the hydrogen atoms of organic molecules were generated theoretically and refined isotropically with fixed thermal factors. The crystallographic data for both the compounds are listed in Table 1 and important bond lengths are listed in Table 2-3. Crystallographic data in CIF format for the compound 1-2 has been deposited as CCDC number 1530374 and 1963564.

Table 1. Crystal Data and Structure Refinements for Compounds 1 and 2. Compound

1

2

C12N2H14Cu3I5

C24N4H28Cu7Br9I2

1011.37

1790.27

P21/n (No. 147)

P-1 (No. 2)

a/Å

10.2541(8)

12.2712(12)

b/Å

17.3775(14)

12.4910(12)

c/Å

13.8194(8)

15.0835(14)

α/º

90

89.6480(10)

β/º

119.865(4)

87.9820(10)

γ/º

90

62.6430(10)

2135.5(3)

2052.0(3)

4

2

3.146

2.897

chemical formula fw Space group

V(Å3) Z Dcalcd (g·cm-3)

Temp (K)

293(2)

293(2)

µ (mm-1)

10.185

13.849

F(000)

1808

1648

Reflections collected

24497

24128

Unique reflections

4905

9279

Reflections (I>2σ(I))

4221

6781

2

1.024

1.025

R1,wR2 (I > 2σ(I))

0.0350/0.1225

0.0434/0.1011

R1,wR2 (all data)

GOF on F

a

0.0422/0.1291

0.0673/0.1100

3

2.049

2.455

3

-2.559

-1.875

∆ρmax (e/Å ) ∆ρmin (e/Å ) a

R1 = ∑||Fo| -|Fc||/∑|Fo|, wR2 = {∑w[(Fo)2 -(Fc)2]2/∑w[(Fo)2]2}1/2

Table 2. Selected bond lengths (Å) for compound 1. Cu(1)-I(4) Cu(1)-I(3) Cu(1)-I(4)#1 Cu(1)-I(5) Cu(2)-I(5) Cu(2)-I(1) Cu(2)-I(3) Cu(2)-I(2) I(4)-Cu(1)-I(3) I(4)-Cu(1)-I(4)#1 I(3)-Cu(1)-I(4)#1 I(4)-Cu(1)-I(5) I(3)-Cu(1)-I(5) I(4)#1-Cu(1)-I(5) I(2)#2-Cu(3)-I(1) I(2)#2-Cu(3)-I(3) I(1)-Cu(3)-I(3)

2.6489(11) 2.6766(11) 2.6777(11) 2.7391(12) 2.5193(11) 2.6953(13) 2.7361(15) 2.8235(16) 112.74(4) 104.26(4) 106.48(4) 105.40(4) 109.98(4) 118.02(4) 121.62(5) 111.67(6) 97.92(5)

Cu(3)-I(2)#2 Cu(3)-I(1) Cu(3)-I(3) Cu(3)-I(2) Cu(1)-Cu(2) Cu(2)-Cu(3) Cu(3)-Cu(3)#2

2.5067(11) 2.6675(14) 2.7901(16) 2.8531(16) 2.7626(16) 2.4446(15) 2.802(3)

I(5)-Cu(2)-I(3) I(1)-Cu(2)-I(3) I(5)-Cu(2)-I(2) I(1)-Cu(2)-I(2) I(3)-Cu(2)-I(2) I(5)-Cu(2)-I(1) I(2)#2-Cu(3)-I(2) I(1)-Cu(3)-I(2) I(3)-Cu(3)-I(2)

115.08(5) 98.58(4) 108.43(5) 97.57(4) 110.68(5) 124.87(5) 117.35(6) 97.50(4) 108.27(4)

Symmetry transformations used to generate equivalent atoms: #1 -x+1, -y, -z+2; #2 -x, -y, -z+1.

Table 3. Selected Bond Lengths (Å) for compound 2. Cu(1)-Br(5)

2.4523(13)

Cu(5)-Br(4)

2.4672(14)

Cu(1)-Br(6)

2.4621(13)

Cu(5)-Br(2)

2.4869(14)

Cu(1)-Br(7)

2.6120(14)

Cu(5)-I(2)

2.7224(14)

Cu(1)-I(1)

2.6388(12)

Cu(5)-I(1)

2.7446(14)

Cu(2)-Br(4)

2.4361(14)

Cu(6)-Br(6)

2.4303(13)

Cu(2)-Br(1)

2.4542(14)

Cu(6)-Br(8)

2.5283(14)

Cu(2)-I(2)

2.6661(13)

Cu(6)-I(2)

2.6167(14)

Cu(2)-Br(8)

2.6670(15)

Cu(6)-I(1)

2.8932(16)

Cu(3)-Br(2)

2.5027(14)

Cu(7)-Br(1)#2

2.4440(15)

Cu(3)-Br(7)

2.5145(14)

Cu(7)-Br(3)

2.4449(15)

Cu(3)-Br(9)

2.5400(14)

Cu(7)-Br(8)#2

2.5466(15)

Cu(3)-I(1)

2.6589(12)

Cu(7)-Br(9)

2.6615(16)

Cu(4)-Br(3)

2.4482(15)

Cu(1)-Cu(3)

2.8175(16)

Cu(4)-Br(5)

2.4635(14)

Cu(1)-Cu(6)

2.9446(17)

Cu(4)-Br(7)

2.5180(14)

Cu(1)-Cu(4)

3.0089(16)

Cu(4)-Br(9)

2.6204(15)

Cu(2)-Cu(6)

2.7118(16)

Cu(3)-Cu(4)

3.0594(17)

Cu(2)-Cu(5)

2.7736(16)

Cu(4)-Cu(7)

2.9106(18)

Cu(2)-Cu(7)#1

2.8932(18)

Cu(5)-Cu(6)

2.7026(17)

Cu(3)-Cu(5)

2.8671(16)

Symmetry transformations used to generate equivalent atoms: #1 x+1, y, z, #2 x-1, y, z. Photocatalytic activity measurements. The photocatalytic activity of as-prepared sample 1 was evaluated by the photodegradation of organic pollutant, such as Rhodamine B (RhB), under visible light irradiation, which origins from the 50 W Xe lamp using a cut-off filter to remove all wavelengths less than 400 nm and more than 780 nm. In a typical photocatalytic process, 30 mg powder sample 1 was added to a 30 mL of 4× 10-5 mol·L-1 solution of RhB, which was magnetically stirred in the dark for one night to ensure adsorption equilibrium between the catalyst and solution. After that, the mixture was exposed to visible light irradiation and about 4 mL mixture was continually taken from the reaction cell at a given intervals during the irradiation. The catalyst was separated from the suspension by high-speed centrifugation and the supernatant clear solution was analyzed on UV-Vis spectrophotometer via examining the optical absorption of RhB at 552 nm. For

collecting the abundant sample to conduct the recycling experiments, two or even more the photocatalytic reactions were performed under the same condition, and the samples were separated through centrifugation, collected, combined and dried in an oven at 80 ºC for 12 h. After that, the same amount of sample was carried out the recycling experiment according to the same method.

Calculation detail. The electronic band structure of compound 1 was calculated by using the crystallographic data with the CASTEP code based on density functional theory (DFT), which adopts the plane-wave basis set for valence electrons and norm-conserving pseudopotential for the core electrons [34]. Hence, Cu-3d104s1, I-5s25p5, C-2s22p2, N-2s22p3 and H-1s1 orbital were used as valence atomic configurations. Other calculating parameters were set by the default values of the CASTEP code, for example, an energy convergence tolerance of 1.0 × 10−5 eV.

Results and discussion Compounds 1 and 2 were solvothermally synthesized in mixed solvent of HI, acetonitrile and methyl alcohol, in which the 2,2-bipy molecule was in situ protonated into [(Me)2-2,2-bipy]2+ cation due to the acidic environment. Such in situ reactions have also been reported in many inorganic-organic materials [35-36]. Furthermore, the KI/KBr was added in the reaction not only used as the source of I-/Br- but also effectively increase the solubility of the CuI/CuBr, which also play an very important role in the formation of the title compounds.

(a)

(b) Figure 1. Detailed view of the 1D [Cu3I5]2- chain (a) and stacking manner of compound 1 along the a-axis (b).

Single-crystal X-ray diffraction analysis revealed that compound 1 crystallizes in the monoclinic space group P21/n (No. 147) and features a 1D anionic [Cu3I5]2- chain separated by [(Me)2-2,2-bipy]2+ cations. In the asymmetric unit of compound 1, there are three crystallographically independent Cu, four I atoms and one [(Me)2-2,2-bipy]2+ cation. All the Cu+ ions are surrounded by four iodine ions with tetrahedral coordination environments. The Cu(1) atom is coordinated by one µ3-I and three µ2-I atoms, and the Cu(2) atom contacts with two µ2-I and two µ3-I atoms, whereas the Cu(3) atom is surrounded by one µ2-I and three µ3-I atoms. The Cu-I bond distances fall in the range of 2.5067(11)-2.8531(16) Å, which are comparable with those of hybrid cuprous iodides, such as [TM(2,2-bipy)3]Cu5I7 (TM = Fe, Co, Ni), [etpy][Cu3I4], [Mepy][Cu2I3], etc [37-38]. Each Cu(1)I4, Cu(2)I4 and Cu(3)I4 tetrahedron is inter-linked via edge- and face-sharing to form a [Cu3I7] trimer, and the neighboring [Cu3I7] trimers are further successively condensed via edge-sharing to form the 1D [Cu3I5]2- chains along the c-axis (Figure 1a). In the 1D [Cu3I5]2- chain, there are abundant short Cu···Cu distances of 2.4446(15)- 2.802(3) Å, which are shorter than the sum

of the van der Waals radii of 3.44 Å indicating the weak metal–metal interactions. The neighboring [Cu3I5]2- chains feature parallel stacking along the b- and c-axis, which are separated and bridged by the [(Me)2-2,2-bipy]2+ cations via C–H···I hydrogen bonds to form a 3D H-bonding network (Figure. 1b). It should be noted that the [Cu3I5]2- chain belongs to a new type of 1D cuprous iodide chain despite of that a large of 1D [CuxIy](y-x)- chains have been reported, such as [Cu2X4]2-, [Cu3X4]-, [Cu3X6]3-, [Cu4X6]2-, [Cu5X7]2-, [Cu6X7]-, etc [18-25]. Single crystal X-ray diffraction analysis revealed that compound 2 crystallizes in the triclinic space group P-1 (No. 2). In the asymmetric unit of compound 2, there are two crystallographically independent [(Me)2-bipy]2+ cations, seven Cu+, nine Br- and two I- ions. All the Cu+ ions adopt the tetrahedral coordination environments with four Br and/or I atoms. The Cu(1), Cu(2) and Cu(3) atoms are coordinated by one I and three Br atoms, and Cu(5) and Cu(6) atoms are surrounded by two I and two Br atoms, whereas Cu(4) and Cu(7) atoms closely contact with four Br atoms. The Cu-Br and Cu-I bond distances fall in the range of 2.4303(13)-2.6615(16) Å and 2.6167(14)-2.8932(16) Å, respectively, which are comparable with those of hybrid cuprous halides, such as [TM(2,2-bipy)3]Cu5I7, [Co(2,2-bipy)3]Cu5Br8, etc [39-40]. Each Cu(1)Br3I, Cu(3)Br3I, Cu(5)Br2I2 and Cu(6)Br2I2 tetrahedron is condensed by face-sharing to form a Cu4Br7I2 ring centered by I(1) atoms. Such Cu4Br7I2 ring is similar to the Cu4I9 unit in [TM(2,2-bipy)3]Cu5I7, (BPO)Ag5I7, [Mn(4,4-bipy)(DMF)3(H2O)]Ag5I7·4,4-bipy, (bmph)Ag5I7, etc [13,15,41-42]. On the other hand, each Cu(2)Br3I, Cu(4)Br4 and Cu(7)Br4 tetrahedron is interconnected via edge-sharing to form a linear Cu3Br7I trimer. The above Cu4Br7I2 ring is bridged by Cu3Br7I trimer via edge-sharing to form the 1D [Cu7Br9I2]2- anionic chain along the a-axis (Figure 2a). The neighboring [Cu7Br9I2]2- chains

feature parallel stacking along the b- and c-axis, which are separated and interlinked by [(Me)2-bipy]2+ cations via C-H···Br hydrogen bonds to form a 3D H-bonding network (Figure 2b). It should be noted that although there are lots of hybrid cuprous halides, very rare phase containing mixed halogens are reported until now.

(a)

(b) Figure 2. Detailed view of the 1D [Cu7Br9I2]2- chain (a) and stacking manner of compound 2 along the b-axis (b).

The purity of powder sample for compounds 1 and 2 has been proved by XRD analysis, in which the experimental pattern is in good agreement with the theoretical simulation (Figure S1). Thermal gravimetric behaviors of compounds 1 and 2 were investigated under a nitrogen atmosphere in the temperature range of 30−800 °C. The TGA curves show that compounds 1 and 2 can be stable up to about 200 °C. The solid state optical absorption spectrum of powder sample 1 and 2 were measured at room temperature (Figure 3). The nature of the bandgaps was determined by fitting functions for

both direct and indirect bandgaps to the data at energies above the Urbach tail region [43]. The data for compounds 1 and 2 are best fit for direct bandgap using Tauc’s function; therefore compounds 1 and 2 are assigned as direct bandgaps of 1.96 eV and 2.22eV, which are accordance with its color of dark red (Figure S3). Obviously, the title compounds have smaller band gap and exhibit red shift of the absorption edge compared with the bulk CuI (2.95 eV). Surely, the photo absorption of compounds 1 and 2 is close to those hybrid cuprous iodides directed by conjugated organic cations, such as [N-Bz-Py]Cu6I8 (2.52 eV), which is due to the high electron affinity and direct participation in adjusting the conduction bands of conjugated organic cations.[29]

(a)

(b)

Figure 3. The solid state UV-Vis optical absorption spectra of compounds 1 (a) and 2 (b). The narrow band gap of the title compound encourages us to study the photocatalytic activity under the visible light irradiation, which is evaluated via photodecomposition of organic pollutant (such as RhB) over the powder sample. Before the photodegradation experiment of compound 1, nor the blank experiment without any photocatalyst or the catalytic experiment without visible light irradiation show any observable decrease in RhB concentration with time. Subsequently, the powder sample 1 was added to the RhB solution, which was magnetically stirred in dark for one night to ensure absorption/desorption equilibrium between the sample and RhB molecule. After that, the

mixture was exposed to visible light irradiation, the purplish red color of the RhB solution slowly change to be colorless under the photodegradation action of sample 1 and the absorption intensity of RhB constantly decrease to nearly zero (Figure 4a). The degradation efficiency is defined as C/C0, where C and C0 represent the remnant and initial concentration of RhB, respectively. As shown in Figure 4b, the degradation ratio of RhB reaches 73% after 60 min irradiation over sample 1, and then reaches nearly 100% after 90 min, resulting in complete decolorization. At the same time, we also studies the visible light driven photocatalytic property of primary CuI under the completely same reaction condition, and the result shows that more than 87% of RhB is still alive after visible light irradiation. Hence, the sample 1 features more excellent degradation ability than the primary CuI. Assuming that the photodegradation procedure belongs to the pseudo-firstorder reaction, the degradation rate of RhB over compound 1 is about 0.021 min−1, which is close to those of hybrid silver or cuprous halides (Table S1). Furthermore, we also examine the photochemical stability of sample 1 due to the critical factor for the wide practical application of photocatalyst, especially for those CuI-based photosensitive materials. So we further perform the cyclic photodegradation experiment for RhB dye over sample 1 under the same condition. The result shows that there is no obvious decrease of photodecomposition efficiencies and rates over sample 1 under the three repeated applications (Figure 4c). At the end of repeated experiments, the XRD pattern of collected sample is almost identical to those of as-prepared sample, indicating the sample 1 belongs to a stable photocatalyst under visible light irradiation (Figure S1a).

(a)

(b)

(c) Figure 4. The absorption spectra of the RhB solution in the presence of compound 1 (a), the corresponding photodegradation efficiency of RhB (b) and the repeated photooxidation of RhB over sample 1 (c).

As well as we known, the primary CuI features slowly photolysis reaction under sunlight and seldom independently be used as the photocatalyst. However, as a stable photocatalyst, the compound 1 requires an effective way of separating the photo-induced electron/hole carries to prevent the reduction of Cu+ ion and increase the photocatalytic activity of hybrid material. To get insight into the photocatalytic mechanism as well as the electron transfer manner, we calculates the electronic band structure of compound 1 based on density functional theory (DFT). It is found that compound 1 has band gap of about 1.74 eV, which is smaller than the experimental optical band gap. Such a discrepancy is due to the discontinuity of the exchange-correlation potential that underestimates the band gap of semiconductors [44-45]. As shown in Figure 5, the top of the valence band (VB) just below the Fermi level is mainly composed of the 5p state of I and the 3d state of Cu, and lowest conduction bands (CB) are contributed by C-2p and N-2p based on the total and partial DOS diagrams. Hence, the photo induced charge transition of the compound 1 mainly occurs between the organic [(Me)2-2,2-bipy]2+ cations and 1D anionic [Cu3I5]2- chains. Based on the theoretical study as well as the similar reported work, we consider that the photo-induced electron on VB band (1D [Cu3I5]2- chain) absorbed photon with enough energy can easily transfer to the [(Me)2-2,2-bipy]2+ cations (CB), leaving hole on the VB. The photogenerated holes not only directly participate in degradation of organic dyes, but also can be transferred to oxide X- ions into X0, which oxidize RhB molecule and become reduced to X- ions again. In general, the photogenerated electrons of the [(Me)2-2,2-bipy]2+ cations are expected to be trapped by O2 in the solution to form superoxide ions (˙O2-) and other reactive oxygen species [46-50].

Figure 5. The total and partial DOS of compound 1.

It should be noted that the conjugated [(Me)2-2,2-bipy]2+ cation play an important role in the photocatalytic stability and activity of compound 1 via the electronic band regulation. On the one hand, the photosensitive organic cations afford great contribute to the CBs of hybrid phase 1 leading to the narrow band gap, that is, the wide visible light absorption. More importantly, the conjugate organic cations feature strong electron accepting and transferring abilities, which is able to not only facilitate the separation of photo-induced carries and promote the photocatalytic activity, but also effectively decrease the electron concentration on anionic [Cu3I5]2- chains and prevent the photo reduction of Cu+ ions, that is, increasing the photochemical stabilities of hybrid material. In summary, we have successfully obtained a new organic-inorganic hybrid cuprous iodide of [(Me)2-2,2-bipy]Cu3I5. The hybrid phase features narrow band gap, wide visible light absorption and effective photo-induced carries separation way, which results in stable visible light driven photodegradation activity for organic pollutant. The organic cations not only greatly tune the electronic band structure but also facilitate the photocatalytic property of title compound. Further

studies on the structural regulation and relationships of structure–photocatalytic property are in progress in our group.

Acknowledgments. We thank the financial supports from the National Nature Science Foundation of China (Nos. 21872081), and Laboratory Open Foundation of Qufu Normal University (sk201722).

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Highlights 1. Two new types of one-dimensional hybrid cuprous halides were prepared and characterized. 2. The [(Me)2-2,2-bipy]Cu3I5 and [(Me)2-bipy)]2Cu7Br9I2 have narrow band gaps of 1.95 eV-2.13 eV. 3. The cuprous halides result in stable photocatalytic properties under visible light irradiation.

Author statements On behalf of all the authors, we hereby certify that the contents of the paper have never been published in another journal. The data in this paper are real and reliable.

Sincerely yours! Zhihong Jing 2020-1-7

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: