Visible Light Hydrogen Evolution over <i>α</i>-MoO</span>_{<span style="font-family:Verdana;">3</span>}<span style="font-family:Verdana;"> and <i>α</i>-MoO</span>_{<span style="font-family:Verdana;">3</span>}<span style="font-family:Verdana;">/ZnO Hetero-Junction

MoO ₃ and 5% MoO ₃/ZnO were prepared by impregnation method using (NH ₄) ₆Mo ₇O ₂₄, 4H ₂O as precursor and ZnO as support. The prepared samples were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), infra red (FTIR) and UV-Vis diffuse reflectance (DRS) spectroscopies and photo-electrochemistry. The XRD pattern showed that the MoO₃ powder treated at 700?C is a single-phase crystallizing in an orthorhombic structure with a direct optical transition ( 2.70 eV ) . The hetero-junction 5% MoO₃/ZnO was photo-electrochemically characterized to assess its feasibility for H₂ production under visible light. The capacitance potential (C^?2 f(E)) characteristic of MoO₃ plotted in Na₂SO₄, (0.1 M) electrolyte indicates n-type conduction with a flat band potential of ?0.54 V_SCE. The photocatalytic activity was performed for the photoreduction of water to hydrogen under visible light illumination. The best performance occurs at pH ~ 7 with an evolved volume of 5.9 mL.

• MoO</span>_{<span style="font-family:Verdana;">3</span>}<span style="font-family:Verdana;">/ZnO
• Hetero-Junction
• Photocatalysis
• Hydrogen
• Visible Light

1. Introduction

Nowadays, the energy supply is becoming a major problem facing our civilization and increasing demand, leading to the exhaustion of natural resources (coal, oil, gas, etc.). For this, it is important to find friendly alternatives process for the future years. So, clean strategies are highly motivated and renewable energies have many advantages due to their availability, cleanness and power [1]. The development of an efficient and low-cost energy carrier has recently alerted scientists to the promising approach of directly converting renewable energy into hydrogen [2] [3] [4]. This approach focuses on the H₂ production from solar energy using photo-electrochemical oxides. In this perspective, hydrogen is a promising energetic vector due to its being used as a clean status, like fuel cells, in both the transport and industrial sectors. However, it differs from other primary energetic sources in that it must be manufactured just before utilization due to the difficult storage or liquefaction. Therefore, both scientific and technological advances are being made, as it has become essential to control the technologies producing renewable and sustainable energy. Hydrogen can be produced by various methods such as water electrolysis [5] [6] [7] [8] [9], coal gasification and methane reforming (steam reforming, oxidation and dry reforming) [10] [11].

MoO₃ is an important photocatalyst that has received considerable attention as an advanced material owing to its attractive physical and chemical properties including its stability at multiple oxidation states, mechanical hardness, thermal stability, superconductivity, and great performance in many catalytic reactions [12]. Recently, numerous inspired schemes and pathways have been developed for the synthesis of nanomaterials for their potential applications in diverse technological fields [13]. Among the important transition-metal oxides such as molybdenum (Mo) based oxides are attractive due to their unique structural and optical properties [14]. Although molybdenum has oxidation states ranging from +2 to +6, oxides exist mainly in two forms namely (IV) and (VI) [15]. The existence of metal-like electronic conductivity of molybdenum(IV) oxide makes them promising in energy-related applications [16]. MoO₃ is a wide bandgap semiconductor with n-type behavior attributed to oxygen deficiency. Their electrochromic, thermochromic, and photochromic properties, have been investigated as smart materials for catalysis [17], sensors and lubricants. MoO₃ is also used as display devices, sensors, smart windows, lubricants, battery electrodes [17] [18] [19]. MoO₃ exhibits three crystallographic forms, orthorhombic (α-MoO₃), monoclinic (β-MoO₃), and hexagonal (θ-MoO₃) [20]. The meta-stable β and θ forms are more studied than the thermodynamically stable α-MoO₃ [21] [22]. The monoclinic variety is transformed to orthorhombic MoO₃ at 400˚C. Crystalline o-MoO₃ has a layered structure of distorted MoO₆ polyhedral sharing edges and corners and one oxygen in each polyhedron is unshared M = O. It is literally known that the phase purity of MoO₃ relied on the adopted synthetic procedure and experimental conditions. Considerable progress has been accomplished recently for the size and phase-controlled synthesis of MoO₃ with optimized catalytic properties [23] [24]. There are many routes based on different starting compounds: polymerization and polycondensation of metal alkoxides, ion-exchange methods and oxidizing reaction of metallic W or Mo with a solution of H₂O [25] [26].

ZnO is n-type semiconductor with a direct wide bandgap energy of ~3.3 eV and large exciton binding energy (60 meV) at room temperature [27]. Its good optical and electrical properties coupled with a low cost, non-toxicity and abundance in nature make it attractive in photo-electrochemical conversion. Its other favorable properties include electrochemical stability and thermal stability as well as good stability in hydrogen plasma [28]. So, the control of size and shape of ZnO nano/micro-structures is becoming an interesting field due to their inherent size and shape-dependent applications [29] [30]. Its unique properties and low production cost make it attractive in chemicals, semiconductors, electronics, and health care industries.

The improvement of ZnO conductivity is commonly done through doping by hetero-valent atoms or oxygen deficiency. Such operations depend on the type of dopant or surrounding atmosphere, to increase the density of free charge carriers [31] [32] [33].

The aim of the present work is focused on the preparation of α-MoO₃ and the hetero-junction α-5%MoO₃/ZnO by impregnation method. The photocatalysts properties were assessed through the hydrogen formation upon visible light. The materials were characterized by XRD, FT-IR, DRS, SEM analysis, and the photo-electrochemistry.

2. Experimental Part

2.1. Materials Preparation

The support ZnO was prepared by nitrate route at pH ~ 10 using Zn(NO₃)₂, 6H₂O as precursor material. The solution was slowly evaporated and the obtained solid is heated, at 500˚C (6 h) with a flow rate of 5˚C/min. The supported material 5 wt% MoO₃/ZnO was synthesized by wet impregnation using (NH₄)₆Mo₇O₂₄, 4H₂O on ZnO support. The suspension was agitated for 1 h, evaporated on a sand bath and dried at 100˚C overnight. Finally, all the samples were calcined at 700˚C for 6 h.

2.2. Characterization Technique

The solides MoO₃ and 5% MoO₃/ZnO were identified by X ray diffraction (XRD) using INEL XRG 3000 diffractometer with CuKα anticathode (λ = 0.15405 nm). Scanning electronic microscopy (SEM) study was carried out with Philips XL30- FEG equipment. The FTIR spectra were recorded with a FTIR Alpha Bruckers spectrometer. UV-Vis diffuse reflectance spectra (DRS) were measured in the range of 200 - 900 nm using a Specord 200 Plus spectrophotomoter with an integrating sphere accessory. The reflectance (R) was converted to absorbance by Kubelka-Munk function:

F ( R ) = ( 1 − R ) 2 / 2 R (1)

The photo-electrochemical study was performed in standard cell with three electrodes; Pt auxiliary electrode, a saturated calomel electrode (SCE) and the working electrode. The studies were inverstigated in neutral electrolyte recorded PGZ301 potentiostat (Radiometer analytical) using Na₂SO₄ (0.1 M). The capacitance-potential (C⁻² f(E)) characterization carried out at a frequency of 10 kHz.

2.3. Photocatalytic Application

The photocatalytic activity of different samples was assessed by the reduction of water to hydrogen under three tungsten lamps (3 × 200 W). The reaction war realized in a Pyrex double walled reactor equipped with a cooling system; the temperature was regulated at 50˚C ± 1˚C. Typically, 100 mg photocatalysis were suspended in 200 mL of Na₂SO₄ aqueous solution (0.1 M). The suspension was bulbbed with nitrogen for 30 min. The hydrogen was quantified volumetrically by water displacement caused by the pressure developed inside the reactor. The gas (H₂) generated was identified by gas chromatography (Agilent Technologies 7890A, GC system) analysis with the retention time of 1.592 mn.

3. Results and Discussion

3.1. Characterization

Figure 1 displays the XRD results for the samples calcined at 700˚C. All characteristic peaks located at 24.1˚, 26.5˚, 28.16˚, 34.2˚, 39.4˚, 49.8˚ and 59.5˚ indicate that α-MoO₃ crystallizes in the orthorhombic system (JCPDS Card N˚ 47-1320) with no impurity. The 2θ peak at 12.8˚ indicates the presence of the orthorhombic phase instead of the monoclinic structure [34].

Figure 1 shows the formation of the zincite phase in agreement with the JCPDS Card N˚ 36-1451. For the system 5 wt% MoO₃/ZnO, the presence of the mixed phases confirms the formation of the hetero-junction. The mean particle sizes (D) are evaluated using the broadening of intense XRD peaks (β):

D = 0.9 λ { β cos θ } − 1 (2)

D is found to be 84 nm. The lattice parameters a and c was calculated using Equation (2):

d h k l 2 = 4 / 3 ( ( h 2 + h k + k 2 ) / a 2 ) l 2 / c 2 (3)

where d_hkl is the inter planar spacing obtained from Bragg’s law, and h, k and l the Miller indices. The lattice parameters are accurately evaluated by the least square method: a = 3.89 and c =7.09 Å, which are slightly higher than those for bulk ZnO (a = 3.16 Å and c = 5.1 Å), may be due to the effects of compressive grains.

The surface morphology of the synthesized samples was observed by the SEM analysis. MoO₃ has a fairly homogeneous structure and a stick-like appearance (Figure 2(a)). ZnO has a homogeneous structure and a spherical appearance (Figure 2(b)). The SEM image shows that the system 5 wt% MoO₃/ZnO has a homogeneous nature with spherical form and regular sizes (Figure 2(c)). It has been observed the grain size of the ZnO changes with doping of Mo.

The FT-IR spectra of MoO₃ and Mo-doped ZnO are regrouped in Figure 3.

The peak at 1600 cm⁻¹ is assigned to the bending vibration of adsorbed water on the material surface while the peak around 420 cm⁻¹ is attributed to the stretching vibration of Zn-O bonds. The presence of MoO₃ depicted evidenced by peaks at 980, 819 and 453 cm⁻¹. The first peak characteristic of the terminal M = O stretching vibration with an indicator of the layered orthorhombic MoO₃ phase. The peak at 819 cm⁻¹ is assigned to the stretching mode of oxygen in Mo-O-Mo bonds, while the broad band at 453 cm⁻¹ to the bending vibration of oxygen atom are linked to three metal atoms. Two peaks at 1544 cm⁻¹ and 1442 cm⁻¹ could be ascribed to the formation of Mo-O-Zn bond. The presence of additional absorption bands in the region (404 - 1110 cm⁻¹) compared to ZnO nanoparticles indicates the presence of Mo-oxides. The M-O frequencies observed for metal oxides are in agreement with the literature [35] [36].

The optical band gap (E_g) is estimated by assuming a transition between the valence and the conduction bands using the relation:

( α h ν ) n = B × ( h ν − E g ) (4)

where h ν is the energy of the incident photon and B is an energy-independent constant. For allowed direct transitions the coefficient n is equal to 2 and for indirect allowed transitions n = 1/2. So, the gap E_g of MoO₃ is found to be 2.70 eV (Figure 4) from the plot of ( α h ν ) 1 / 2 versus h ν and by extrapolating the linear portion of the absorption edge to ( α h ν ) 1 / 2 = 0 [37]. This value is in agreement with the yellow color of the material. ZnO exhibits an indirect transition with a band gap of 3.10 eV (Figure 4).

The photo-electrochemical (PEC) characterization was performed in the dark and under irradiation at a scan rate of 10 mV s⁻¹ in the region [−1, 1 V]. The study was conducted in the same conditions of the photo-catalytic tests (see below). A good electrochemical stability of the material is observed with a dark current (J_d) less than 2 mA/cm². The increase of the photo-current (J_ph) along the anodic polarization is characteristic of n-type conductivity where the electrons are the majority charges carriers (Figure 5). The potential of H₂O/H₂ couple is −0.48 and −0.49 V respectively onto MnO₃ and ZnO. The value is obtained by extrapolating the tangent line over the slope and prolonging it to zero current in the current-potential curve.

The (C⁻²f(E)) plot of the semi-conductor/electrolyte junction is regrouped in Figure 6. The potential V_fb is obtained according to the following equation:

1 C 2 = 2 ε ε o e N D ( V − V f b − k T e ) (5)

where e is electron charge (1.6 × 10⁻¹⁹ C), ε the dielectric constant, and N_D the electrons density.

The positive slopes indicating n type [38] behavior where the majority charges carriers are electrons. The potential E_fb of MnO₃ and ZnO is 0.61 and −0.61 V, respectively, these values are derived from the potential intercept of potential axis (C⁻² = 0).

3.2. Photocatalytic Hydrogen Generation

The synthesized photocatalysts are successfully applied for H₂ production under visible light irradiation in neutral solution (0.1 M Na₂SO₄) using S 2 O 3 2 − as hole scavenger (10⁻³ M) under magnetic stirring (200 rpm). It is shown that the H₂ production over MoO₃ and 5 wt% MoO₃/ZnO as a function of the catalyst mass in the range (50 - 300 mg) indicated that a maximum is reached with a catalyst mass of 200 mg. The H₂ amount is 5.9 mL for MoO₃ and 2.4 mL for 5 wt% MoO₃/ZnO (Figure 7). The low value of H₂ produced over the heterosystem indicated the absence of synergetic effect between MoO₃ and ZnO. The enhanced photoactivity of MoO₃ under visible illumination can be attributed to the defect states introduced within the gap region during the mechanical treatment on the catalyst surface. However, the problem is that such defect sates can also work as carriers traps, thereby yielding a decrease of the quantum yields

Figure 8 regrouped the results of the hydrogen generation of versus pH with the catalyst mass of 200 mg for three different pHs (~ 7, 10 and 12) in presence of S 2 O 3 2 as hole scavenger. So, H₂ evolution decreases with raising pH and the

best photoactivity is obtained on MoO₃ at pH ~ 7 while on the hetero-junction 5 wt% MoO₃/ZnO only 1.4 mL is obtained at pH 10. The photocatalytic performance of MoO₃ is close to the work on delafossite reported by Bellal et al. [39].

4. Conclusion

The photocatalytic hydrogen generation was evaluated using the synthesized α-MoO₃, ZnO and their hetero-junction. The prepared materials were characterized by XRD, FT-IR, UV-Vis DRS, SEM and photo-electrochemical analysis. The operating conditions were optimized to improve the hydrogen generation under visible light, and the best photo-activity was observed with a catalyst mass of 200 mg in Na₂SO₄ solution. It was found that α-MoO₃ has the potential to produce hydrogen compared to the heterojunction due to both the light absorption in the visible region and the absence of synergy.

Acknowledgements

The authors thank Dr. A. Djadoun and Dr. R. Brahimi for their technical assistance. The financial support of this work is provided by the Faculty of Chemistry.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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