α-MoO₃ nanoparticles: solution combustion synthesis, photocatalytic and electrochemical properties

Abstract

Nanoparticles of ultra-porous MoO₃ were synthesized in a single step by a solution combustion reaction using molybdenum metal powder as precursor for the first time. The effects of the preparation conditions, such as the temperature and precursor concentration, on the crystalline phase and morphology of the products were studied systematically. The analytical techniques SEM, TEM, PXRD, TGA, FTIR, and SAED were used to characterize the morphology, composition, and structure of the as-prepared products. The TEM images of MoO₃ show the sizes of the particles to be in the range of 2–10 nm. Electrochemical characterization of MoO₃ was carried out in 0.5 M H₂SO₄. The specific capacitance and electrochromic properties of MoO₃ were studied. The high photocatalytic activity of MoO₃ was investigated using methylene blue azo dye at various concentrations. MoO₃ showed the ability to degrade 100% of methylene blue present at high concentrations of about 75 mg L⁻¹.

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1. Introduction

The synthesis of transition metal oxides and their use in advanced applications represent an exciting and rapidly expanding area of research. Several techniques are being used to control the size of transition metal oxides, which is important for their potential applications.¹ In particular molybdenum oxide has attractive and unusual chemistry produced by its multiple valence states.^2,3 Molybdenum oxide at the nanoscale has shown high activity and is being used in diverse applications such as cathodes in rechargeable batteries, field emission devices, solid lubricants, superconductors, thermoelectric materials, electrochromic devices and as a photocatalyst.^4–8

Molybdenum can form the binary oxides MoO₂ and MoO₃. Molybdenum trioxide has several polymorphs, such as the thermodynamically stable orthorhombic α-MoO₃, metastable monoclinic β-MoO₃ and metastable hexagonal h-MoO₃. The crystalline structure of orthorhombic α-MoO₃ shows a unique layer structure made up of chains of octahedra that share corners. Two such chains are connected by sharing two edges of the octahedra to form a double chain. These double chains are then connected together in the third dimension (perpendicular to the plane of the double chain) by sharing corners to form a sheet-like structure. Thus, for each octahedron, three oxygen atoms are shared by three octahedra of the same double chain, two are shared by two octahedra of adjacent double chains, and one is unshared. This unshared unit is commonly referred to as a Mo=O unit. Finally, these two-dimensional sheets are stacked on top of each other with rather weak interactions, i.e., van der Waals' forces, between the layers.⁹ MoO₃ has a wide band gap of 2.5 to 3.2 eV, is an n-type semiconductor and is a promising material for photocatalytic degradation of organic molecules both under UV and visible light. It has been used in various different technical, industrial and scientific applications. Due to its distinctive electrochromic, thermochromic and photochromic properties, it has been investigated over the past decades for use in smart materials, gas sensors, lubricants, electrochemical storage materials, catalysts and host materials for intercalation.^10–13

The formation of +4 and +6 oxidation states of molybdenum in molybdenum oxide with the required phase and the novel characteristic properties differing from their bulk counterpart mainly depends on the method of synthesis. Considerable research effort has gone into synthesizing MoO₃ nanostructures with specific morphologies, sizes, crystal structures, and dopants, using various synthesis techniques including sol–gel, combustion, hydrothermal synthesis, chemical vapour deposition and pulsed laser ablation.^14–21 Among these methods, the solution combustion route is promising since one can control the formation of product by controlling several parameters such as oxidizer-to-fuel ratio, volume of the precursor solution, reaction temperature, oxygen/air partial pressure, additional oxidizers, mixture of fuels and ignition temperature. This process is capable of producing larger-scale materials faster than the other techniques, and at a lower temperature.

Redox-active metal oxide materials have become the centre of attention for supercapacitor studies.²² The process of charge storage is a combination of two mechanisms in the case of metal oxides:²³ these materials store energy through highly reversible surface redox reactions in addition to electric double layer capacitance, and this combination is termed pseudocapacitance.^24,25 Redox-active metal oxides have gained considerable interest in the areas of electrochromics and batteries.^26,27 The electrochromic effect exhibited by these materials has made them very useful in the development of displays and smart windows.^28,29

For the present work, MoO₃ was synthesised within a few minutes by a solution combustion method, starting with dissolving Mo metal powder in hydrogen peroxide and using sucrose as a fuel. The particle size varies in the range of 2–10 nm. Electrochemical studies were used to examine the specific capacitance and electrochromic properties of the synthesized materials. MoO₃ displayed excellent photocatalytic activity for the degradation of methylene blue under UV light.

The present combustion-based synthesis using molybdenum metal as the precursor and sucrose as the fuel has several advantages including short reaction time, use of a low-cost precursor, and high exothermicity; the reaction lasts for a few seconds and leads to controlled growth of particles that produce floppy end products.

2. Experimental

2.1 Synthesis

All chemicals were purchased from Merck Ltd. and used without further purification. Synthesis of MoO₃ was carried out using two different sources of Mo: a precursor solution of Mo metal was used in “synthesis route 1” and a precursor solution of ammonium heptamolybdate pentahydrate (NH₄)₆Mo₇O₂₄·5H₂O (AHM) was used for “synthesis route 2”.

2.1.1 Synthesis route 1. An aqueous solution of peroxopolymolybdic acid was prepared by dissolving 0.2 g of molybdenum metal powder in 3 mL of hydrogen peroxide (28%) in a 100 mL beaker. About 0.37 g of sucrose was added into this solution as a fuel (oxidizer : fuel ratio = 1 : 1). The mixture was heated on a hot plate at 150 ± 5 °C for 2–3 min to obtain a homogeneous aqueous solution. The aqueous solution was then placed into a muffle furnace maintained at 470 ± 10 °C in air. A vigorous flammable reaction with voluminous formation of froth led to the final products in one minute and this powder is denoted as M1.

2.1.2 Synthesis route 2. For this synthetic route, the precursor solution was prepared by dissolving 0.5 g of AHM in 5 mL distilled water in a 100 mL beaker. Upon warming, a clear light green solution was formed. To this solution, 1.71 g of DL-malic acid was transferred, and the greenish colour turned dark blue. This aqueous mixture was then placed into a muffle furnace maintained at 470 ± 10 °C. After about 5 min, formation of froth followed by a smouldering combustion was observed. The powder obtained by this route is denoted as M2.

2.2 Electrochemical procedures

Electrochemical measurements were carried out using three-electrode cell systems. A glassy carbon electrode (GCE), a platinum electrode and a saturated calomel electrode were used as working, counter and reference electrodes respectively. A uniform suspension of 5 mg mL⁻¹ of synthesized metal oxide in double distilled water was obtained by ultrasonicating the mixture for 20 min. A volume of 6 μL of this suspension was drop-casted onto the GCE and left to dry in the ambient air. Note that the GCE had been previously washed with 1, 0.3 and 0.05 micron alumina slurries and then with distilled water.

A clear and transparent 2 cm × 1 cm × 0.7 mm glass electrode was coated with indium tin oxide (ITO) and washed with alcohol and deionized water. An aqueous suspension of metal oxide was drop-casted onto the glass electrode and dried under ambient conditions.

2.2.1 Cyclic voltammetry (CV) procedure. 10 mL of 0.5 M H₂SO₄ was placed in the electrochemical cell to function as an electrolyte, and nitrogen gas was purged for 15 minutes to remove the dissolved oxygen. Using the three-electrode system, voltammograms were recorded.

2.3 Photocatalytic MB degradation

The photocatalytic activity of the as-synthesized molybdenum oxide (M1) was evaluated by measuring the photocatalytic degradation of methylene blue (MB) in water under the illumination of UV light. A 120 W high-pressure mercury lamp with a wavelength of 253 nm was used as the UV irradiation source. In a typical degradation experiment, 250 mL of a 75 mg L⁻¹ MB solution and 100 mg of prepared molybdenum oxide (M1) were added into a 500 mL Pyrex dish. The pH of this mixture was ∼7. At regular time intervals of 5 min, 3 mL aliquots were sampled and centrifuged to remove the oxide particles. The degradation of the organic dye was monitored by measuring the absorbance of the aliquot solution using a UV-visible spectrophotometer with de-ionized water as the reference. The characteristic absorption of MB at a wavelength of 664 nm was chosen as the parameter to be monitored for the photocatalytic degradation process.

3. Characterization

The powder XRD pattern was recorded on a PANalytical X'pert PRO X-ray diffractometer with a graphite monochromatized Cu Kα radiation source (λ = 1.541 Å). Morphologies and particle sizes of combustion-derived powders were examined using a JEOL-JSM-6490 LV scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) by a JEOL JEM 2100 operating at 200 kV. IR spectra were recorded using a Bruker Alpha-T FTIR spectrometer (Diamond crystal ATR mode, resolution 4 cm⁻¹, 400–4000 cm⁻¹). Thermogravimetric and differential thermal analyses were performed from room temperature to 1000 °C with a heating rate of 10 °C min⁻¹ using an SDT Q600 V20.9 Build 20 apparatus. Cyclic voltammetry experiments were performed using a CH Instruments electrochemical work station (Model CHI 619B, CH Instruments, TX, USA) in a standard three-electrode cell.

4. Results and discussion

The powder X-ray diffraction patterns of as-prepared MoO₃ powders (M1 and M2) are shown in Fig. 1. All of the Bragg reflections of M1 and M2 powders could be indexed to an orthorhombic phase of MoO₃. The sharp diffraction peaks of MoO₃ at (020), (110), (040), (021) and (060) indicate a highly crystalline orthorhombic structure (α-phase). The intensity of the peak of the diffraction from the (021) planes is very strong, as shown in Fig. 1a, indicating a crystal orientation along (001). As shown in Fig. 1b, the high intense peak of (020) planes of the powder XRD pattern suggests that the obtained material has an anisotropic orientation with respect to the (010) face.³⁰ The broadening of the PXRD pattern of the M1 powder indicates the smaller crystallite sizes. The crystallite sizes calculated using the Williamson–Hall plot obtained by profile fitting of the XRD spectra of the M1 and M2 MoO₃ powders were found to be 18 nm and 50 nm, respectively.

Fig. 1 PXRD patterns of MoO₃ powders: (a) M1 powder and (b) M2 powder.

The differences in the morphologies of the as-synthesized M1 and M2 MoO₃ powders were studied using SEM micrographs. Fig. 2a shows the SEM image of the M1, which exhibits macroporosity with fine grains. The BET specific surface area was found to be ∼4.97 m² g⁻¹. In the SEM image of Fig. 2b, M2 can be seen to be quite agglomerated despite being porous.

Fig. 2 SEM images of MoO₃ powders: (a) M1 powder and (b) M2 powder.

Fig. 3 shows the TEM images, SAED pattern and size distribution histogram of the MoO₃ (M1) powder. From the TEM images (a and b), it is observed that the particles are well distributed and then less agglomerated. The abundance of particles with different sizes is also shown in the size distribution histogram (Fig. 3d). There is a narrow particle size distribution with the highest frequency in the range of 6 ± 2 nm. The histogram was obtained by analyzing several frames of similar bright field images using IMAGE J software. The selected-area electron diffraction (SAED) pattern (Fig. 3c) shows the polycrystalline nature of the MoO₃ (M1) powder. A TEM image of the M2 powder is shown in ESI (Fig. S5†) and it is found that the particles are in the range of 50 to 100 nm.

Fig. 3 TEM images (a and b), SAED pattern (c) and size distribution histogram (d) of the MoO₃ (M1) powder.

FT-IR was performed to determine the chemical bonding states between molybdenum and oxygen atoms in MoO₃. Fig. 4 shows the IR spectrum of MoO₃ (M1) measured in the 4000–400 cm⁻¹ region. It can be seen that the as-prepared MoO₃ shows three peaks at 553, 848 and 985 cm⁻¹. The peak at 985 cm⁻¹ is due to the terminal Mo=O bond, which is an indicator of the layered orthorhombic MoO₃ phase. The peak at 848 cm⁻¹ is attributed to the Mo–O–Mo vibrations of the Mo⁶⁺ ions and the broad band at 553 cm⁻¹ is due to the bending vibration of an oxygen atom linked to three metal atoms.³¹

Fig. 4 IR spectrum of MoO₃ (M1) powder.

TGA was performed to determine whether carbon is present in the as-synthesised MoO₃ (M1) powder. Fig. 5 shows that there is a weight loss of ∼1.54%, up to 700 °C, which could be accounted for by loss of adsorbed water.

Fig. 5 TGA of the as-synthesised MoO₃ (M1) powder.

4.1 Electrochemical studies

By drop-casting MoO₃ (M1) aqueous suspension onto the GCE working electrode and by following the above-described CV procedure, 25 continuous cyclic voltammograms were recorded in a potential window from −0.8 to +0.6 V with a scan rate of 30 mV s⁻¹. Three sets of well-resolved and reversible redox couples were observed at −0.032, −0.409 and −0.55 V as shown in Fig. 6. Similar studies with the same experimental conditions were carried out on M2 and commercial MoO₃ for comparison, as shown in ESI Fig. S1 and S2.†

Fig. 6 Continuous cyclic voltammograms (25 cycles) of the GCE drop-casted with MoO₃ (M1) powder in 0.5 M H₂SO₄ with a scan rate 30 mV s⁻¹.

The three sets of redox peaks have also been reported in the literature. But the differences in the peak current and potential corresponding to the major redox peak are due to different experimental conditions and different electrodes employed.³² In the above-described potential window, hydrogen ions insert into the M1 film during the cathodic half cycle, which results in the formation of hydrogen molybdenum oxide bronzes [eqn (1)]. During the reverse cycle, withdrawal of hydrogen ions takes place to form the low-valent molybdenum oxide [eqn (2)].³³ Though this process is reversible, there is a decrease in the peak currents, which can be observed due to the trapping of a certain percentage of the inserted hydrogen ions.³⁴

MoO₃ + xH⁺ + xe⁻ → MoO_3−x (OH)_x (0 < x < 1.6) (1)

MoO₃ + 2ye⁻ + 2yH⁺→ MoO_3−2y + yH₂O (0 < y < 1) (2)

The specific capacitance of the M1-drop-casted GCE was determined by recording the CV in the potential window 0.2 to 1.2 V with a 40 mV s⁻¹ scan rate by following the above-described procedure. Similarly, CVs were recorded for M2 and commercially obtained MoO₃ for the purpose of comparison. The cyclic voltammograms obtained for synthesized MoO₃ systems were approximately rectangularly shaped with one redox peak in each (Fig. 7). Capacitance and specific capacitance (C_sp) were calculated using eqn (3) and (4).¹⁷ C_sp of the M1, M2 and commercially procured MoO₃ systems were found to be 108.88, 77.09 and 41.04 F g⁻¹, respectively.

C_sp = 2C_dl/S (3)

where, C_sp – specific capacitance; C_dl – double layer capacitance; S – quantity of the material used for the study (in this particular case, quantity of material drop-casted onto the electrode).

Fig. 7 Overlaid cyclic voltammograms of GCEs drop-casted with M1 (black line), M2 (blue line) and commercially obtained MoO₃ (red line) in 0.5 M H₂SO₄ with a 40 mV s⁻¹ scan rate.

Double layer capacitance was calculated using the formula:

C_dl (dE/dt) = (I_a − I_c)/2 (4)

where, C_dl – double layer capacitance; dE/dt – rate of change of potential (scan rate) at which the voltammogram was recorded; I_a – anodic current; I_c – cathodic current.

The redox reactions expressed as eqn (1) and (2) explains the electrochromic property of MoO₃. As expressed in eqn (1), cathodic polarization at −2.0 V of M1-casted ITO-coated glass causes the intercalation of H⁺ ions, which in turn converts the white film to green. The intensity of the green color can be increased by polarizing the film at a higher cathodic overpotential such as −3.0 V. The reverse of the above, i.e., anodic polarization, causes the deintercalation of inserted H⁺ ions that leads to the bleaching of the colored film. Further anodic polarization at +3.0 V causes the complete bleaching of the green color as shown in Fig. 8a. The corresponding transmittance spectra were recorded for the different polarizations applied. The spectra agree well with optical photographs taken and with the above explanation. With the increase in cathodic polarization potential, the green color becomes more intense, and as a result a decrease in the transmittance was observed. Conversely, an increase in anodic polarization potential bleaches the color of the film, causing the M1 film to become more transparent (Fig. 8b). M2 and commercial MoO₃ also form the molybdenum bronze-like M1, due to the intercalation of H⁺ ions on applying potentials such as −2.0 and −3.0 V, as shown in ESI Fig. S3 and S4,† respectively. But the colour intensity of M1 is greater than that of M2, which in turn is greater than that of commercial MoO₃ at the same applied potentials. This difference in colour intensity indicates the greater percentage of insertion of H⁺ into MoO₃ in the case of M1. Similarly, less bleaching of colour was observed in the case of M2 and commercial MoO₃ than in M1. This can be explained by incomplete deintercalation of inserted H⁺ ions in M2 and commercial MoO₃ due to less porosity.

Fig. 8 (a) Optical photographs of MoO₃ (M1)-casted ITO-coated glass. (b) corresponding UV-vis spectra of the M1-coated ITO glass.

4.2 Photocatalytic activity

The photocatalytic degradation of MB was used as a test reaction to investigate the photocatalytic performance of MoO₃ (M1) nanoparticles. There are several reports of photocatalytic degradation of MB³⁵ and here we are investigating this reaction under UV irradiation. The degradation was monitored by UV-vis absorption spectroscopy by using the characteristic light absorption of MB at a wavelength of 664 nm at a prefixed time interval.

Initially, with constant concentration of MB dye, the influence of the amount of photocatalyst was investigated in the range of 50 to 250 mg. Around 100% degradation was observed with a dye concentration of about 75 mg L⁻¹ for 100 mg of catalyst.

Fig. 9a shows I/I_o versus irradiation time, where I and I_o are the intensities of the absorbance peak at 664 nm at the reaction time and the initial time. With lower catalyst dosage the degradation was very poor; with 50 mg of catalyst, only ∼10% of the MB degraded. Even the increase in the catalyst dosage to more than 100 mg shows low degradation. This might be due to the scattering of UV light by the excess of photocatalyst which reduces the generation of charge carriers. The other reason could be that the turbidity, which is due to the aggregation of the catalyst particles caused by the high catalyst dosage, leads to a decrease in the penetration of light.

Fig. 9 Photocatalytic degradation of MB: (a) at constant concentration of 75 mg L⁻¹ over a range of catalyst loads, (b) at constant catalyst dosage of 100 mg over a range of concentrations and (c) comparison of MoO₃ (M1) with commercial MoO₃.

As the catalyst dosage was optimised, the effect of concentration was investigated. Fig. 9b shows that as the dye concentration was increased to 100 mg L⁻¹, there was a large decrease in the degradation rate. Hence the optimum catalyst dose of 100 mg MoO₃ can degrade 75 mg L⁻¹ of MB in 60 min. But the commercial MoO₃ shows nearly 18% of the degradation even after 60 min.

5. Conclusion

In summary, we have established a combustion reaction that could produce orthorhombic MoO₃ nanoparticles. As-made MoO₃ powders (M1 and M2) exhibited electrochromic properties besides photocatalytic activity in the degradation of methylene blue. Another interesting point regarding MoO₃ (M1) powder is that its fine particle sizes in the range of 2–10 nm result in a specific surface area of ∼4.97 m² g⁻¹. Hence, the high photocatalytic activity could be correlated with the narrow size distribution of the synthesised MoO₃ (M1) powders.

Acknowledgements

One of the authors, G.T. Chandrappa thanks Department of Science and Technology (DST), Government of India, New Delhi for financial support to carry out the present research work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05135a

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