Photo-electrochemical and physical characterizations of a new single crystal POM-based material. Application in photocatalysis

Accepted Manuscript Photo-electrochemical and physical characterizations of a new single crystal POMbased material. Application in photocatalysis D. Meziani, K. Abdmeziem, S. Bouacida, M.Trari PII:

S0022-2860(16)30678-0

DOI:

10.1016/j.molstruc.2016.07.006

Reference:

MOLSTR 22709

To appear in:

Journal of Molecular Structure

Received Date: 11 April 2016 Revised Date:

1 July 2016

Accepted Date: 4 July 2016

Please cite this article as: D. Meziani, K. Abdmeziem, S. Bouacida, M.Trari, Photo-electrochemical and physical characterizations of a new single crystal POM-based material. Application in photocatalysis, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.07.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Photocatalytic degradation of methylene blue on [(H2pip)3][α-PW12O40]2.4H2O supramolecular material.

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Photo-electrochemical and physical characterizations of a new single crystal POM- based material. Application in photocatalysis. D. Meziani 1, K. Abdmeziem 1 ∗, S. Bouacida 2,3, M.Trari 4 USTHB, Faculty of Chemistry, Laboratory of Electrochemistry-Corrosion, Metallurgy and Inorganic

Chemistry, BP 32 El-Alia, 16111 Algiers, Algeria 2

Département Sciences de la Matière, Faculté des Sciences Exactes et Sciences de la Nature et de la Vie,

Université Oum El Bouaghi 04000, Algérie. 3

Unité de Recherche de Chimie de l’Environnement et Moléculaire Structurale (UR CHEMS) Université

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Mentouri Constantine, 25000 Algérie. 4

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Laboratory of Storage and Valorization of Renewable Energies, Faculty of Chemistry, U.S.T.H.B., BP

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32 El-Alia, 16111 Algiers, Algeria

Abstract

A new inorganic-organic hybrid material [(H2pip)3][α-PW12O40]2.4H2O, prepared by hydrothermal method, was structurally characterized by single-crystal X-ray diffraction. The compound based on a Keggin-type polyoxotungstate and piperazine (pip) displays an hybrid framework built from two (α-Keggin)3- polyoxoanions and three (H2pip)2+ hydrogen-bonded

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fragments, forming 3-D supramolecular architecture. The diffuse reflectance spectrum shows two optical transitions directly (3.27 eV) and indirectly (3.12 eV) allowed. The electrical conductivity follows an exponential law, indicating a semiconducting comportment with activation energy of 14 meV. The Mott-Schottky characteristic, plotted in Na2SO4 (0.5 M) solution indicates n-type

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conduction with a flat band potential of -0.084 VSCE and electrons density of 4.24×1018 cm-3. As application, the photo-degradation of methylene blue (MB) upon UV irradiation was successfully

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achieved by OH• radicals. The improved activity is attributed to the potentials closeness of the valence and conduction bands with the radical levels. Keywords: Single crystal, Keggin polyoxometalate, Photoelectrochemical, Photocatalysis, Methylene blue.

1. Introduction: ∗

To whom correspondence should be addressed:

E-mails : [email protected] Tel / Fax : +00 213 21 24 73 11 / +00 213 21 24 80 08

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Since the first H3PW12O40 phosphotungstic acid discovered earlier by Keggin, polyoxometalates (POMs) species have attracted a great interest as building blocks in the design of new inorganic-organic hybrid supramolecular materials [1]. In this research field, different kinds of polyoxoanions, transition metal ions and organo-nitrogen ligands (including imidazole,

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pyridyl and tetrazole ligands, etc.) have been used [2-4]. The chemistry of polyoxometalates combined with small organic molecules provides knowledge about the interaction of organic molecules with the Keggin surface through hydrogen bonding [5, 6]. Indeed, hydrogen bond is an important interaction in the supramolecular networks since it allows the design of materials with

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various architectures and enhanced functional properties, thus offering a wide range of applications in photocatalysis, materials science, catalysis, sensors and medicine [7-10]. Among

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the applications, the photocatalysis occupies an important place since it does not require any sophisticated equipment and works under mild operating conditions [11]. It is well known that POMs and more especially the tungstophosphoric acid (H3PW12O40) show good photocatalytic performances in the dyes degradation [12, 13]. When POMs are illuminated by incident photons (hν) with energy greater than their band gap (Eg), electrons are excited from the valence band (VB) to the conduction band (CB). The generated

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electron-hole (e-/h+) pairs allow the formation of radicals responsible of advanced oxidation process (AOP) of the organic matter [14, 15]. On the other hand, there are only few works on POM-based hybrid materials with piperazine or its derivates as organic ligands in the literature and their photoelectrochemical (PEC) properties have been little investigated.

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In this work, we report the synthesis, structural characterization, physical and photoelectrochemical properties of a new inorganic–organic [(H2pip)3][α-PW12O40]2.4H2O single

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crystal, constructed from Keggin-type [α-PW12O40] polyoxotungstate and piperazine organic units. As application, the photocatalytic degradation of methylene blue (MB) was studied under UV irradiation.

2. Experimental

2.1 Hydrothermal synthesis All chemicals were commercially purchased and used as received. In a typical synthesis, H3PW12O40 (2.88 g, 1 mmol) and piperazine (0.19 g, 1 mmol) were dissolved in 10 mL of distilled water under constant agitation. The mixture was homogenised for 3 h at room 2

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temperature (pH ~ 2) and then transferred in 23 mL-Teflon-lined stainless steel autoclave. The crystallisation was then performed under autogenous pressure at 180 °C during 72 h. After cooling to ambient temperature, white transparent crystals were recovered by filtration, washed thoroughly with distilled water and dried at 60 °C overnight (50% yield based on W). The

close to the experimental one: P, 0.82; W, 70.07. 2.2 Materials and methods

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calculated composition of [(H2pip)3][α-PW12O40]2.4H2O (6096.6): (wt%) P, 0.98; W, 69.9, was

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The powder X-ray diffraction (XRD) pattern was performed on a Philips X'Pert Pro MDP diffractometer with Cu Kα radiation (λ= 1.54056 Å). Scanning electron microscopy (SEM) images were taken using a JSM-6700F microscope, operating at 5 kV. The FTIR spectrum was

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recorded with a Nicolet avatar 330-FTIR spectrometer in the range (400-4000 cm-1), using the KBr technique. The UV–Vis spectrum of the powder was recorded with a Jasco V-650 spectrophotometer equipped with an integrating sphere in the range (200-750 nm). The thermal analysis (TG) was performed on a Perkin Elmer STA 6000 instrument under N2 atmosphere at a heating rate of 10 °C min-1. The electrochemical measurements were carried on in a standard Pyrex cell at 25 °C using a PGZ 301 potentiostat (Radiometer analytical). A platinum foil was

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used as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. 2.3 Crystal structure determination

Crystallographic data of the compound were collected at 150 K on a Bruker APEXII

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diffractometer with a graphite-monochromated Mo Kα radiation (λ= 0.71073 Å). An empirical absorption correction to the intensities was carried out using SADABS [16]. The structure was

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resolved using sir 2002 program [17], and refined by the full-matrix least-squares technique on F2 by using SHELX-97 software [16]. Anisotropic thermal parameters were applied to all non-hydrogen atoms. The (H2pip)2+ molecules located in the plane (101) present a disorder which is refined with PART instruction. The hydrogen atoms were introduced as riding atoms in calculated positions and constrained with CH = 0.93 Å and Uiso (H) = 1.2 Ueq (C). N-H bonds are excluded, given their theoretical value of 0.87 Å. The hydrogen atoms of water molecules could not be found from the residual peaks and

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were directly included in the final molecular formula. The crystal data and structural refinements are gathered in Table 1.

3. Results and discussion

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3.1 Structure description

The single-crystal X-ray diffraction reveals that the synthesized compound consists of two discrete polyoxoanions [PW12O40]3− and three diprotonated piperazine (H2pip)2+ cations per unit cell, crystallizing in the triclinic system with the P-1 space group (Fig. S1). The [PW12O40]3−

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anion has a typical α-Keggin structure, consisting of a central PO4 tetrahedron surrounded by four vertex-sharing W3O13 trimers. Each W3O13 group is composed of three WO6 octahedra linked via

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triangular arrangement by sharing edges. There are four types of oxygen atoms with different coordination geometry in the cluster : the terminal oxygen atoms (Ot), central oxygen atoms (Oc), bridging oxygen atoms of two octahedra sharing an edge (Ob) and bridging oxygen atoms between two octahedra sharing a corner (Ob’), whose bond distances are in the ranges: W-Ot 1.681–1.717, W-Oc 2.408–2.465, W-Ob 1.897–1.944, W-Ob’ 1.875–1.941Å (Table S1). For PO4 groups, the O–P–O angles are in the range (109.25–109.77°) (Table S2) with P–Oc distances

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varying from 1.519 to 1.541 Å (Oc represents the tetrahedral oxygen atoms in distorted PO4 tetrahedron). Hydrogen-bond interactions between Keggin anions and small organic molecules are specific and important since they help to stabilize the structure (Table S3). As shown in Fig. S2, each α-Keggin group is surrounded by eight neighboring (H2pip)2+ ligands

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and connects to them by hydrogen bonds. Each (H2pip)2+ cation donates several typical hydrogen bonds formed by N-H…..O and C-H…..O interactions to terminal or bridging oxygen atoms from

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two [PW12O40]3- anions. These interactions are mainly in the range (2.12-2.60 Å) (Table S3). The interactions of lattice water molecules with (H2pip)2+ ligands are given in Fig. S3, there are four hydrogen bonds varying from 1.88 to 2.52 Å (Table S3) with the oxygen atoms of water molecules. The two disordered (H2pip)2+ ligands are linked to O2w, O3w and O4w atoms involving hydrogen bonds, while the six others are connected only to O1w (Fig. S3). All these bonds and interactions generate a three-dimensional supramolecular lattice (Fig. 1). 3.2 Characterization

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The XRD pattern is in good agreement with the simulated one, obtained from single crystal X-ray diffraction, thus confirming the phase purity of the sample (Fig. S4). The difference between the intensities of the patterns is due to preferred orientations of the powder during the

formula: D = K (β cos θ)-1

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data collection [18]. The crystallite size (D ~ 34 nm) is calculated from the empirical Scherrer

(1)

Where K (= 0.94) is the shape factor and β (radian) the broadening of the intense XRD peak.

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Assuming nonporous and spherical crystallites, a specific surface area of 51.17 m2 g-1 is calculated from the relation (= 6/ρ D), ρ being the experimental density (= 3. 44 g cm-3).

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The SEM image (Fig. S5) shows regular cuboid single crystals in the size range (50-150 µm). Both the size and form of the crystals are suitable for single-crystal X-ray diffraction. TG plot of the compound, recorded in the range (30–700 °C) under N2 flow (Fig. S6), shows a gradual decrease of the weight (exp. 2.65 %) up to 150 °C due to desorbed water. A second weight loss up to 370 °C accounting for 1.25 % of the total weight is attributed to the departure of four crystalline water molecules, in agreement with the calculated value (calc. 1.18 %). The two-

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step loss with a plateau region in the range (370–630 °C) is due to the decomposition of (H2pip) ligand (exp. 4.33%; calc. 4.43%). The weight loss from the TG curve supports the molecular formula of the synthesized compound.

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The FTIR spectrum of the compound is shown in Fig. S7. The characteristic peaks at 1079, 983, 894 and 806 cm-1 are assigned to ν(P–Oa), ν(W–Ot), ν(W–Ob–W), and ν(W–Oc–W) of PW12O40-3

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polyanions, respectively [19-21]. The following bands are characteristic of the organic ligand vibrations : νC-N (1200-1400 cm-1), ν ring C-C (1430-1600 cm-1), νC-H (2700-3100 cm-1), νN-H (3243 cm-1), the latter is shifted toward lower frequencies, upon forming hydrogen bonds. However, the band assignment in some of these regions is a laborious task, due to an eventual mixing with some deformation vibrations. For example, the peaks around 1500 cm-1 may be attributed to deformation vibration δCNH while that at 2375 cm-1 represents the δ (NH2)+ deformation vibration of the protonated amine. The band at 3541 cm-1 is attributed to the stretching vibration of lattice water [22].

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The diffuse reflectance (DR), recorded between 200 and 900 nm, is presented in Fig. S8. The spectrum exhibits two bands at 254 and 316 nm, characteristic of the Keggin type and assigned respectively to the transitions Ot → W and Ob,c → W ligand metal charge transfer (LMCT) in the polyoxoanion structure [23]. The optical gap (Eg) and the nature of the transitions are determined

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from the Tauc relation [24]: (αhν)m = A(hν - Eg)

(2)

Where A is a constant and α the optical absorption coefficient. The transitions of [(H2pip)3][α-

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PW12O40]2.4H2O, determined from the extrapolation of the linear plots (αhν)m with the hν-axis (Fig. 2), are found to be 3.12 eV (m= 0.5, indirect) and 3.27 eV (m= 2, direct). These values are

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close to those cited for POM’s materials [25].

The variation of log (σT) vs. reciprocal temperature (Fig. 3) indicates a semi conducting behavior, the conductivity data are well fitted by a small polaron model [24]: σT = σo exp {Ea/kT} with activation energy (Ea) of 14 meV. Hence, all donors are ionized at ambient temperature (kT ~ 26 meV) and the enhanced conductivity mainly comes from the electron mobility (µ e), which is thermally activated (µ e = 1.72×10-6 cm2 V-1 s-1)1.

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The knowledge of the energetic positions of the valence and conduction bands is a prerequisite for the photocatalytic study [26], and the photoelectrochemistry is widely used to provide the semiconducting properties of the material and to build the energy band diagram of the junction [27].

The

photo-electrochemical

properties

of

[(H2pip)3][α-

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semiconductor/electrolyte

PW12O40]2.4H2O are performed in neutral medium (Na2SO4, 0.5 M). The current-potential J(E) curves are recorded both in the dark (Jd) and under illumination (Jph) and the data are used for

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plotting (Jph - Jd)2 against the potential (E) according to the Gartner relation [28]: (Jph - Jd)2 = Const × α2 W2 (E - Eon)

(3)

The current Jph increases toward the anodic direction (Fig. 4, inset), indicating n-type conduction. The intersection of the linear part with the potential-axis gives the photo-current onset potential Eon (= 0.226 V). However, the flat band potential (Efb) is accurately determined from the interfacial capacitance using the Mott–Schottky relation [29]:

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 C = [2/(εε eAN )](E − E )

(4)

Where A is the electrode surface area, εo the permittivity of vacuum, e the electron charge and ND the electrons density. The permittivity of [(H2pip)3][α-PW12O40]2.4H2O (ε ~ 700) is obtained from the dielectric measurements on pellets. The positive slope of the plot confirms the n-type

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behavior (Fig. 4); the potential E (-0.076 V) and the density ND (4.24×1018 cm-3) are provided

 from the intersection with the potential axis and the slope of the plot {C = f (E)} respectively

[30]. The small difference between the potentials Eon and Efb (~ 0.15 V) indicates the absence of in the electrochemical scale according to the relation [31]:

(5)

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P = 4.75 + e E + E

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surface states within the gap region. The potential Efb outlines the position of the conduction band

where 4.75 is the free energy of the reference electrode (SCE) with respect to vacuum, and the P value (-4.66 eV) corresponds to a potential of -0.084 V. Hence, the energy of VB is deduced from the relation (P - Eg = -7.93 eV/3.18 V). Such results are close to those cited by Li et al. [32]. The high energetic position P of the conduction band and the wide gap indicate that the valence band is mainly made up of O2-: 2p orbital while the conduction band derives from W6+: 5d character

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[33]; the optical transition is attributed to a charge transfer. The low electrons density ND produces a depletion wide δ of 88 nm. 

! (""#$ )

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(6)

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δ=

The δ value, calculated for an optimal band bending (E-Efb) of ~ 0.4 V, extends over several unit cells and this is attractive in photocatalysis since it allows the separation of a large number of (e-

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/h+) pairs. Indeed, the photocatalytic yield depends on the light penetration into the semiconductor, and the low density (ND) makes the depletion width large (Eq. 6) so that most generated incident photons (e-/h+) pairs contribute to the photoactivity. 3.3 Photocatalytic activity

In view of large scale pollution and increasing costs of conventional techniques for the water treatment, the exploitation of the solar energy is strongly encouraged. POM hybrid materials exhibit a good photocatalytic performance for dyes photodegradation [34]. Methylene

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blue (MB), a dye model, is used for evaluating the photoactivity of [(H2pip)3][α-PW12O40]2.4H2O under UV irradiation. The experiments were performed according to the following protocol: 10 mg of photocatalyst powder were dispersed in 100 mL of MB aqueous solution (10 mg/L). Before irradiation, the system was magnetically stirred in the dark for 30 min to ensure the

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adsorption equilibrium. The suspension was then irradiated with UV lamp (4 W, max. emission: 254 nm); 3 mL were collected every 20 min of irradiation, followed by centrifugation (3000 rpm, 3 mn) to separate the photocatalyst particles. The remaining MB concentration (C) was determined by measuring the absorbance at λmax (= 664 nm). Under illumination the absorption

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peak of MB decreased from 1.9 to 0.22 (Fig. 5); the relative absorbance of MB (C/Co) versus the reaction time (t) was plotted and given in Fig. S9. The change in the concentration indicates MB

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photo-oxidation, since only little change was observed by photolysis under the operating conditions. The results show that 89% of MB has been decomposed after only 60 min of UV irradiation.

The crystallites in colloid systems cannot be polarized and the additional condition in photocatalysis for n-type semiconductor is that its free potential (~ 0.3 V) for [(H2pip)3][αPW12O40]2.4H2O must be more anodic than the flat band potential E (- 0.076 V). The

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illumination of the compound leads to a charge transfer O2-: 2p → W6+: 5d giving the formation of excited-state species (POM*) with formation of (e-/h+) pairs [35, 36]. The carriers recombination is an undesired reaction which competes with MB oxidation unless the (e-/h+)

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pairs are separated by the electric junction field.

On the basis of the energy diagram (Fig. 6), drawn from the physical and PEC characterizations, the radicals OH• (0.145 V, reaction 8, 9) and (2.54 V, reaction 10) [37, 38],

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generated upon illumination of the catalyst, are responsible of the mineralization of the organic matter by (AOP) [39, 40]. Indeed, the OH• species are energetically positioned below the conduction band (-0.084 V) and above the valence band (3.18 V), respectively, permitting a facile (e-/h+) pairs transfer to the interface and increasing lifetime. Both electrons and holes are trapped on the surface to form OH• radicals, responsible of MB oxidation. The degradation of MB can be achieved according to the following sequences: , POM + hν → POM (e() , h+) )

(7)

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(8)

H2O2 + hν → 2 OH•

(9)

+ • h, +) + H2Oads → H + OH

(10)

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 O2 + 2e() + 2 H+ → H2O2

It is worthwhile to outline that water does not compete with MB oxidation for the photoholes, because the potential of O2/H2O is more anodic than the valence band. The results of the present work showed that [(H2pip)3][α-PW12O40]2.4H2O possesses a good photocatalytic activity for the photodegradation of MB. It is therefore expected to be promising for other organic

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molecules.

In continuation of our research, synthesis is in progress with varying the organonitrogen

with expected photocatalytic properties.

Conclusion

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ligand and incorporating d-block transition metal, to design supramolecular host-guest structures

In this work, we presented the crystal structure of a new inorganic-organic hybrid single

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crystal material [(H2pip)3][α-PW12O40]2.4H2O, synthesized by hydrothermal method. The optical, physical and photoelectrochemical characterizations indicated semiconducting properties with ntype behavior and allowed to draw the energy band diagram. The photo-degradation of MB under UV light was successfully achieved, due to the potentials closeness of the valence and conduction

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bands with the potentials of the OH• radical.

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Supplementary material

Crystallographic data (excluding structure factors) for the title compound have been deposited at the Cambridge Crystallographic Data Centre, number is CCDC 1430840. These data can be obtained, free of charge, via the following website: www.ccdc.cam.ac.uk/data_request/cif./cif.

Acknowledgments

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The authors thank MHESR (Ministry of Higher Education and Scientific Research) Algeria, for the financial support. References

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Table1. Crystal data and structure refinement parameters.

[(H2pip)3][α-PW12O40]2.4H2O

Fw

6096.6

Temp / K

150

Crystal system

Triclinic

Space group

P-1

a/Å

10.3592

b/Å

10.5175

c/ Å

19.2456

β/° γ/ ° V / Å3 Z / Z'

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α/°

92.996

91.605

2028.52 2/1

R1a[I ˃ 2 sigma(I)]

0.0484

wR2b(all data)

0.0499

GOF

1.0070

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0.0397

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 = ∑ | | − | |  / | |  /

b

104.166

Rint


 = ∑| | − | |/| |

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Figures captions

Fig. 1. The 3-D Supramolecular architecture of [(H2pip)3][α-PW12O40]2.4H2O. Fig. 2. Optical transition of [(H2pip)3][α-PW12O40]2.4H2O : (a) The direct optical transition,

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(b) The indirect allowed transition. Fig. 3. The thermal variation of the conductivity of [(H2pip)3][α-PW12O40]2.4H2O.

Fig. 4. The Mott-Schottky plot of [(H2pip)3][α-PW12O40]2.4H2O in Na2SO4 solution (0.5 M). Inset: the determination of the potential Eon.

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Fig. 5. The absorption spectra of MB in presence of [(H2pip)3][α-PW12O40]2.4H2O, under UV irradiation.

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Fig. 6. The energetic diagram of [(H2pip)3][α-PW12O40]2.4H2O at pH ∼ 7.

ACCEPTED MANUSCRIPT ► A new single crystal POM-based hybrid compound was synthesized by hydrothermal route. ► [(H2pip)3][α-PW12O40]2.4H2O exhibits n-type conduction and two optical transitions. ► The valence and conduction bands are made from O2-:2p; W6+:5d orbitals respectively.

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► The energy band positions of the compound lead to dye photo-degradation.