Y-zeolite

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 0 7 0 e2 0 7 8

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Photocatalytic hydrogen evolution on new mesoporous material Bi2S3/Y-zeolite Abderrezak Abdi a, Alain Denoyelle b, Nadine Commenges-Bernole b, Mohamed Trari c,* a

Laboratoire d’Electrochimie et de Corrosion, Ecole Militaire Polytechnique, BP 17, BEB, 16111 Algiers, Algeria LEPMI, Grenoble INP/UJF/CNRS, BP 75, 38402 Saint Martin d’He`res Cedex, France c Laboratory of Storage and Valorization of Renewable Energies, USTHB, BP 32, 16111 Algiers, Algeria b

article info

abstract

Article history:

The novel semi conducting configuration n-Bi2S3/Y-zeolite is elaborated by exchange reac-

Received 9 September 2012

tion between H/Y-zeolite and Bi3þ. The precipitation of Bi2S3 is achieved with thiourea, and

Received in revised form

activated by ultra sound probe at a frequency of 20 kHz for 60 min. X-ray diffraction and

14 November 2012

Raman spectroscopy confirm the formation of Bi2S3 which crystallizes in an orthorhombic

Accepted 16 November 2012

symmetry. The scanning electron microscopy (SEM) shows uniform and rough surface with

Available online 27 December 2012

fine Bi2S3 crystallites. The specific surface area (SBET ¼ 87.82 m2 g1) is wseven times greater

Keywords:

characteristic gives a flat band potential of 0.465 VNHE. The electrical conductivity of Bi2S3 is

Hydrogen

correlated to the optical gap (Eg ¼ 1.40 eV) and photo-electrochemical characterization to

Bi2S3

establish the energetic diagram of the hetero-system Bi2S3/Y-zeolite/KOH solution. The

H/Y-zeolite

latter predicts a spontaneous hydrogen evolution upon visible light. An enhancement of

Photo-electrochemistry

twice with respect to Bi2S3 is obtained in presence of S2 O2 3 as holes scavenger. The best

than that of Bi2S3 (9.57 m2 g1) prepared under the same conditions. The MotteSchottky

performance (292 mmol H2 (g catalyst)1 h1) occurs at pH w13.1 with a light-to-chemical energy yield of 0.12%.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The conversion of the solar energy into electric energy and/or chemical fuels is increasingly oriented toward the development of new semiconductor (SC) materials with novel configurations [1e3]. Hydrogen is universally recognized as a clean energetic vector and its production becomes one of the central topics in the energy research [4,5]. It has attracted much attention as a way of storing the solar energy and remains still the subject of intense researches owing to the depletion of fossil reserves. Some alternatives to achieve the water splitting have been reported among which the

photoelectrolysis [6]. n-type SC must have light absorption in the visible region, a long-term chemical stability and a flat band potential (Vfb) as cathodic as possible [7]. The chalcogenides A2B3 (A ¼ Bi and Sb, B ¼ S, Se and Te) are attractive owing to their low cost and high absorption coefficients (w104 cm1) [8]. Among the congeners, Bi2S3 has received great intention due to its applications including thermoelectric cooling technologies, optoelectronic and IR devices [9]. With a black color, it absorbs over the whole solar spectrum up to 1000 nm and has been prepared with various morphologies including nano-rods and nano-composites [10]. To our knowledge, there are no papers on n-Bi2S3 supported on

* Corresponding author. Tel.: þ213 21 24 79 55; fax: þ213 21 24 80 08. E-mail address: [email protected] (M. Trari). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.085

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 0 7 0 e2 0 7 8

zeolite and the photocatalytic performance has not been reported before now. Zeolites constitute a large group of more than 100 natural minerals and various forms are used as photocatalysts and catalyst supports [11]. They are hydrated alumino silicates My/z[(SiO2)x(AlO2)y],nH2O with micro porous crystalline structure, M denotes an exchangeable metal ion with a valence z. They extend over three dimensional lattice of tetrahedral units TO4 (T ¼ Si, Al) interconnected through oxygen atoms [12]. Incorporation of Al3þ makes the framework negatively charged, and requires the presence of heterovalent cations within the structure in order to keep the electro-neutrality [13]. Zeolites are suitable for loading the chalcogenides because of their large surface areas, pore size, ion-exchange capability, affinity for heavy metals, optical transparency to UVeVis radiations (>240 nm) and polarization strength of channels [14,15]. H/Y-zeolite has a high Si/Al ratio and low cation content which make it thermally and chemically stable. The cations are located in the sodalite super cage and double hexagonal rings. Indeed, the super cages can accommodate many cations, even those with high hydrated radii, due to their aperture (8  A) and large diameter (13  A) [16] and could provide enough space for assembling semi-conductor clusters. It has yet been reported that CdS nanoparticles supported on H/Y-zeolite allow efficient hydrogen photo-production [17]. In this work, we report the preparation and the characterization of Bi2S3 supported on zeolite and its application for H2-production under visible light. This is done in the hope of increasing the efficiency by absorbing more sunlight and enhancing the charges separation.

2.

Experimental

All reagents used in this work are of analytical quality (purity > 99%) and the solutions are made up with doubly distilled water. The synthesis of Bi2S3/Y-zeolite is carried out via two step process. The exchanged Bi/Y-zeolite is prepared by mixing 250 mL of Bi(NO3)3 (0.48 mmol L1) to 1 g of H/Yzeolite (chemical formula: H53.3Na1Al54.3Si134.3O377.2) in acidic solution. The pH is adjusted at 1.5 by addition of HNO3 where the specie Bi3þ predominates and the solution is heated at 60  C under magnetic stirring (300 rpm). To determine the exchange capacity of the zeolite, aliquots are withdrawn at periodic time intervals. In the second step, Bi2S3/Y-zeolite is prepared in ammonia solution (0.1 N) from 1 g of Bi/Y-zeolite and 0.36 g of thiourea (SC(NH2)2) as sulfide source [(Bi/Yzeolite: SC(NH2)2) ¼ 1:1.5]. The synthesis is activated by an ultrasound probe (750 W) at a frequency of 20 kHz. The temperature is maintained constant at 20  C by water circulation. Bi2S3/Y-zeolite is recovered as black powder, thoroughly washed with water and ethanol, and dried at 70  C overnight.

2.1.

Materials characterization

2.1.1.

H/Y-zeolite exchange capacity

The exchange capacity (G) of H/Y-zeolite is determined by measuring the absorbance of Bi3þ in the supernatant solution by atomic absorption (Shimadzu AA-6300); the samples are

2071

diluted 10 times. The linear calibration curve is plotted to match closely the composition of the solutions. G is calculated from the relations: Gð%Þ ¼

qe ¼


C0  Ce $100; C0

ðC0  Ce Þ $100 m

(1)

(2)

where C0 and Ce are the initial and equilibrium concentrations of Bi3þ (mmol/L), qe the amount of exchanged Bi3þ (mmol/g), V the volume of the solution and m the mass of the adsorbent (g).

2.1.2.

Bi2S3/Y-zeolite photocatalyst

The phase Bi2S3/Y-zeolite is identified by both X-ray diffraction (MINIFLEX II) using Cu Ka radiation (l ¼ 0.1506 nm) in the 2q range (5e80 ) and a Jobin-Yvon Raman spectrometer with Ar þ ion laser beam (514 nm). The diffuse reflectance spectra are recorded with a UVeVis spectrophotometer (Shimadzu UV-2401 PC) operating between 300 and 1100 nm, BaSO4 is used as reference. The specific surface areas are determined by the BET method on a Quantachrome Nova surface analyzer 3200. The surface morphology of the powder is investigated by scanning electron microscope (SEM) using a Quanta 600 FEG unit. Elemental analysis is carried out with Link, ISIS-300, Oxford EDAX detector. Differential scanning calorimetry (DSC) is performed under nitrogen atmosphere with a differential thermal analyzer (Netzsch Phoenix) over the temperature range (25e500  C) at a heating rate of 3  C/min. The electrical conductivity (s) is measured by the two probe method in the range (25e600  C) on disc-shaped pellets sintered in evacuated Pyrex ampoule (<1 mbar) at 300  C. The intensityepotential J(V) characteristics are plotted in a three electrode cell, with a large area Pt counter electrode (CE) and a saturated calomel electrode (SCE). For convenience, the potentials are given with respect to normal hydrogen electrode (NHE). The electrode potential is monitored with a PGZ 301 (Radiometer potentiostat) and the electrolyte (KOH 1 M/ Na2S2O3 0.1 M) is continuously bubbled by nitrogen. The capacitance measurement is performed at 1 kHz.

2.2.

Hydrogen photoproduction

The hydrogen formation is performed in a closed doublewalled Pyrex reactor thermo-stated at 50  1  C. 250 mg of Bi2S3/Y-zeolite are dispersed in 250 mL of the solution (1 M KOH þ 0.1 M Na2S2O3). The suspension is irradiated with the visible output of an assembly of three tungsten lamps (200 W), under magnetic stirring. The light intensity of 29 mW cm2 (2.09  1019 photons s1) is measured with a commercial light-meter (Testo 545). The energy of the incident light is determined with a digital light meter (Test 545). Before each run, the solution is bubbled with nitrogen to prevent uptake of photogenerated holes by dissolved O2. Evolved hydrogen is identified by gas chromatography (TCD Shimadzu IGC121 ML) containing two 4 m carbosieve B columns (1/8 inch, 100e200 meshes) and argon as carrier gas. No gas other than hydrogen and traces of water are detected.

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Results and discussion

The Bi3þ exchanged with H/Y-zeolite is carried out at 60  C, with an initial concentration of 0.48 mmol L1. The synthesis occurs via two step process. Bi3þ moves through the pores and channels of H/Y-zeolite to be exchanged with Hþ according to the scheme: 3þ

10



þ 3SCðNH2 Þ2 þ 6OH /Bi2 S3 =Y  zeolite þ 6H2 O

þ 3CN2 H2

(4)

w 60 min are taken as adequate equilibrium time. The X-ray diffraction pattern of Bi2S3/Y-zeolite is given in Fig. 2 along with that of Bi2S3. These results indicate the phase formation of Bi2S3 on the zeolite framework. The peaks are assigned to the orthorhombic unit cell (SG: Pbnm) with the lattice constants a ¼ 11.35  A, b ¼ 11.05  A, and c ¼ 3.85  A in agreement with the  JCPDS card N 17-0320. The average crystallite size (w6 nm) is estimated from the full width at half maxim FWHM: n

¼ 0:94lðbcos qÞ1

351

(3)

It is expected that this new synthetic approach can be applied to synthesize porous nano-composites with high photocatalytic activity. Fig. 1 shows that the exchange occurs simultaneously inside (slow process) and outside (rapid) the sodalite cages. The maximal uptake adsorption (G ¼ 67.8%) is reached within 30 min, corresponding to 6.50 mmol g1 (qe). The second step involves the precipitation: 3þ 2BiðzeoliteÞ

141 421 002

211

Bi2S3

H/Y-zeolite

þ 3Hþ ðzeoliteÞ þ BiðsolutionÞ /BiðzeoliteÞ þ 3HðsolutionÞ 3þ

Bi2S3/Y-zeolite

Intensity (a. u.)

3.

130

The volume is measured through water displacement in an inverted graduated burette.

221

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 0 7 0 e2 0 7 8

o

(5)

where b (rd.) is the broadening of the intense XRD peak. Meanwhile, there is total disappearance of the Y-zeolite peaks. H/Y-zeolite contains both sodalite cage and super cage A), which confine the Bi2S3 particles. Therefore, the (13  formation of Bi2S3 takes place not only in the pores of Y-zeolite but also on the surface. The hetero-system displays a broad diffraction peak corresponding to the (130) reticular plane

20

30

40

50

60

70

80

2 θ ( degrees ) Fig. 2 e X-ray diffraction patterns of H/Y-zeolite, Bi2S3 and Bi2S3/Y-zeolite.

which agrees with typical mesoporous silica [18], Laser Raman spectroscopy is a powerful tool for the structure investigation [19]. Little is known on Bi2S3 [20] and no Bi2S3 supported on zeolite framework has been reported before now. The spectrum (Fig. 3) exhibits four vibrational peaks associated with BieS bonds at 323, 493, 601 and 969 cm1, in agreement with the literature [21,22]. The additional peak at 1093 cm1 is not known, but may be attributed to surface optical phonon modes. It is well known that the different morphologies can result in wave numbers shift due to the difference in the size (quantum size effect) and surface phonon modes [23]. To investigate the location of the guest materials in the composite, we have measured the specific surface area (SBET) and pores volume of Bi2S3/Y-zeolite. The results (Fig. 4) show that the surface SBET (¼629 m2 g1) of the host Y-zeolite decreases drastically down to 87.82 m2 g1. Similarly, the volume of the pores decreases from 0.364 to 0.121 mL g1. Such pronounced decrease clearly demonstrates that Bi2S3 is located in the cages of Y-zeolite and obstructs the channels of the zeolite, thus impeding the diffusion of N2 throughout the channels. Assuming cylindrical pore geometry, the average pore size (rp) is given by the relation:

10

4.5

323 cm 601 cm 493 cm

4.0

8

Intensity (x 10 / a. u.)

3.5

969 cm 6

(a)

1093 cm

4

Bismuth (III) concentration (x 10 mmol)

5.0

3.0

67.8% of Bi ions exchanged

2.5

equilibrium zone 2.0

4

(b) 2

1.5 0

10

20

30

40

50

Time (min)

60

0

200

400

600

800

1000

1200

1400

-1

Fig. 1 e Variation of adsorption uptake of Bi3D on H/Yzeolite, C0ðBi3D Þ [0:48 mmol=L; pH 1.5, T [ 60  C.

Raman shift (cm ) Fig. 3 e Raman spectra of Bi2S3/Y-zeolite and H/Y-zeolite.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 0 7 0 e2 0 7 8

Adsorption Desorption

240

(a) 220

70

(b)

60

3

-1

Adsorbed Quantity (cm g STP)

80

200

50 180

40

(c)

30

0.0

0.4

0.8

20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure ( P/P0 )

Fig. 4 e N2 adsorption/desorption plots of (a) H/Y-zeolite, (b) Bi2S3/Y-zeolite and (c) Bi2S3.

rp ¼ 2Vp =SBET

(6)

According to the International Union of Pure and Applied Chemistry (IUPAC), the prefix meso- refers to the region (2e50 nm), macro- >50 nm, and micro- <2 nm. With an average pore size of w3 nm, Bi2S3/Y-zeolite is classified as a meso-porous material. It is important to note that the results are interesting, despite the decrease of the specific surface area of the zeolitic framework, insofar they are several times greater than that of Bi2S3 prepared under the same conditions (SBET ¼ 9.57 m2 g1 and Vp ¼ 0.066 mL g1). The SEM pictures (Fig. 5) show rough surface with fine particles of Bi2S3 on zeolite framework. The EDAX spectrum (Fig. 6) clearly shows the presence of Bi (18.08 at.%), S (25.49 at.%) of Bi2S3 and Si (12.76 at.%), Al (4.82 at.%) and O (38.85 at.%) of Y zeolite framework. The molar ratio Bi/S reveals that Bi is a little more than S. The presence of excess bismuth in the hetero-system

2073

may be attributed to the existence of a sodalite cages closed by Bi2S3 formed on the surface which prevents Bi3þ cations to react with S2 within the cages. The first loss in the TGA plot of Bi2S3/Y-zeolite (Fig. 7) is attributed to the thermo-desorption of zeolitic water and impurities (310e525 K) whereas the decomposition of Bi2S3 occurs via two step processes and confirmed by DSC run (Fig. 8). Indeed, an endothermic peak and two small irreversible exothermic peaks are observed in the same temperature range as TGA. Fig. 9a shows the UVeVisible absorption spectra of both Bi2S3/Y-zeolite and H/Y-zeolite. The incorporation of Bi2S3 into the zeolite framework has a remarkable influence on the light absorption. The transition in Bi2S3, is allowed by the Laporte’s law and occurs between ns-orbital representing the high occupied molecular orbital (HOMO) and np-orbital, the lower unoccupied molecular orbital (LUMO). This charge transfer is classified as ligand-to-metal charge transition (LMCT). The gap of Bi2S3 depends on the particle size of the semiconductor (quantum effect) and lies between 1.25 and 1.9 eV depending on the synthesis method. Band gap SCs have the dependence of the optical coefficient (a) on the light energy hn [24]:   n ðahnÞ ¼ A hn  Eg

(7)

A being a constant and n is equal to 2 or 1/2 for the allowed direct or indirect transitions respectively. By extrapolating the linear portion to the hn axis (Fig. 9b), a gap of 1.40 eV for Bi2S3/ Y-zeolite is obtained, close to that cited in the literature [25]. A further transition, indirectly allowed is observed at 1.30 eV. Fig. 10 gives the thermal variation of the electrical conductivity of the system Bi2S3/Y-zeolite. The charge localization leads to a temperature dependence which can be described by various aspects. The curve is subdivided into three regions. In the first region (285e400 K), s increase slowly with temperature, where the number of ionized donor shallows mainly determines the electrons concentration (ND). This naturally occurs as a result of the transition of the carriers from the impurity levels to the conduction band and the conductivity follows an exponential law: s ¼ s0 expf  Ea =kTg

(8)

with activation energy (Ea) of w0.1 eV and a resistivity s300 K of 2.1  103 U cm. In the second region, an SC-metal-transition occurs and the conduction is governed by both the electrons concentration and the mobility, in agreement with the experimental results of previous works. Indeed, Shaban et al. [26] reported that the fall in the electrical conductivity is due to the decrease of the mobility while the density in this temperature region remains nearly constant. Above 660 K, the intrinsic conduction begins and s increases sharply where the charge localization is attributed to lattice distortion (small polaron hopping) suggesting conduction due to sulfurvacancy. Ge et al. [27] reports that at 673 K, the sulfide deficiency in Bi2S3 generates vacancies VS and causes increase in the electron concentration:   Bi2 S3 /Bi2 S3x þ 2x e ðCBÞ þ x=2 S2

Fig. 5 e SEM micrograph of Bi2S3/Y-zeolite.

(9)

The conduction occurs by electron hopping between mixed bismuth valences accommodated in octahedral environment. Formally, each removed sulfide generates two electrons:

2074

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 0 7 0 e2 0 7 8

Fig. 6 e EDS spectra of Bi2S3/Y-zeolite.

 SS 40:5S2 þ V s þ 2e

(10)

The species are written according to the Kroger-Vink notation. The charge carriers may be though of as being Bi2þ in a back ground sea of Bi3þ. So, the transport properties are altered by sulfur deficiency and n-Bi2S3 can be used as photoanode. The low mobility mh300 K (2.54  102 cm2 V1 s1), defined as the mean drift velocity in an electric field of unit force, is due to the obstruction of S2 ions to the hopping process between mixed valences Bi2þ/Biþ.

3.1.

Photo-electrochemical properties

The photo-electrochemical properties of Bi2S3/Y-zeolite are studied in alkaline solution (1 M KOH/ S2 O2 3 0.1 M). The J(V) characteristics in the dark and under visible light are shown in Fig. 11. The photocurrent (Jph) increases toward the anodic direction confirming the n-type conduction. The photocurrent onset potential (Von) is taken as the potential below which no photocurrent could be observed. However, the potential Von is accurately determined from the Gartner relation:

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 0 7 0 e2 0 7 8

J2ph ¼ Cons  a2 W2 ðV  Von Þ

100 800

600

400

Second decomposition step in Bi S

Bi S decomposition :

TGA

T (K)

weight loss

Reduction of sulfur in Bi S

60

200

40

DTG

0

1

2

3

0

4

 kT C2 ¼ f0:53 0 3 NA g Vfb  V  e

time (h) Fig. 7 e Combined TGA/DTG vs. temperature of Bi2S3/Yzeolite material.

670 K

762 K

Endo

DSC (mW mg )

-1.2

386 K

-1.6

-2.0 300

400

500

600

700

800

Temperature (K)

Fig. 8 e Typical thermogram of DSC vs. temperature of Bi2S3/Y-zeolite.

a

(12)

by extrapolating the plot to C2 ¼ 0 and the slope of the linear part respectively (Fig. 12). The positive slope confirms the n type behavior of Bi2S3. The permittivity of Bi2S3 (3 ¼ 120 at 1 kHz) is taken from ref. [30]. The photo-current onset potential is shifted to the negative side. The difference between the potentials Von and Vfb suggests the presence of some surface states within the gap region. The potential of H2O/H2 couple can be positioned anywhere, depending on the O2 concentration; its accurate position is given by plotting the J(V) characteristic under the same working conditions (N2 saturated solution). As noticed above, the valence band of Bi2S3 is of S2: 3p character (HOMO) (sulfur is less electronegative than oxygen) while the conduction band (CB) derives mainly from Bi3þ: 6s2 orbital. Hence, the photoelectrons have a strong ability to reduce H2O into gaseous H2, making efficient the utilization of the solar spectrum by reducing the gap. The potential Vfb outlines the energetic position of Bi2S3-CB with respect to vacuum: PCB ¼ 4.50 þ e Vfb e Ea. The PCB value (3.74 eV/0.77 V) is more negative than the H2O/H2 level allowing a favorable H2-liberation. Furthermore, the potential of H2O/H2 level changes with pH (0.06 V pH1) whereas the potential Vfb is not or only

-0.4

-0.8

(11)

The plot of J2ph (Fig. 11, Inset) intercepts the x-axis at the potential Von (¼0.67 V), close to that reported by some of us [28]. Such negative value should give rise to a large band bending with a zero (e/hþ) pairs recombination. The maximum Jph value reached at w0.46 V with a plateau region, is unaffected by the agitation of the solution, this implies that the recombination rate of (e/hþ) pairs is kept at a low level and the overall process is controlled by the interfacial reactions and electron transport within Bi2S3 [29]. However, the flat band potential (Vfb ¼ 0.465 V) and the donor density (ND ¼ 1.17  1018 cm3) are evaluated from the MotteSchottky plot:

Thermodesorption of zeolitic water and impurties

80

2075

b

Fig. 9 e UVevisible absorption spectra of (a) H/Y-zeolite and Bi2S3/Y-zeolite and (b) the direct allowed optical transition of Bi2S3/Y-zeolite.

2076

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 0 7 0 e2 0 7 8

0.2

-2

C /µF

-2

cm 4

0.3

0.1

Vfb = - 0.465 V 0.0 -1.0

-0.8

Fig. 10 e Logarithm of the electrical conductivity of Bi2S3/Yzeolite vs. reciprocal absolute temperature.

-0.6

-0.4

-0.2

0.0

Potential (VNHE ) Fig. 12 e The MotteSchottky plot of Bi2S3 in (KOH 1 M) solution.

slightly dependent on the adsorption of ions (e.g. Hþ, OH); Riley et al. [31] reports neglected zeta potentials of Bi2S3 nanoparticles (22 mV). This property has been exploited to have an optimal band bending at the interface Bi2S3/S2 O2 3 electrolyte. The potential of Bi2S3-VB (þ0.63 V) is deduced from the relation: PVB





¼ Vfb  Ea  Eg e

3.2.

(13)

Photoactivity

The energy band diagram of the junction Bi2S3/Y-zeolite/ solution (Fig. 13), predicts clearly a spontanous hydrogen photoproduction. In the absence of electroactive species, the photoholes are consumed to corrode Bi2S3. Hydrogen production on Bi2S3/Y-zeolite photocatalyst in KOH solution (pH w 13.5) containing S2 O2 3 as hole scavenger is successfully tested under visible-light irradiation compared with Bi2S3. The potential of O2/H2O is more positive than Bi2S3/Y-zeolite-VB and oxygen cannot compete with S2 O2 3 oxidation.

To ensure maximum photocatalytic performance, the experiments are performed at the optimal temperature (50  C) and the hydrogen energy conversion efficiency is given by [32]: 

h ¼ 2 number of H2 mol s1 photons flux s1

(14)

the factor 2 comes from the fact the H2 formation is two electron event. The electrons exchange is determined by the relative positions of unoccupied states pertained to adsorbed H2O, and Bi2S3-CB (0.76 V). An efficient charge transfer is expected if both the energetic states show an optimum band bending of w0.3 V and the driving force is closely related to the redox potential of H2O/H2 couple. The undesired (e/hþ) recombination represents the major energy-wasting step; the photo-corrosion may be kinetically prevented by using S2 O2 3 as hole scavenger which promotes the charges separation. With a redox potential less positive than Bi2S3-VB potential 2 (þ0.63 V), the redox couple S2 O2 6 =S2 O3 (wþ0.08 V) is ideal as hole scavenger to prevent the recombination and SO2 3

4

Energy eV

In the dark Under ullimination

0

- 4.50

2

-0.4 Potential (V

-0.2

0.0

0.2

)

-4

Fig. 11 e The J(V) characteristics of n-Bi2S3/Y-zeolite plotted both in the dark and under illumination in (KOH 1 M, 0.1 M Na2S2O3), scan rate 4 mV sL1. Inset: the determination of the potential Vfb through the J2ph vs. the potential.

E = 0.1 eV

-0.5

2-

-2

Current (mA cm )

-0.6

∇

-0.8

2-

-1.0

S4O6 /S2O3 H2O/H2

CB = - 0.77 V 0

Eg = 1.40 eV

0

VB = + 0.63 V

n-Bi2S3/Y-zeolite

+0.5

Potential (VNHE)

Fig. 13 e The energy diagram of n-Bi2S3/Y-zeolite/S2 O2L 3 electrolyte junction.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 0 7 0 e2 0 7 8

a reduced gap. The composite material has a high specific surface area with meso-porous texture. The hydrogen production is twice on the hetero-system Bi2S3/Y-zeolite. This improvement is attributed to the high surface area, mesoporosity and synergistic effect which result in a narrow band gap, and promotes the charges separation. The development of such photocatalytic configuration is elaborated via one reaction and would promote usage of solar light for cleaner energy production system. The zeolite structure has a significant role to play in the photocatalytic properties and is not only a simple support matrix.

Bi2S3/Y-zeolite

H2 (mmol / g catalyst)

0.20

2077

Bi2S3

0.16

0.12

0.08

0.04

Acknowledgment

0.00 0

10

20

30

40

50

60

Time (min) Fig. 14 e H2 evolution vs. illumination time on Bi2S3 and Bi2S3/Y-zeolite in (KOH 1 M, Na2S2O3 0.1 M) solution.

oxidation proceeds fast enough to keep the holes concentration below the critical threshold required to trigger the photo driven corrosion and the oxide approaches complete stability photo-corrosion. It is worthwhile to mention that the concentration of Bi3þ in the final solution is too small for a reliable determination. Under the experimental conditions, Bi2S3/Y-zeolite (292 mmol g1 h1, h ¼ 0.12%) exhibits much enhanced photocatalytic activity than Bi2S3 (114 mmol g1 h1, h ¼ 0.05%) with a quantum yield of 0.045% (Fig. 14). S2 O2 3 oxidation to S2 O2 6 takes place via hole injection in VB:  þ Bi2 S2 3 =Y  zeolite þ hn/eCB þ hVB

(15)

 2H2 O þ 2e CB /H2 þ 2OH

(16)

2  þ 2S2 O2 3 þ 2hVB S4 O6 þ 2OH

(17)

The difference of activity is attributed to the high specific surface area of Bi2S3/Y-zeolite, thus providing more PEC active sites. The enhanced performance is also due to the high optical coefficient absorption of Bi2S3 (w104 cm1) which yields a large penetration length. Some authors [33] reported an enhancement in the hydrogen production, after supporting the photocatalyst in the zeolite structure. They suggested that the zeolite matrix acts as a templating agent by controlling the surface area and the morphology of the semiconducting particles during the phase formation. The tendency to saturation of the H2-formation over illumination time is due to the competitive reduction of the end product S4 O2 6 (reaction 17). Indeed, the initial activity is nearly restored when using a new S2 O2 3 solution.

4.

Conclusion

We have successfully prepared a novel photocatalyst configuration Bi2S3/Y-zeolithe by chemical way. The valence band consists of less electronegative S2: 3p orbital leading to

The authors would like to thank Mr. A. Crisis for the Raman spectroscopy at the Laboratory of Electrochemistry and Physical Chemistry of Materials and Interfaces (Grenoble) for his experimental assistance.

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