Novel Fabrication and Enhanced Photocatalytic MB Degradation of Hierarchical Porous Monoliths of MoO3 Nanoplates

Porous monoliths of MoO3 nanoplates were synthesized from ammonium molybdate (AHM) by freeze-casting and subsequent thermal treatment from 300 to 600 °C. Pure orthorhombic MoO3 phase was obtained at thermal treatment temperature of 400 °C and above. MoO3 monoliths thermally treated at 400 °C displayed bimodal pore structure, including large pore channels replicating the ice crystals and small pores from MoO3 sheets stacking. Transmission electron microscopy (TEM) images revealed that the average thicknesses of MoO3 sheet were 50 and 300 nm in porous monoliths thermally treated at 400 °C. The photocatalytic performance of MoO3 was evaluated through degradation of methylene blue (MB) under visible light radiation and MoO3 synthesized at 400 °C exhibited strong adsorption performance and best photocatalytic activity for photodegradation of MB of 99.7% under visible illumination for 60 min. MoO3 photocatalyst displayed promising cyclic performance, and the decolorization efficiency of MB solution was 98.1% after four cycles.

visible radiation. The results showed that the as-synthesized MoO 3 exhibited high-efficiency catalytic as well as adsorption performance, and the decomposition efficiency of 30 mg/L MB was 98.8% under illumination for 30 min, which was far superior to the decomposition efficiencies reported in literature.

Results and Discussion
Phase analysis and morphology observation. X-ray diffraction patterns (XRD) of porous ammonium molybdate (AHM) after thermal treatment between 300 and 600 °C in Fig. 1 shows the formation of α-MoO 3 . Porous AHM, thermally treated at 300 °C shows the presence of h-MoO 3 diffraction peaks at the 2θ of 9.69°, 19.45° and 29.355° in addition to diffraction peaks of α-MoO 3 . Porous AHM thermally treated at 400 °C shows that all the diffraction peaks of the synthesized products correspond to only α-MoO 3 . On increase in thermal treatment temperature of porous AHM to 500 and 600 °C, the intensities of (020), (040) and (060) diffraction peaks increased (Fig. 1). It suggests that the α-MoO 3 crystals grow preferentially along (0k0) direction with increase of thermal treatment temperature. The average crystallite sizes of α-MoO 3 obtained at different thermal treatment temperature are estimated by using Scherrer's equation (L = 0.89λ/β cos θ) 32 and are shown in Table 1. Figure 2 shows the morphology of as-synthesized α-MoO 3 treated at different temperatures. MoO 3 synthesized at 300 °C is composed of foam-like cellular structure (Fig. 2a). At low thermal treatment temperature, the α-MoO 3 crystals show lower crystallinity and do not show rod-like or sheet-like morphology typical of α-MoO 3 33 . On increase of thermal treatment temperature to 400 °C, bimodal pore structure is visible (Fig. 2b) including large porous channels replicating from ice sublimation and small pores originating from stacking of α-MoO 3 nanosheets. As shown in Fig. 2c, when the thermal treatment temperature is further raised to 500 °C, α-MoO 3 sheets show typical crystalline morphology with the average sheet thickness of 300 nm. However, when the thermal treatment temperature increased to 600 °C, α-MoO 3 morphology changed greatly (Fig. 2d). It can be clearly seen that α-MoO 3 powder present a belt-like structure with an average thickness of 2 μm and a length of about 25 μm. This morphology evolved because of the growth of α-MoO 3 crystal along both a axis and b axis. SEM results confirm that the synthesis temperature has a significant impact on the morphologies of α-MoO 3 . Figure 3 shows the energy dispersive spectrum (EDS) and elemental mapping of porous MoO 3 synthesized at 400 °C and demonstrate the homogeneous distribution of Mo and O elements.
Transmission electron micrographs (TEM) of the MoO 3 single crystals in Fig. 4 show that α-MoO 3 nanoplates synthesized at 300 °C have irregular morphology with an average diameter of 200 nm and thickness between 20 and 40 nm. Similarly, α-MoO 3 synthesized at 400 °C (Fig. 4b) exhibited plate thickness of about 50 nm. When the temperature rises to 500 °C, the crystals further grow to an average thickness of 300 nm (Fig. 4c). However, α-MoO 3 nanoplates change to nanorods at the thermal treatment temperature of 600 °C (Fig. 4d). The nanorods are 7 μm long. TEM results further confirm that synthesis temperature has a significant impact on the crystal size and morphology of α-MoO 3 .
FT-IR analysis. Figure 5 shows the surface functional groups of α-MoO 3 products determined with FT-IR.    The texture properties of porous MoO 3 . MoO 3 porous structure was determined with N 2 adsorption/desorption method at 77 K. The adsorption isotherms are classified as type IV-isotherms according to Brunauer-Deming-Deming-Teller (BDDT) classification (Fig. 6a). It induces that the porous structure of the monolith contains mesopores and macropores. The mesopore size distribution is illustrated in the corresponding pore size distribution in Fig. 6b. The BET surface area, pore volume and BJH desorption average pore size of monoliths synthesized at different thermal treatment temperatures are summarized in Table 2. It shows that MoO 3 synthesized at 400 °C has the highest BET surface area of 25.62 m 2 /g and a total pore volume of 0.13 cm 3 /g. In comparison, MoO 3 synthesized at 500 and 600 °C has low BET surface area and pore volume. It suggests that with increase of sintering temperature the MoO 3 grains grow to micron size and results in reduction in pore volume and BET surface area.

XPS analysis.
To further study the composition and chemical state, MoO 3 synthesized at 400 °C was analyzed by X-ray photoelectron spectroscopy (XPS) analysis. Figure 7a shows that the peaks in the spectra were assigned to Mo, O, and C. The C element results from the adventitious hydrocarbon from XPS instrument itself 34 . No other impurities were found. The binding energies in the XPS analysis were obtained by referencing the C 1s signal at 284.5 eV. Figure 7b shows two peaks located at 232.8 eV and 236 eV can be indexed to the Mo 3d 1/2 and Mo 3d 3/2 signals, respectively, which can be assigned to Mo 6+ valence state. The O 1s spectra of the sample were provided in Fig. 7c. The intense peak centered at 530.8 eV was attributed to O 2− anions. In addition, the binding energy at 531.6 eV belongs to hydroxyl or water molecules that are absorbed on the surface of the sample 35 .
UV-Vis spectra analysis. Figure 8a shows the UV-Vis spectra collected from MoO 3 prepared at different temperatures. When the preparation temperature was lower than 500 °C the absorbance of MoO 3 for UV-light was increased and the absorption edge shows red shift which could be due to the grain growth and gradually increased particle size of MoO 3 . The band gaps of the samples were calculated using the Kubelka-Munk method 36 .      PL analysis. Figure 9 shows the photoluminescence spectra of the porous MoO 3 in the wavelength range between 400 and 800 nm under the excitation of 325 nm at room temperature. Two peaks at 440 nm and 481 nm in the emission spectrum is observed, corresponding to the recombination between the conduction bands and the valence bands. The increase of PL intensity corresponds to fast recombination of electron-hole pairs, indicating decrease of photocatalytic activity. In addition, an extra weak emission at 713 nm is observed when the temperature higher than 500 °C. The result shows the existence of a IB between the conduction bands and the valence bands. The existence of IB also led to recombination of electron-hole pairs and reduced the photocatalytic performance 38 . The relative positions among the CB, VB, and IB, as well as the two emissions are schematically shown in the inset of Fig. 9.
Adsorption and photocatalytic properties of the samples. The adsorption of contaminates molecules is a prerequisite for good photocatalytic activity 39 . Figure 10a shows the variation of the methyl blue (MB) concentration during its adsorption. It can be seen that the porous MoO 3 established adsorption-desorption equilibrium in 30 min. In addition, the adsorption amounts of the catalysts decreased as the thermal treatment temperature increases. Figure 10b shows the photocatalytic activity of the as-synthesized MoO 3 . The as-synthesized MoO 3 at 300 °C has the best adsorption performance and faded rate reached 98.4% after stirred for 30 min in the dark. MoO 3 synthesized at 400 °C also has a high adsorption performance with decolorization efficiency of 53.1% under dark for 30 min and the rapid degradation efficiency with decolorization efficiency of 95.0%, 98.4% and 99.2% under visible illumination for 20, 30 and 60 min, respectively, which is higher than standard photocatalyst of TiO 2 (P25). The stability of MoO 3 was investigated for four cycles (Fig. 10c), and MoO 3 synthesized at 400 °C remained active across   Table 3.
Where k is the degradation rate, C 0 is the initial concentration of MB, and C t is the concentration of MB at reaction time t and MoO 3 synthesized at 400 °C has the fastest reaction rate (k = 0.147 min −1 ). The presence of the hierarchically porous structure increases the surface area and enhances the surface adsorption of water and hydroxyl groups. Water and hydroxyl groups can react with the photo-induced holes on the surface of the catalyst to produce hydroxyl radicals, which is a strong oxidizing agent to degrade organic compounds and therefore improves the photocatalytic activity. MoO 3 synthesized at 400 °C showed excellent photocatalytic performance and could potentially be used for photodegradation of pollutants under visible-light radiation.
In order to study the mechanism of the photodegradation, the corresponding effective scavengers were added to the reaction, namely isopropyl alcohol (IPA), triethanolamine (TEOA) and benzoquinone (BQ), respectively. IPA was employed to trap ·OH, TEOA scavenges h + and BQ scavenges O 2 ·− 41 . As shown in Fig. 10d, the addition of IPA could induce the depression effect on the photodegradation of MB solution. Therefore, we can conclude that hydroxyl radicals were the main active species in the reaction systems.
Mechanism of the photocatalytic process. According to the results of the above, we propose a possible photocatalytic mechanism of porous MoO 3 , which is illustrated in Fig. 11. The conduction band and valence band potentials of the semiconductor were calculated by the following equation 42 :  where X is the absolute electronegativity of the semiconductor, which was defined as the geometric average of the absolute electronegativity of the constituent atoms, E e is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), and Eg is the band gap energy of the semiconductor 43,44 . Therefore, the conduction and valence band positions of MoO 3 synthesized at 400 °C were calculated to be 0.47 eV and 3.32 eV, respectively. When MoO 3 particles are irradiated with visible light photogenerated electrons-hole pair formed and photons can migrate to the catalyst surface and initiate redox reactions with the adsorbed H 2 O or -OH generating hydroxyl radicals (·OH) 45 . The major reaction steps in this photocatalytic mechanism are summarized as follows: Characterization of MoO 3 . The crystal structure of MoO 3 powder was characterized by X-ray diffraction (XRD, Bruker D8 advance, Germany) with a Cu-K α radiation source and settings of 30 mA and 40 KV at a . The composition of the MoO 3 was analysis by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, USA). The transmittance and reflectance spectra of the films were recorded using a UV-Vis spectrophotometer (Lambda 750s, USA) in the spectral range 300-800 nm. Photoluminescence spectra of the samples were recorded using a Fluoscence spectrophotometer (VARIAN 3000, USA).

Photocatalytic properties characterization.
Methylene blue (MB) was chosen as the representative organic pollutant to evaluate photocatalytic performance. The adsorption properties were performed in this study. 30 mg of photocatalyst powder was added to a 50 mL MB aqueous solution (30 mg/L) in the dark. During the adsorption, exactly 5 mL of suspension were taken from the reactor at given time intervals.
A 150 W halogen lamp with a controlled voltage of 220 V was used as the energy source for photocatalysis. The distance between the surface of solution and the lamp was approximately 15 cm. Each 0.05 g of the as-synthesized MoO 3 was dispersed in 100 mL of 30 mg/L MB solutions and stirred for 30 min in the dark to establish absorption-desorption equilibrium before testing. At periodic time intervals, 5 mL aliquots were sampled and ultimately centrifuged to extract particles. The percentage degradation of the dye was calculated via the following equation: Where C 0 is initial absorbance of the dry solution before degradation, and C t is absorbance of the dye solution at time t 10 .