1. Introduction

The necessity of the clean water supply and green energy has been aggrandized globally. Heavy metals, industrial waste and dyes are the main sources of water pollution [1]. TiO₂ stands as a potential candidate for the degradation of organic dyes, because of its inherited characteristics. However, the wide band gap (3.2 eV) and rapid recombination of photo-generated electrons imposes constraints for its utilization in the photo degradation processes [2]. In order to subdue these constraints, numerous strategies have been adopted for nanocomposites synthesis of TiO₂ with metals, nonmetals, noble metals and semiconducting materials [3–11]. Thin films of TiO₂ and MoO₃ have also been demonstrated to be as a good photocatalyst [12, 13]. Experiments were performed in order to investigate the hybridize TiO₂ with MoO₃ via different synthesis techniques [14–22]. In 2010, core-shell TiO₂ and MoO₃ nanostructures were synthesized, in which TiO₂ served as a core and MoO₃ as a shell [23, 24]. Recently, a significant improvement in photochromic properties has also been successfully attempted via TiO₂-MoO₃ nanostructures. Two types of solvents were used, in which the nanostructures exhibited 66.6% higher photochromic coloration efficiency in ethanol than in water [25]. However, these techniques are surfactants based and required thermal treatments in order to avoid bi-products.

Here we provide a substitutive approach for nanocomposites synthesis with femtosecond laser ablation in water. The objective of current investigation is the synthesis of MoO₃-TiO₂ nano-composites with novel approach of pelletization, which was followed by laser ablation in water. In this approach, MoO₃ was mixed with TiO₂ and pressed into pellets with the help of cold isostatic press (CIP) technique. The stable nanocomposites were synthesized with femtosecond laser ablation of the pellets in pure distilled water. Besides, the nanocomposites were applied for Methylene Blue (MB) dye degradation and current-voltage study to investigate the photocatalytic and photovoltaic behaviors of the generated nanocomposites.

2. Experimental Protocols

The powder of MoO₃ in (1, 3, 6 and 9 atm. %) was mixed with TiO₂. For each pellet, 1.5 g of the mixed powder was pressed with hydraulic press. In order to overcome fragility of the material in water, the generated pellets were pressed with Cold Isostatic Press (CIP) by applying pressure of 200 MPa. As a consequence, the obtained pellets were of enough hardness and have no brittleness for nanocomposites synthesis with laser ablation in water. In the following text, the samples with MoO₃ contents of 1, 3, 6 and 9 atm. % mixed with TiO₂ are named as 1% MoO₃-TiO₂, 3% MoO₃-TiO₂, 6% MoO₃-TiO₂ and 9% MoO₃-TiO₂, respectively. During the experiment, an ultrashort Ti: Sapphire laser operating at 800 nm, with repetition rate of 1 kHz and 70 fs pulse duration was utilized in current investigation. Laser pulses with 0.2 mJ pulse energy were focused perpendicularly on the pellets surface with the help of 20 cm-plano convex lens, which resulted in a spot size of 0.45 cm². The pellets performed of 2 mm thickness and 18 mm diameters were obtained. The target density (TD) of the pellets was 78%. The pellets were staged inside a glass vessel, which was filled with 7 ml deionized water (18 MΩ). The liquid column above the pellet surface was 6 mm. During nanocomposites synthesis, the stage containing glass vessel was rotated at a speed of 8 revolutions per minute (rpm). The stage was allowed to move 0.5 mm horizontally after each cycle of rotation. This process of laser ablation was kept running for 30 minutes duration. As a result, stable nanocomposites colloids with violet appearance for more than two weeks were obtained.

The Crystallinity of the synthesized nanocomposites colloids was probed with X-Ray diffraction (XRD) using Cu Kα radiation. While the absorption properties were investigated with UV-Vis spectro-photometer, which was equipped with diffuse reflectance port. The generated nanocomposites colloids were centrifuged at 9,000 rpm for 9 minutes, which was followed by drying the residue at 70 °C for 5 hours. As a result 3.1 mg of the nanocomposites powders were achieved. Almost same quantity of weight loss was examined by weighing the pellets before and after laser ablation. The synthesized nanocomposites were utilized for XPS, UV-Visible, photovoltaic and photocatalytic measurements. Transmission Electron Microscope (TEM) operating at 300 kV was utilized to ascertain the size of the generated nanocomposites colloids. For this analysis a few drops of the synthesized colloids were put on a Cu grid, which was followed by TEM and EDS analyses. An autolab potentiostat equipped with a 100 W xenon lamp was utilized for photovoltaic measurements. The current density-voltage (J-V) and electrochemical impedance spectroscopy (EIS) measurements were carried out under xenon lamp.

The valence state of the nanocomposites was ascertained with X-ray photoelectron spectroscopy (XPS, thermo Escalab 250 Xi). The measurements were carried out with Al Kα as a radiation source, comprising hυ = 1486.6 eV energy. While the values of binding energy were referenced to carbon C 1s peaks ranging from 284.6 to 285.2 eV. The photocatalytic assay of the synthesized nanocomposites was inspected by MB dye degradation with visible light irradiation. For this measurement, 4.4 mg of the catalyst and 0.44 mg of the MB concentrations were maintained in 40 mL water. The degradation measurements were carried out using mercury vapor lamp with 150 W power and wavelength from 420 nm to 800 nm for four hours. The visible light from the lamp was focused perpendicularly on the glass vessel. After continuous irradiation with mercury vapor lamp, the aliquots of samples were taken and centrifuged to eliminate photo-catalysts particulates. The degradation efficiency was ascertained with UV-Vis spectrometer. In order to investigate the photovoltaic properties of the nanocomposites, the mass concentration of 1.5 g/500 ml was maintained. The solution was dropped on fluorine doped tin oxide (FTO) glass substrate, which served as a working electrode followed by drying in an oven at 80 °C. The Ag-AgCl and Pt plate were rendered as a reference and counter electrodes respectively in one mole NaOH electrolyte solution.

3. Results and Discussion

Synthesis of nanomaterial with laser ablation in liquids is governed by many factors and it is difficult to ascertain complex process of nanomaterials synthesis in liquids [9]. However, in our experiment when laser pulses hit the pellets surface, the dense clusters of the ablated species were ejected from the target. These species were accompanied by the generation of shock waves in the vicinity of laser impact. Therefore, the pressure and thermal gradients were amplified, which augmented the reaction of ablated species with the surrounding liquid. During the continuous process of laser ablation more species were ablated and the pressure gradient goes on increasing. The water layer restricts the forceful ejection of ablated species, which causes the confinement to the ablated species. A stage occurred when the confined zone could not sustain and the dense clouds of nanocomposites were ejected in water.

According to Schmitz et al [26] for nanomaterials synthesis with laser ablation in liquids the target density (TD) is one of the important factors. They speculated that the target porosity might have an impact on the particles crystallinity. In our experiment the TD was close to 80%, which might be supportive for the nanocomposites synthesis with 0.2 mJ pulse energy in water. The surface morphology of the synthesized nanocomposites is exhibited in Fig. 1. It can be recognized from Fig. 1(b) that spherical shaped TiO₂ nanoparticles are decorated with MoO₃ nanoparticles. The average size of the nanocomposites was 4.81 ± 0.79 nm, while the size ranged from 2 to 34 nm. The co-existence of crystalline TiO₂ and MoO₃ was revealed from HR-TEM images in Fig. 1(c). The fringe separations of 0.35 nm for TiO₂ and 0.23 nm for MoO₃ were determined from HR-TEM images. The elemental presence was analyzed with energy dispersive X-ray spectroscopy (EDS) mapping in Figs. 1(d)-1(f). The EDS results represented the TiO₂ nanoparticles in Fig. 1(e), while the existence of MoO₃ nanoparticles was revealed in Fig. 1(f). Therefore, the EDS results witness the co-existence of TiO₂ and MoO₃ in the synthesized nanocomposites. It means that pelletization with CIP followed by laser ablation provided a facile route for nanocomposites synthesis. Furthermore, the current mode of nanocomposites synthesis was without the replacement of target during laser ablation. In this approach, no subsequent heat treatment was mandatory for pelletization of the oxides mixture. Besides, during synthesis of the nanocomposites with replacement of one target by the other, a part of laser energy is wasted because of interaction with nanoparticle colloids from first target [27].

 

Fig. 1 TEM images of the TiO₂-MoO₃ nanocomposites for 100 nm (a), and 50 nm (b), HR-TEM image of the nanocomposites (c), selected area EDS elemental mapping (d), Ti Kα1 (e), and Mo Kα1 (f).

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Therefore, the current method for nanocomposites synthesis with laser ablation is free from replacement of the target and more laser energy can be saved. The XRD patterns of the nanocomposites with different contents of MoO₃ are depicted in Figs. 2(a) and 2(b). It can be presumed from Fig. 2(a) that diffraction peaks of XRD pattern for TiO₂ nanoparticles contains anatase phase with diffraction angles at 25.2°. While MoO₃ diffraction peaks appears at 12.7° and 25.6° for 6%MO₃-TiO₂ and 9% MO₃-TiO₂, which is indicated in Figs. 2(a) and 2(b). The diffuse reflectance spectra (DRS) in Fig. 2(c) shows that the absorption edges of the nanocomposites were shifted towards visible region of the solar spectrum. Such shift indicates that sufficient quantity of MoO₃ has been mixed with TiO₂ by laser ablation. The band gap energy (Eg) was calculated using Eg = 1239.8/λ, which was 3.10 eV for pure TiO₂ nanoparticles. Similarly, the band gaps calculated for 1%MoO₃-TiO₂, 3% MoO₃-TiO₂, 6% MoO₃-TiO₂, and 9% MoO₃-TiO₂ were 2.98, 2.92, 2.85, and 2.78 eV respectively. It means that the band gap of TiO₂ can be decreased with current approach. The XPS spectrum of the nanocomposites generated with laser ablation of 3% MoO₃-TiO₂ is depicted in Fig. 3(a). The spectrum exhibits that the Ti, Mo, O and C elements were present on nanocomposites surface. This is in line with the TEM and EDS results, in which co-existence of TiO₂ and MoO₃ has been witnessed.

 

Fig. 2 XRD patterns of (a) TiO₂, 1% MoO₃-TiO₂, 3% MoO₃-TiO₂, 6%MoO₃-TiO₂ and 9% MoO₃-TiO₂ nanocomposites, (b) Enlarged image showing MoO₃ peak at 25.2° and 25.6°. (c) DRS spectra of TiO₂, 1% MoO₃-TiO₂, 3% MoO₃-TiO₂, 6% MoO₃-TiO₂ and 9% MoO₃-TiO₂ nanocomposites, (d) Band gap calculation for MoO₃-TiO₂ nanocomposites.

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Fig. 3 XPS spectrum of the nanocomposites (a), deconvoluted spectrum of Mo 3d3/2 into 231.6 and 232.2 eV (b), Ti 2p spectrum showing 457.8 eV and 463.8 eV binding energies (c).

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The deconvoluted peaks for Mo3d in Fig. 3(b) at 231.6 and 232.2 eV denote the Mo⁵⁺ and Mo⁶⁺ states, which offers their key role in band gap modification [28].In Fig. 3(c), the binding energies for Ti 2P3/2 and Ti 2P1/2 at 457.8 and 463.8 eV indicate the Ti⁴⁺ oxidation state [29]. The oxidation state is an important ingredient in dye degradation, which can be ascribed to the mixing of MoO₃ with TiO₂ as a result of laser ablation.

The photovoltaic assay of the nanocomposites was ascertained in terms of the photo conversion efficiency for dye sensitized solar cells. Figure 4 describes the photovoltaic performance of nanocomposites for dye synthesized solar cells. The values of photovoltaic and photocatalytic parameters are summarized in Table 1.The best performance of current density-voltage is obtained from 3%MoO₃-TiO₂ as shown in Fig. 4(a). Besides, from EIS analysis in Fig. 4(b) the smallest arc radius was derived from 3% MoO₃-TiO₂ sample as well. It means that better charge transfer and less resistance Rct have been offered by nanocomposites synthesized with 3% MoO₃-TiO₂.

 

Fig. 4 J-V graphs of the nanocomposites (a) and nyquist plots of the nanocomposites (b) for different MoO₃ concentrations mixed with TiO₂.

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Table 1. Photovoltaic and photocatalytic parameters of the nanocomposites

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The photocatalytic performance of the synthesized nanocomposites was evaluated in terms of MB dye degradation under visible light irradiation for four hours. The nanocomposites show higher photocatalytic efficiency compared with pure TiO₂ nanoparticles. In Fig. 5, the 3%MoO₃-TiO₂ exhibited stronger photocatalytic efficiency. It can be presumed that 3% MoO₃-TiO₂ provided e^––h⁺ separation and interfacial charge transfer during dye degradation [30,31]. After 4 hours irradiation with visible light only 5% of MB dye was left as shown in Fig. 5. It is also shown in Fig. 5 that dye degradation of nanocomposites was hindered with the addition of MoO₃ above 3%. This is ascribed to exponential enhancement in the recombination process as the distance between e^––h⁺ is reduced [17].

 

Fig. 5 MB dye degradation under visible light for TiO₂ mixed with different MoO₃ concentrations.

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In previous work by the other researchers the best performance for dye degradation was observed with 5% Mo concentration [21]. However, in our experiment the best performance for dye degradation was examined with 3% MoO₃ concentration. This is because the dye degradation reaction is termed as a surface reaction, which might be dependent on the mode of nanocomposites synthesis. For the nanocomposites synthesized with chemical methods the photocatalytic activity is hampered because of the surface contamination with residual reagents [32]. In contrast, the synthesis of the nanocomposites with laser ablation is deemed as relatively green technique, which does not require residual reagents during the synthesis process. Therefore, the nanocomposites synthesis with laser ablation could have better surface reaction for dye degradation and the photocatalyst with 3% MoO₃ concentration exhibited best performance for MB dye degradation. Overall, the current approach for nanocomposites synthesis based on laser ablation of the pelleted sample provided a promising substitute for nanocomposites generation. This approach unveils the channels of interest for nanocomposites synthesis with laser ablation of other materials in powder form.

4. Conclusion

In summary, a surfactant free approach was devised for MoO₃-TiO₂ nanocomposites synthesis with pelleted powder samples. TEM analysis exhibited the TiO₂ nanoparticles decorated with MoO₃. The photovoltaic as well as photocatalytic characteristics of the nanocomposites were investigated. The nanocomposites manifested an efficient photo-conversion performance and light harvesting capability for MB dye degradation under visible light irradiation. This remarkable performance was attributed to better e^––h⁺ separation by MoO₃-TiO₂ nanocomposites. Further research can be conducted by mixing more compounds for photo conversion and photocatalytic applications.