Trans. Nonferrous Met. Soc. China 25(2015) 3271-3278

Structure, morphology and opto-magnetic properties of Bi₂MoO₆ nano-photocatalyst synthesized by sol-gel method

V. UMAPATHY^1,2, A. MANIKANDAN³, S. ARUL ANTONY³, P. RAMU⁴, P. NEERAJA⁵

1. Research and Development Centre, Bharathiar University, Coimbatore, Tamil Nadu 641 046, India;

2. Caplin Point Laboratories Limited, Chennai 600 017, India;

3. Postgraduate and Research Department of Chemistry, Presidency College, Chennai 600 005, India;

4. Department of Chemistry, Meenakshi Academy of Higher Education and Research, Chennai, Tamil Nadu 600 009, India;

5. Department of Chemistry, Adhiyamaan College of Engineering, Hosur, Tamil Nadu 635 109, India

Received 17 November 2014; accepted 23 March 2015

Abstract: Bismuth molybdate (Bi₂MoO₆) nano-particles (NPs) were synthesized using bismuth nitrate, ammonium molybdate, citric acid and ethyl cellulose by a simple sol-gel method. The structure, morphology, opto-magnetic and photocatalytic properties of the obtained powder were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray (EDX), ultraviolet-visible diffuse reflectance spectra (DRS), photoluminescence (PL) spectra and vibrating sample magnetometer (VSM) techniques. The XRD, FT-IR and EDX results indicate that the resultant powder is pure and single phase crystalline Bi₂MoO₆ with orthorhombic structure. The HRSEM image shows that the morphology of obtained powder consists with well defined nano-particles structure. The VSM results show superparamagnetic behavior of the obtained nano-particles. The photocatalytic activity of Bi₂MoO₆ nano-particles was performed. The addition of TiO₂ catalyst enhances the photocatalytic activity of Bi₂MoO₆ nano-particles. The catalysts Bi₂MoO₆, TiO₂ and mixed oxide catalyst Bi₂MoO₆-TiO₂ nano-composites (NCs) were tested for the photocatalytic degradation (PCD) of 4-chlorophenol (4-CP). It is found that the PCD efficiency of Bi₂MoO₆-TiO₂ NCs is higher than that of pure Bi₂MoO₆ and TiO₂ catalysts.

Key words: Bi₂MoO₆; nanostructure; sol-gel synthesis; optical properties; magnetic properties; photocatalyst

1 Introduction

Nanostructured semiconductor materials have attracted considerable attention in nanoscience and nanotechnology, due to their unique physical-chemical properties compared with those of the same bulk materials [1]. Recently, metal molybdates materials have been widely used in photoluminescence, microwave applications, optical fibers, scintillator materials, humidity sensors and photocatalysis [2,3]. However, bismuth oxide, Bi₂MO₆ (M=W, Mo), nanomaterials are of special interest, due to their dielectric, ion-conductive, luminescent and catalytic properties [4,5]. The unique properties of nanomaterials are not only dependent on the compositions but also on both size and shapes of materials which are scientific interest in many practical and technological applications [6-8]. Nowadays, Bi₂MO₆ nonmaterial has been used as an excellent solar-energy-conversion material for water splitting and photocatalytic degradation of organic compounds [9-14]. Among many metal molybdates, bismuth molybdate (Bi₂MoO₆) is an important photocatalyst for the degradation of organic compounds.

Various methods have been used to prepare Bi₂MoO₆ nanostrutures, such as hydrothermal [15], solid-state reaction [16], co-precipitation [17] and amorphous complex precursor [18] methods. However, using the above conventional methods, the Bi₂MoO₆ nano-particles (NPs) were produced comparatively large in size with irregular morphology and inhomogeneous, because MoO₃ has a tendency to vaporize at high temperatures. In this study, Bi₂MoO₆ nano-particles were prepared by a simple sol-gel method using ethyl cellulose as the surfactant. Ethyl cellulose is a derivative of cellulose in which some of hydroxyl groups on the repeating glucose units are converted into ethyl ether groups. The number of ethyl groups can vary depending on the manufacturer. Ethyl cellulose contains hydroxyl group in its individual unit, which plays an important role in the dispersion process of Bi₂MoO₆ particles. This hydroxyl group forms an ester linkage with citric acid, which forms big polymeric structure that traps the metal oxides and water molecules and thus prevents the agglomeration of particles. The remarkable advantage of sol-gel method over the above method is the simplicity of the preparation procedure. Sol-gel method exhibits many advantages, such as low process temperature and high control of pure products.

Nanostructured photocatalyst materials have gained much interest, due to their potential application in environmental purification [11,19], solar energy conversion [20] and H₂ production by water splitting [9]. Recently, many studies have been carried out to exploit new visible light-driven photocatalysts. The photo- catalytic activities of Bi₂WO₆ have been revealed by KUDO and HIJII [9], TANG et al [21] and ZHANG and ZHU [22]. KUDO et al [23,24] found out that Bi₂MoO₆ was able to carry out the photocatalytic O^2- evolution under visible light irradiation. The photocatalysis offers superior technology for the removal of different toxic organic compounds from water by using TiO₂ catalyst, which has been widely studied [25], because of its financial/economic and ecologically safe opportunity for solving the energy and pollution problems [26]. TiO₂ is an excellent photocatalyst, because of its high activity, low cost and good stability. In addition to TiO₂, other nanosized mixed oxides, such as tantalates [27,28], vanadates [29,30] and tungstates [11,19,31-34], have been reported for ultraviolet (UV) and visible light photocatalytic activities by many researchers.

Bi₂MoO₆ is an excellent material with visible light- driven photocatalytic activity for water splitting and decomposition of organic pollutants [35]. The improvement of photocatalytic activity of Bi₂MoO₆ can be achieved by doping with TiO₂ catalyst in order to reduce the charge carrier recombination. Such advanced Bi₂MoO₆-TiO₂ mixed catalyst extends its application through the generation of new catalytic sites, due to a strong interaction between them. Therefore, it is highly interesting and desirable to study the photocatalytic activity of Bi₂MoO₆-TiO₂ mixed oxide. HANKARE et al [36] have reported ZnFe₂O₄, TiO₂-ZnFe₂O₄, TiO₂- Al₂O₃-ZnFe₂O₄ photocatalysts for the degradation of methyl red and thymol blue. In this study, Bi₂MoO₆ nano-particles prepared by a simple sol-gel method. It was observed that Bi₂MoO₆ nano-particle can be used as a photocatalyst for the efficient bleaching and mineralization of 4-chloro phenol (4-CP) under visible light irradiation.

2 Experimental

2.1 Materials and methods

All the chemicals were of analytical grade obtained from Merck, India, and were used as received without further purification. Ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O), bismuth nitrate (Bi(NO₃)₃·5H₂O), citric acid and ethyl cellulose were used as the raw materials. Ethyl cellulose powders were sprinkled slowly into deionized water under continuous stirring to avoid the clumping of material in water. Bismuth nitrate and ammonium molybdate in stoichiometric ratio and citric acid were dissolved in deionized water separately. These solutions were added to ethyl cellulose solution at 50 °C to form sol. This sol is then heated slowly to 90 °C under constant stirring to obtain a wet gel. Then, the gel product was calcined at 650 °C for 2 h. It was ground in a mortar to form a final product of fine powder.

2.2 Characterization techniques

The characterizations of the obtained Bi₂MoO₆ nano-powder were conducted using various techniques to verify the phase formation, crystallite size, distribution and to explore other parameters of interest. The structural characterization of Bi₂MoO₆ nano-particles was performed using Rigaku Ultima X-ray diffractometer equipped with Cu K_α radiation (λ=1.5418 ). The surface functional groups were analyzed by Perkin Elmer Fourier transform infrared (FT-IR) spectrometer. The morphological studies and energy dispersive X-ray analysis (EDX) of Bi₂MoO₆ NPs have been performed with a Jeol JSM6360 high resolution scanning electron microscope (HRSEM). The UV-visible diffuse reflectance spectrum (DRS) was recorded using Cary100 UV-visible spectrophotometer to estimate their band gap energy (E_g). The photoluminescence (PL) properties were recorded at room temperature using Varian Cary Eclipse Fluorescence Spectrophotometer. The magnetic measurements were carried out at room temperature using a PMC MicroMag 3900 model vibrating sample magnetometer equipped with 1 T magnet.

2.3 Photocatalytic reactor setup and degradation procedure

All photochemical reactions under identical conditions were carried out in a self-designed photocatalytic reactor. This model consists of eight medium pressure mercury vapor lamps (8 W) setting in parallel and emitting wavelength of 365 nm. It has a reaction chamber with specially designed reflectors made of highly polished Al and built in cooling fan at the bottom and black cover to prevent UV leakage. It is provided with the magnetic stirrer at the center. The open borosilicate glass tube of 40 cm in height and 12.6 mm in diameter was used as a reaction vessel. The irradiation was carried out using only six parallel medium pressure mercury lamps. The solution was aerated continuously by a pump to provide oxygen and for the complete mixing of solution. Prior to the photocatalytic experiments, the adsorption of 4-CP on Bi₂MoO₆ and TiO₂-supported Bi₂MoO₆ nano-photocatalyst was carried out by mixing 100 mL aqueous solution of 4-CP with fixed mass of the respective photocatalyst. A known amount of commercial TiO₂ (Degussa P-25) was added to a known amount of Bi₂MoO₆ and finely ground in a mortar and pestle for 30 min so as to obtain a mixture of Bi₂MoO₆-TiO₂ in the required mole fraction. The photocatalytic degradation (PCD) was carried out by mixing 100 mL aqueous solution of 4-CP and fixed mass of Bi₂MoO₆ nano-photocatalyst. The PCD of 4-CP was also carried out with Bi₂MoO₆-TiO₂ mixed oxide. Therefore, the interactions of Bi₂MoO₆ with TiO₂ can be assumed to take place at the grain boundaries. The PCD efficiency was also calculated for pure oxides (Bi₂MoO₆ and TiO₂) and mixed oxide (Bi₂MoO₆-TiO₂). All solutions prior to photolysis were kept in dark by covering with Al foil to prevent any photochemical reactions. The PCD efficiency (η) was calculated from the following expression:

η=(C_i-C_t)/C_i×100%                           (1)

where C_i is the initial concentration of 4-CP, C_t is the concentration of 4-CP after time t (min).

3 Results and discussion

3.1 Powder X-ray diffraction (XRD)

The crystal structure and phase analysis of the samples were characterized by powder X-ray diffraction (XRD) pattern. The XRD pattern of as-synthesized Bi₂MoO₆ powder shown in Fig. 1 is indexed to orthorhombic Bi₂MoO₆ according to the JCPDS database No. 21-0102 [15,37]. No impurities of secondary phases such as Bi₂O₃, MoO₃, and others were detected in the XRD pattern of product. The very high peak intensity suggests that the material is highly crystalline. This indicates the complete transformation of the precursor into orthorhombic Bi₂MoO₆ phase. The average crystallite size of Bi₂MoO₆ sample was calculated using Debye Scherrer formula given in Eq. (2):

                               (2)

where L is the crystallite size, λ is the X-ray wavelength, θ is the Bragg diffraction angle and β is the full width at half maximum (FWHM). The average crystallite size L calculated from the diffraction peaks is found to be 35-38 nm.

Fig. 1 XRD pattern of Bi₂MoO₆ NPs

The lattice parameter of the sample was calculated using the formula given in Eq. (3):

               (3)

where h, k and l are Miller’s indices. The calculated lattice parameters are found to be a=5.498 , b=16.118 , and c=5.401 , which are in good agreement with the JCPDS file card No. 21-0102. Similar values (a=5.502 , b=16.210 , and c=5.483 ) have been reported earlier by ADHIKARI et al [37].

3.2 FT-IR spectral analysis

Figure 2 shows the Fourier transform infrared (FT-IR) spectrum of Bi₂MoO₆ nano-powders. The FT-IR spectrum contains a broad band between ~3200 and ~3500 cm^-1 which is due to the hydroxyl (O—H) stretching mode [38]. A weak band appearing at 2137 cm^-1 may be due to the combination band of C—H or O—H stretching. However, a sharp band at 1646 cm^-1 is due to the presence of O—H bending vibration of water molecule. The spectrum of Bi₂MoO₆ sample shows absorption bands between ~650 and 850 cm^-1 which is mainly due to Mo=O stretching vibration. However, the peak appearing at 720 cm^-1 is ascribed to Mo(VI)—O tetrahedral stretching and the peak at 499 cm^-1 corresponds to Bi(III)—O octahedral stretching vibration.

3.3 Scanning electron microscopy (SEM) studies

The surface morphology of Bi₂MoO₆ sample was examined by high resolution scanning electron microscopy (HRSEM). Figure 3(a) shows the HRSEM image of Bi₂MoO₆ sample which consists of agglomerated particle-like nanostructure. The formation of agglomerated particle-like nanostructure may be due to the attachment of magnetic nature of nano-crystals. The elemental composition and purity of Bi₂MoO₆ sample were also analyzed by energy dispersive X-ray (EDX) analysis. Figure 3(b) shows the EDX spectrum of Bi₂MoO₆ sample which shows the presence of Bi, Mo and O by the appearance of Bi, Mo and O peaks without any other characteristic peaks. Hence, the EDX results are perfect evidences to propose that the prepared sample does not contain any other elements and is indeed free from other impurities.

Fig. 2 FT-IR spectrum of Bi₂MoO₆ NPs

Fig. 3 HRSEM image (a) and EDX spectrum (b) of Bi₂MoO₆ NPs

3.4 Optical properties

UV-visible absorption spectroscopy is an important technique for characterizing the optical properties of as-prepared Bi₂MoO₆ nano-powders. The UV-visible diffuse reflectance spectrum (DRS) of Bi₂MoO₆ nano-particle is shown in Fig. 4(a). The broad absorption band centered at 230 nm is attributed to the O^2- to Mo⁶⁺ charge transfer of the isolated MoO₆ sites [39]. The steep shape of DRS spectrum indicates that the visible light absorption is arisen from the band-gap transition instead of impurity levels [24,40]. The color of Bi₂MoO₆ sample is yellow, in accordance with the extension of its absorption edge to 478 nm. The steep absorption edge is at 478 nm, corresponding to a band gap energy E_g of about 2.59 eV (the inset in Fig. 4(a)), indicating that the sample exhibits an intense absorption in the visible light range. The DRS analysis was used to study the relation of crystallite size and band gap of the semiconductors. The band gap energy (E_g) of the samples can be evaluated using the Kubelka-Munk model. It allows the calculation of the absorption coefficient (α) by the measurement of the UV-visible diffuse reflectance. The kubelka-Munk function, F(R), is directly proportional to the absorption coefficient (α) and the value is estimated from the following equation [41]:

                         (4)

where α is the absorbance, R is the reflectance. A graph is plotted between [F(R)hυ]² and hυ, and the obtained intercept value is the band gap energy of the sample, as shown in the inset in Fig. 4(a). The estimated band gap value of Bi₂MoO₆ sample is 2.59 eV. The E_g of 2.59 eV is much closer to 2.53 eV [13], 2.60 eV [42] and 2.58 eV [43], for which the samples were synthesized by conventional/ microwave-assisted solvothermal, molten salt route and citrate method, respectively. Therefore, the observed different band gaps by different preparation routes could not result from the quantum-size effect, but can be ascribed to their different degrees of crystallization [18,34].

Fig. 4 UV-visible diffuse reflectance spectrum (a) and PL spectrum (b) of Bi₂MoO₆ NPs

However, the color of the sample is light yellow and it is suggested that the visible light absorption is due to the transition from the valence band consisting of O 2p orbitals to the conduction band derived from the primary Mo 4d orbitals in MoO₆ octahedra and the secondary Bi 6p orbitals [24].

The room temperature photoluminescence (PL) spectrum of Bi₂MoO₆ nano-particles is shown in Fig. 4(b). The PL spectrum was recorded in order to study the defects and other impurity states of semiconductors. The PL spectrum shows an emission band in the UV region at around 385 nm, which is attributed to the near band-edge emission of Bi₂MoO₆, indicating the quantum confinement effect [44-46]. However, the luminescence peaks were observed in the visible region at around 422, 458, 485, 510 and 535 nm, which are mainly due to the presence of radiative defects and oxygen vacancies of Bi₂MoO₆ nano-particles.

3.5 Magnetic properties

Figure 5 shows the magnetic hysteresis (M-H) loop of Bi₂MoO₆ nano-particles with the field sweeping from -15 to +15 kA/m at room temperature. The obtained Bi₂MoO₆ nano-particles show superparamagnetic behavior. The Bi₂MoO₆ nano-particles are important magnetic materials. The Bi₂MoO₆ nano-particles obtained by sol-gel method show remarkable super- paramagnetic behavior with relatively high saturation magnetization (M_s) than those obtained by other methods, which may be due to the synthesis route and conditions, the type of precursors, calcinations, etc. The saturation magnetization (M_s), remnant magnetization (M_r) and coercivity (H_c) values of the sample Bi₂MoO₆ are 2.55×10^{-4 A}·m²/kg, 0.44×10^{-4 A}·m²/kg and 142.57 kA/m, respectively. However, the magnetic properties of materials are influenced by many factors, such as size, crystallinity and surface structure.

Fig. 5 Magnetic hysteresis (M-H) loops of Bi₂MoO₆ NPs

3.6 Photocatalytic properties

It has been generally accepted that the crystallinity, size, shape and morphologies of the nano-materials are important factors that influence their photocatalytic activity. It is important to prepare Bi₂MoO₆ nano- photocatalyst to understand the catalytic activity of TiO₂ catalyst supported Bi₂MoO₆-TiO₂ nano-composites. This study made an attempt to reveal the relationship between optical and photocatalytic properties of pure metal oxides (Bi₂MoO₆ and TiO₂) or mixed metal oxide (Bi₂MoO₆-TiO₂) and a series of experiments were carried out with 4-CP in aqueous suspension with the light wavelength of 365 nm.

The effect of TiO₂-supported Bi₂MoO₆ nano- photocatalyst on the PCD efficiency was evaluated, as shown in Fig. 6. The control experiment was carried out in the absence of catalyst by irradiating the solution with UV radiation (photolysis). The degradation of 4-CP due to photolysis is found to be less than 10%. The PCD efficiency of Bi₂MoO₆ is low when compared with that of TiO₂. The PCD efficiency of TiO₂-supported Bi₂MoO₆ (i.e., Bi₂MoO₆-TiO₂) photocatalyst is higher than that of pure Bi₂MoO₆ photocatalyst. It is found that the photocatalytic activity of single phase Bi₂MoO₆ is enhanced when it is coupled with TiO₂ catalyst to form a composite catalyst [47, 48]. Though, the band gap of Bi₂MoO₆ is smaller (2.59 eV) than that of TiO₂ (3.2 eV) and it is a visible light active catalyst, which exhibits lower photocatalytic activity, due to its lower valence band potential compared with that of TiO₂ [36]. When TiO₂ and Bi₂MoO₆ are coupled and irradiated with UV-visible light, the photocatalytic activity is improved, though the charge carriers can migrate to Bi₂MoO₆ due to higher valence band potential of TiO₂. The 4-CP photocatalytic degradation occurs by the hydroxyl radicals attacking the phenyl groups of 4-CP. It is believed to be initiated through the attacks by hydroxyl radicals at the phenyl groups of 4-CP, which may result in the formation of intermediates that may be mono- hydroxylated or dihydroxylated 4-CP and followed by the cleavage of two phenyl groups into intermediates. In addition, hydroquinone is the major intermediate.

Fig. 6 PCD efficiency of TiO₂-supported Bi₂MoO₆ photocatalyst

The photo-degradation kinetics of 4-CP with and without Bi₂MoO₆-TiO₂ nano-photocatalyst in the presence of UV light was evaluated using the pseudo first-order rate equation:

ln(C_t/C_o)=-k₁t                               (5)

where C_o is the initial concentration (mg/L), C_t is the concentration (mg/L) at time t, t is the UV light exposure time and k₁ is the first-order rate constant. The values of k₁ in relation to 4-CP concentrations of 50 mg/L and 1000 mg/L in the presence of Bi₂MoO₆-TiO₂ catalyst are 1.67×10^-2 and 0.79×10^-2 min^-1, respectively, whereas the values of k₁ of the same reaction without catalyst are found to be 0.63×10^-2 and 0.29×10^-2 min^-1, respectively. Higher rate constant achieved using Bi₂MoO₆-TiO₂ catalyst can be attributed to the combined effects of the adsorption of 4-CP molecule over catalyst surface followed by oxidation using the generated hydroxyl radicals and the direct attack of photo-generated holes [49].

4 Conclusions

1) Bi₂MoO₆ nano-particles were synthesized via a simple sol-gel method using ethyl cellulose as the surfactant. The XRD results indicate pure single phase crystalline with orthorhombic structure of Bi₂MoO₆. The HRSEM image shows that the morphology of the product consists with well defined nano-particles structure with agglomeration.

2) The VSM results show superparamagnetic behavior. The PCD efficiency of Bi₂MoO₆ is low when compared with that of TiO₂ catalyst. The PCD efficiency of TiO₂-supported Bi₂MoO₆ (i.e., Bi₂MoO₆-TiO₂) photocatalyst is higher than that of pure Bi₂MoO₆ photocatalyst. These results indicate that Bi₂MoO₆–TiO₂ nanostructures may find applications in water pollution control.

3) Compared with other synthetic methods, sol-gel method is a facile, low-cost pathway to prepare novel Bi₂MoO₆ nano-architectures.

4) The orthorhombic Bi₂MoO₆ and Bi₂MoO₆-TiO₂ nano-photacatalysts show photocatalytic efficiencies of 91.64% and 97.02%, respectively, for the degradation of 4-CP under the UV-visible light irradiation.

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溶胶-凝胶法制备Bi₂MoO₆纳米光催化剂的结构、形貌和光磁特性

V. UMAPATHY^1,2, A. MANIKANDAN³, S. ARUL ANTONY³, P. RAMU⁴, P. NEERAJA⁵

1. Research and Development Centre, Bharathiar University, Coimbatore, Tamil Nadu 641 046, India;

2. Caplin Point Laboratories Limited, Chennai 600 017, India;

3. PG and Research Department of Chemistry, Presidency College, Chennai 600 005, India;

4. Department of Chemistry, Meenakshi Academy of Higher Education and Research, Chennai, Tamil Nadu 600 009, India;

5. Department of Chemistry, Adhiyamaan College of Engineering, Hosur, Tamil Nadu 635 109, India

摘  要：采用硝酸铋、钼酸铵、柠檬酸和乙基纤维素为原料，通过溶胶-凝胶法制备钼酸铋(Bi₂MoO₆)纳米颗粒。通过X射线衍射(XRD)、傅里叶变换红外光谱(FT-IR)、高分辨扫描电镜(HRSEM)、能谱分析(EDX)、紫外-可见漫散射光谱(DRS)、光致发光光谱(PL)和振动样品磁强计(VSM)等手段对制备的粉末结构、形貌、光磁性和光催化性能进行表征。XRD、 FT-IR和EDX结果表明，制备的粉末是具有斜方晶结构的纯单相晶体Bi₂MoO₆。HRSEM图像显示，制备的粉末具有很好的纳米颗粒结构。VSM结果显示制备的纳米颗粒具有超顺磁性。Bi₂MoO₆纳米颗粒的光催化活性实验发现TiO₂催化剂的添加提高Bi₂MoO₆纳米颗粒的光催化活性。催化剂Bi₂MoO₆、TiO₂和混合氧化物催化剂Bi₂MoO₆-TiO₂纳米复合材料对4-氯酚的光催化降解(PCD)，发现Bi₂MoO₆-TiO₂纳米复合材料的PCD效率比纯Bi₂MoO₆和TiO₂催化剂的PCD效率高。

关键词：Bi₂MoO₆；纳米结构；溶胶-凝胶合成法；光学性能；磁性能；光催化剂

(Edited by Mu-lan QIN)

Corresponding author: P. NEERAJA; Tel: +91-9487819133; E-mail: researchneeraja@gmail.com

DOI: 10.1016/S1003-6326(15)63948-6