Fabrication of Ordered and Controlled MoO_x 3D-Nanocubes by Anodization

At microscale surface area to volume ratio is large as compared to bulk matter due to which nano/micro materials possess unique optical, magnetic, interfacial, electrical and chemical properties characteristics. The nanomaterial is very attractive in biomedicine, energy, pharmacy, food, agriculture, and environmental applications [1,2]. Recently transition metal oxides are widely investigated due to wide range of applications [3]. Molybdenum oxides (MoOx) are one of the most attractive transition metal oxides due to their special structural characteristics and low band gap (2.5-2.89eV). MoOx has two simple binary oxides; MoO3 and MoO2. MoO3 has excellent properties like low metallic electrical resistivity, high melting point, and high chemical stability. However, MoO2, MoS (converted from MoOx) has been used in many applications such as a catalyst for alkane isomerization, oxidation reactions, and as a gas sensor. It is also a promising anode material for Li-ion batteries and antibacterial activity [4-10]. Mo compounds have potential in supercapacitor and hydrogen production application [11,12].

Recently MoOx nanoparticles are fabricated by anodization in 0.02M HCl electrolytes. However, only few papers are available on the synthesis MoOx nanocubes by solid state and other methods. But still the synthesized nanocubes were dispersed very faraway and was random size and orientation of the same concept is applied here by adding NH4F to do some surface engineering by cutting edges of the particles by “F” ions [13].

In this study, MoOx nanocubes have been fabricated using the anodization technique for the first time. As “F” ion has the capability to produce pores/ nanotubes in transition metals (Ti. Ta, Fe) because it works like scissor at nano scale [14]. Also, the suitable concentration of NH4F to the electrolyte gives cohesion to the oxides film with the substrate rather it will detach the film in the electrolyte.

Anode was made of molybdenum foil of 1cm width, 2cm length and 0.1mm thickness. Pure graphite sheet (2cm width and 3cm in length and of 0.3cm thickness) has been used as cathode. Samples were washed by dipping in acetone and ethanol for 20min at 50 ℃ temperature. Then samples were cleaned by DI water for 20min at 50 ℃ and dried by air blower. Electrolyte was prepared by mixing 1M KOH, 0.4M NH4F in 20ml of EG and 2ml water stirred by magnetic stirrer for 1 hour at room temperature. The experiment was conducted for different anodization time in the presence of applied voltage of 12V, and the maximum current set was 0.8A, temperature was monitored with the help of digital thermocouple. The experimental arrangements of the anodization current are shown in the Figure 1 and the process parameters are given in Table 1.

Figure 1: Anodization current vs time for all the samples and inset photo is the setup of power supply, connecting wires, anodization bath, cooling bath and sample holder.

Table 1: Anodization parameters for the preparation of samples.

Figure 2:S1 anodized for 2min a) 50μm b) 5μm.

After anodization in electrolyte containing 0.4M NH4F the presence of MoOx cubes is confirmed by Scanning Electron Microscopy (SEM) as clear from the Figures 2-4. It is observed that with the increase in anodization time the formation of cubic shape particles increases and results in increase of particle size. Particle size for the sample (S1) anodized for 2min is about 1μm as can be seen in Figure 2. Particle size for sample (S2) anodized for 5min is 2μm as can be seen in the Figure 3. Similarly, particle size for sample 3 anodized for 7min is 3-4μm as shown in Figure 4. Energy Dispersive Spectroscopy (EDS) was used to measure the elemental composition of the anodized. Peaks of molybdenum, oxygen, potassium, and fluorine are shown in Figure 5. Presence of oxygen reveals the formation of MoOx cubes.

Figure 3: S2 anodized for 5min a) 50μm b) 5μm.

Figure 4: S3 anodized for 7min a) 50μm b) 5μm.

One step anodization is one of the best techniques to produce ordered MoOx cubes using NH4F, KOH in ethylene glycol and DI water as electrolyte. The anodization time controls the size of the cubes. By increasing anodization time the cubes size and its quantity increases drastically and uniformly. Anodization causes the nucleation of cubes uniformly at initial stages and for very long time it divides the surface into patches of cubes clusters.

Support from staff of NILOP and PTPRI is highly acknowledged.

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.

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